To triumph over their competition, athletes must be the fastest, strongest or nimblest contenders in their respective events. Record breakers must then push even further, surpassing the limits of known human capability.

But besides their physical prowess, athletes harness a unique set of mental skills that allow them to succeed in their respective disciplines. Numerous studies have shown that athletes' brains differ from nonathletes' brains.

Here are some examples of how sports uniquely shape athletes' brains.

Related: What is doping? And which performance-enhancing drugs are banned?

Visual cue processing

The ability to quickly soak up visual information and make decisions accordingly is a crucial skill for athletes, especially those who play team sports, such as soccer or basketball.

A 2013 study in the journal Scientific Reports revealed that professional ice hockey, soccer and rugby players are better visual learners than people with lower-level abilities in the same sports.

The pros were compared with "elite amateurs" — in this case, U.S. college athletes and players from a European Olympic sport-training center. They were also compared with nonathlete university students. Compared with both groups, professional athletes performed better, and improved faster, on a task that tested their ability to focus on and track objects moving across a screen. In other words, their brains are more skilled at processing "dynamic visual scenes," or the world moving around them, the study authors found. The elite amateurs were also better at this than the nonathletes.

This knowledge could be used to enhance an athlete's training and determine the best time for them to return to their sport following an injury, Jocelyn Faubert, author of the 2013 Scientific Reports study and a professor at the University of Montreal School of Optometry, told Live Science. For example, assessing how efficiently an athlete can process visual information and not make judgment errors could prevent them from coming back too early and putting themselves in danger, he said.

Muscle memory

Acrobatic athletes, such as divers and gymnasts, need to be really good at performing sequences of movements without consciously thinking about it — a phenomenon colloquially known as "muscle memory."

A 2023 study in The Journal of Neuroscience revealed that the brain plans and coordinates repetitive movements like those performed by athletes and trained musicians by quickly "zipping" and "unzipping" crucial information about them. At first, the sequence and timing of the steps are programmed separately in the brain, but with training, these individual elements become seamlessly integrated into one burst of coordinated brain activity. This process involves a network of neurons in the cortex — the outer layer of the brain — that regulate movement.

Related: 'Breaking,' aka breakdancing, is in the Olympics for the 1st time — here's the brain science behind it

Predictions

In baseball, a batter must make quick and accurate predictions about the fate of each ball the pitcher throws. For example, will it enter the strike zone, and how fast will it come at them?

It turns out that, depending on what the batter predicts, their brain activity changes. Specifically, neurons within a region of the brain called the left ventral extrastriate cortex vary in these scenarios, according to a 2022 study in the journal Cerebral Cortex. This is likely because of batters' unique ability to relate visual cues about a pitcher's movements to the potential path of the ball, the authors said.

Structurally speaking, research has also shown that professional divers, for example, have a thicker superior temporal sulcus (STS) than novices. The STS is a region of the brain that plays an important role in the perception of movement of other living things and it also helps decipher the intentions behind those movements. This makes sense in the context of diving, as these athletes often learn by watching other divers' performances, the authors said. And, of course, this is true of many sports.

Balance

Acrobatic athletes, such as gymnasts, have remarkable proprioceptive skills, or the ability to sense where their bodies are in space. An intricate network of neurons in the cerebellum, a region at the base of the brain, enables these athletes to rapidly course-correct in the air or keep their balance on an apparatus when a trick doesn't go according to plan.

If this safety net malfunctions — as famously happened to esteemed U.S. gymnast Simone Biles when she got the "twisties" during the 2020 Tokyo Olympics — it can cause these athletes to lose control of their bodies in midair, with potentially deadly consequences.

Focus and attention

Athletes must be able to appropriately divide their attention and dynamically switch between different ways of thinking. For example, during a match, a soccer player who is dribbling the ball one way may need to quickly switch direction if approached by a player from the opposing team.

The cognitive skills needed to switch your attention also extend to tasks in daily life, such as listening to a podcast while cleaning the house. A 2022 study in the International Journal of Sport and Exercise Psychology provided evidence that athletes are much better at this than nonathletes are.

Notably, athletes trained in team sports that require aerobic or high-intensity interval training had particularly enhanced skills in this area. They stood out for their cognitive flexibility and their ability to appropriately allocate attention, researchers found.

At this point, it's unknown why athletic training influences cognition this way, Art Kramer, co-author of the International Journal of Sport and Exercise Psychology study and director of the Center for Cognitive and Brain Health at Northeastern University in Boston, told Live Science. To find out, you'd need to do a long-term study or a randomized controlled trial in which some kids are put into athletic training while others are not and then monitor them over time. But such a study would be unethical because some kids would be completely denied access to sports, he said.

Resistance to aging in the brain?

The cognitive benefits of athletic training may also extend throughout life. Perhaps no one exemplified this better than the late Canadian track-and-field athlete Olga Kotelko, who held more than 30 world records.

Before she died in 2014 at age 95, Kramer and colleagues studied her brain in the lab.

As we age, the "white matter" — the connections between neurons in different regions of the brain — deteriorates. However, the team found that Olga — despite being in her mid-90s at the time — had strikingly intact white matter, comparable to that of less-active women who were more than three decades younger.

Olga was also quicker at responding to cognitive tasks than other nonagenarians who were tested in a separate, independent study, and she had better memory than them, the team found.

Of course, general conclusions can't be drawn from one athlete. However, as the team said, there is "only one Olga." For this reason, she afforded scientists a unique glimpse into the long-term impacts of athletic training on the brain.

It is important to note though that not every elite-level sport is associated with people surviving into old age, or staying sharp into their 90s like Olga did. Scientists are still figuring out which sports bring about such benefits and which don't.

Related: Olympic athletes dive into the Seine — days after it was deemed too contaminated with poop for safe swimming

Training of the next generation

Looking forward, encouraging brain training in athletes from an early age may lead to even more sporting gains.

"We're at a point with training athletes, in particular, where the human body can't really go much further, but there's so much more we can actually do with cognition," Kylie Steel, a sports scientist at Western Sydney University in Australia, told Live Science.

In an article in The Conversation, Steel and colleagues argued that coaches should focus more on training athletes' cognitive abilities, such as their memory and decision-making skills. This is especially important during the earlier years of life, when the brain is more malleable, they posited.

In a ball sport like soccer, this training could involve asking players to use their nondominant foot to kick the ball.

"If we can try and encourage a lot more training associated with that [cognitive enhancement] — particularly in the junior years — by the time they get to those later years, tactically, they'll be more skilled," Steel proposed.

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You've reached mile 20 (kilometer 32) of your marathon and suddenly a wave of exhaustion crashes over you, making you feel dizzy and causing your legs to feel like lead. One more step seems impossible.

You've likely "hit the wall" — but what does this mean?

Hitting the wall occurs when the body becomes abruptly and debilitatingly tired during exercise, causing people to slow down or stop working out completely.

The condition usually occurs in people who are competing in long-distance, endurance sports, such as cycling, swimming and running. This is because it is caused by a depletion in the body's store of glycogen — a type of glucose that is stored in liver and muscle tissue.

Related: What causes you to get a 'stitch in your side'?

During high-intensity exercise, our muscles use glycogen as a primary energy source by breaking it down into glucose, which powers movement. However, our muscles can only store a certain amount of glycogen. This means that toward the end of a long race, if someone has not replaced that glycogen with food or drink, the body starts running on empty.

For instance, a marathon is 26.2 miles (42.2 km) long. The average person stores around 1,800 calories worth of glycogen in their muscles and will generally burn around 100 calories per mile (1.6 km) that they run. Therefore, one would typically hit the wall around the 18- to 20-mile (29 to 30 km) mark.

Symptoms of hitting the wall include intense exhaustion, muscle cramping and an increase in heart and breathing rate as the body tries to "keep things going," Laura Richardson, a clinical associate professor in applied exercise science and movement science at the University of Michigan, told Live Science.

Some individuals may just need to slow down and walk for a bit, while others have to stop and rest completely, Richardson said. Regardless, everyone must replenish their depleted glycogen store.

Whether and how quickly a person hits the wall can be influenced by many factors, including sex, age and fitness level, Dr. Samuel Dona, a sports medicine physician at the University of Maryland Medical System, told Live Science.

A 2021 study found that in a sample of 928 marathon runners, 28% of men hit the wall versus 17% of women. This could partly be because men are more likely to overestimate their marathon abilities and run too fast initially, the authors concluded.

Professional athletes, like those who compete in Olympic endurance events, are less likely to hit the wall than regular folk. That's because they have more race experience and a better understanding of what formula works best to stop it from happening, Heather Vincent, director of the University of Florida Health Sports Performance Center, told Live Science.

Some of this formula comes down to proper training, such as doing different intensity exercises, from slow walks to hill work, that encourage plenty of variability in heart rate, Richardson said. This trains the body to learn to use fuel at different intensities while strengthening the heart and lungs, she added. Furthermore, increasing mileage slowly helps prevent people from hitting the wall.

Drinking fluids and consuming a carbohydrate-rich diet can also maximize glycogen stores, — a phenomenon known as "carbohydrate loading," Vincent said.

For long races, however, most people will need to replenish glycogen stores with food and drink — something like an energy bar or gel every 30 minutes, Dona said.

This article is for informational purposes only and is not meant to offer medical advice.

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An event premiering at the 2024 Paris Olympics showcases athletes performing rhythmic footwork, popping into inverted poses supported on one hand, and spinning on the tops of their heads.

That's right: Breaking, widely popularized as "breakdancing," has made its Olympic debut.

As with other athletic events that involve intricate movements and explosive acrobatics, some remarkable brain science underpins dancers' ability to perform this style. In particular, such dance training triggers changes in the vestibular system, a sensory system that's crucial for our sense of balance.

This system is responsible for tracking the acceleration of the head through space as it moves, said Maxime Maheu, an assistant professor at the University of Montreal and a researcher at the Center for Research in Interdisciplinary Rehabilitation. Thus, the vestibular system is especially important for dance styles that involve rotations, such as pirouettes.

"I would tend to say that breakdancers would probably show similar findings," Maheu told Live Science in an email. "As long as the vestibular system is challenged repetitively, I would think we could find similar results."

Related: At what age does athleticism peak in different sports?

The science behind breaking

Breaking originated in the Bronx in New York City in the 1970s and remains a core element of hip-hop culture to this day. It incorporates movements performed standing up and others performed on the ground, as well as moves that transition between the two levels.

It features freezes, which are "powerful and impactful poses" in which the breaker often balances on their hands, elbows or head, said Chadwick Gaspard, an Los Angeles-based dancer who's currently working with Jacob Jonas The Company. Breakers also whip out power moves that involve rapidly flipping and twisting their bodies; spinning on top of their heads; or swinging their legs through the air. "It's like the 'wow' or 'statement piece' of breaking," Gaspard said of power moves.

Especially as you watch power moves unfold, you might wonder how Olympic breakers keep their bearings as they rapidly spin and twist. That's where the vestibular system comes in.

Maheu studies the inner workings of the vestibular system, partly to help patients with chronic dizziness and other conditions that disrupt balance. He's conducted several studies comparing dancers to people without dance training to see if the dancers' unique nervous systems could inform vestibular rehabilitation programs.

He's looked at the vestibulo-ocular reflex (VOR), an involuntary reflex that moves the eyes to stabilize the visual field during head movements. The VOR kicks in when you focus on a fixed point — say, a poster on a wall — while quickly turning your head. However, the VOR is suppressed when you track a moving object — like a passing car — while simultaneously moving your head in the same direction.

In a 2023 study in the Journal of Neurophysiology, Maheu and colleagues worked with professional ballet dancers with 10 to 20 years of experience. The dancers were asked to focus on a fixed point while a researcher quickly turned their heads to one side. In a comparison group of nondancers, the eyes moved at an equal velocity to the head but in the opposite direction to keep their gaze on target. However, dancers' eyes moved slightly faster than the head did for the first millisecond it was moving.

Related: What's happening inside Simone Biles' brain when the 'twisties' set in?

This suggests the VOR can potentially be enhanced with training. "We don’t know for sure how dance training may explain this result, but it could offer interesting new paths to follow for vestibular rehabilitation," Maheu said.

In addition, a 2018 study by the same group found that dancers can more easily suppress their VOR when needed, compared with nondancers. In this study, the dancers had been trained in a range of styles — including salsa, ballet, modern and hip-hop — and were asked to track a moving target as their heads were also moved.

"They did not completely switch off the VOR but were able to move their eyes earlier to correct [their focus]," Maheu said. This difference between the nondancers and dancers grew with experience; dancers with more than 10 years of training showed the most efficient VOR suppression.

The team ran another study with these same dancers, but this time, the dancers and nondancers were pitted against each other in a balance test. This test is designed to reveal whether a person is relying more on visual cues, sensations from the body or the vestibular system to maintain their balance. The dancers appeared to rely less on vestibular reflexes and more on other cues, especially vision, to outperform the nondancers.

These latter two studies included only a small group of people, so that makes their results less certain. However, this overarching idea that dance training changes the vestibular system is also backed up by brain scans.

For instance, a 2014 study of ballet dancers, figure skaters and slackliners (who practice something similar to tightrope walking) found similarities between these athletes' brains and the brains of people with vestibular impairments. The study looked at the density of white matter tracts — the insulated wires that connect brain cells — and found that areas of lower-than-average density overlapped in the two groups' brains. These overlapping tracts were involved in a wide range of functions, including sensation processing and movement control.

Why would people with severe balance problems show the same white-matter patterns as certified balancing experts? One explanation is that "both [groups] might need to suppress or reinterpret vestibular input to maintain balance," the study authors suggested. For instance, "ballet dancers reduce vestibular responses to increase balance during a pirouette."

In short, reacting too strongly to vestibular cues during a turn might be detrimental, making a person dizzier.

Related: Will Olympic athletes ever stop breaking records?

Beyond the vestibular system

It's likely that these distinct changes in the vestibular system would show up in highly trained breakers, too, Maheu proposed. But, of course, it isn't just the vestibular system that enables breakers to perform impressive feats.

Similar to elite gymnasts such as Simone Biles, breakers repeatedly practice sequences of movements, gradually increasing the complexity to build confidence and muscle memory. Studies suggest that, over time, complex movement sequences become encoded in a single, efficient burst of brain activity. This may help to explain why trained athletes don't have to consciously think about every little thing their bodies are doing to stay in control.

When learning to do head spins, breakers start with quarter turns and half turns before working up to full turns and multiples, said Gaspard, who started breaking about a decade ago, training in South Florida with the Street Masters Crew and others.

"As you keep increasing the amount of turns that you can do — at first you'll be dizzy, but at some point, like there'll be less and less dizziness until you start drilling out like a crazy amount," he said.

In regard to being upside down, "maybe within a few weeks or months, being inverted isn't as daunting," Gaspard added.

The most exciting part of seeing breaking in the Olympics is that "this culture that I've been a part of is getting a lot of limelight and exposure to a wider audience," Gaspard said. Breakers have gained new acclaim and work opportunities in recent years and now, "their art is in the Olympics."

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Behind every Olympian is an ensemble of coaches, physiotherapists, nutritionists and doctors who help keep the athlete on top of their game.

In particular, physicians at a major international competition face a unique set of challenges — such as quickly responding to injuries mid-event and supporting athletes' recovery afterward.

Dr. Thomas Kremen is one of the physicians currently at the 2024 Paris Olympics supporting the USA Basketball men's team. The job involves caring for a roster of the world's most famous basketball players, including Stephen Curry, LeBron James and Anthony Edwards.

Kremen is an orthopedic surgeon and sports medicine specialist at University of California, Los Angeles (UCLA) Health, as well as a former collegiate competitive swimmer. Before supporting USA Basketball, Kremen worked as a physician with USA Swimming for 10 years, traveling with the team to the Tokyo Games. He works with UCLA intercollegiate athletics, as well as the Los Angeles Lakers basketball team and Los Angeles Chargers football team.

Live Science spoke with Kremen about what it is like to be a doctor at the Olympics, how physicians like himself prepare for the Games and what he is most looking forward to in Paris.

Related: 'Simone is a very, very rare bird': Experts discuss the science behind Simone Biles' gymnastic prowess

EC: How is it decided which team you're going to be working with?

TK: That's a complex question, but I think it's sort of partly availability, partly experience.

From swimming, obviously, I had had a longitudinal relationship with them, and leading up to the Tokyo Olympics I had several international games that I had traveled with the team as sort of a preparation to make sure that, for all the things we organized for each of these Games, we put it into practice about one year prior to the Games. Just to work out the kinks and make sure we have all the various details optimized.

In 2019, in preparation for the planned 2020 Games, we had traveled to World Championships together with the exact same medical team — so athletic trainers, massage therapists, chiropractors, performance staff, strength and conditioning people, as well as our team of doctors, which was myself, as well as a primary care physician.

For the COVID year, obviously, there was a lot of tweaking because a lot of restrictions came in, and the Games were postponed for a year. But very similar to that, for the USA Basketball games last summer, we participated in the FIBA World Cup in Manila with the same medical infrastructure, just to kind of be prepared for these Games this year.

Related: What's the heaviest weight a person can lift?

EC: How do you prepare players for their games?

TK: There's a couple elements there — there's certainly, "How does the medical team prepare for these big international competitions that are obviously high profile?" And then, "How do the players prepare?"

As a physician, I'm less involved in their day-to-day preparation and more [involved] in making sure that we strategize how we would deal with anything that could come up.

So, the players are used to dealing with keeping themselves in shape and ready for competition. And there's a whole team of players … a whole lot of things that go into making sure that they're actually ready to perform at the highest, highest, elite level. And everyone wants to be bigger, better, faster, stronger. And these athletes are used to doing that day in and day out. So for them, it's almost like continuing on their — I don't want to say day-to-day — but their high, high level, day-to-day elite performance.

From the medical standpoint, we're just always trying to make sure that we have whatever medical resources we could need readily available, so that we can take care of any potential common medical problems, or those rare but more concerning medical problems. We can take care of them expeditiously so that the athletes have the opportunity to perform and do what they need to do.

EC: What are the most common kinds of injuries that you see in these athletes, and how do you treat them?

TK: Yeah, it's not all that mysterious, to be honest. I mean, you've seen basketball players.

Common injuries are like muscle strains and ankle sprains, and that sort of thing are the vast majority of the injuries. And so these players have obviously been playing this sport since they were children, and they've all had these injuries over time. And so those are not particularly new, but you still have to deal with them and their unpredictability to some extent.

So the common things are still common, even at the Olympic level or the school collegiate level.

Although some people may not consider basketball a contact sport, it is still a contact sport because the players are jumping and landing. So a lot of bruises from hitting your elbow on the court when you go up for a layup and can't land perfectly. And there's bruises from hitting the floor, jumping and landing injuries and things like tendonitis, or something gets flared up — ankle sprains, muscle sprains, that kind of thing.

Related: What's the most dangerous sport in the world?

EC: What would you say is the most rewarding part of your job?

TK: It's always great to be able to empower athletes to demonstrate their amazing, amazing skill. And one of the reasons why these athletes are so famous is because their performance level is unbelievable and very, very impressive.

So as a former athlete — albeit at a much, much less level than these particular athletes — but after being an athlete, you always want to be able to let people participate in and put all their hard work to use or to showcase that. And so with these guys, it's great to be able to make sure that they can perform at the best of their ability. And then when you add in the [Summer] Olympic Games, which happens once every four years, and representing our country on top of just the sports in general, it's even that much more special.

EC: What are you most looking forward to about working at the Olympic Games?

TK: Seeing all these different people from extremely different backgrounds. In some cases, you know, some countries are very resource-rich and have a lot of different opportunities at their disposal; other countries may not. But when you get onto the basketball court, none of that matters, you know, and it's kind of an amazing equalizer.

The other thing I think is amazing is —- you know, I'm at a major university where we're always trying to improve how we deliver healthcare to everyone, not just elite professional basketball athletes, but really everybody. And so using these things we learn to make the athletes bigger, better, faster, stronger, like everyone's trying to get to in this elite level — taking those lessons and using that to be innovative at the treatment of our day-to-day patients at UCLA is something that I think is what you always try to have in the back of your mind.

Editor's Note: This interview has been condensed and edited for clarity.

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Editor's note: Since this story was published July 31, two Olympic triathletes have fallen ill following races in the Seine, although it's unclear if the illnesses were directly related to exposure to the river. Due to the sicknesses, Belgium’s triathlon mixed relay team withdrew from the Aug. 5 race and Switzerland's team changed its race lineup. 

Rain showered down on athletes, onlookers, can-can dancers and opera singers at the 2024 Paris Olympics opening ceremony on July 26, and more rain over the weekend reportedly drove an uptick in disease-causing bacteria in the Seine river.

This decline in water quality prompted officials to cancel the swimming portion of a training session for the triathlon events, and the men's race, scheduled for Tuesday (July 30), was postponed. Finally, on Wednesday (July 31), both the women's and men's triathlons were allowed to proceed, with athletes plunging headfirst into the Seine for a 0.9-mile (1.5 kilometers) swim before jumping onto bikes for the next leg of the race.

France and Great Britain snagged gold in the women's and men's races, respectively. But even as the athletes take the podium, there's more river swimming to come, as the mixed relay and marathon swimming events are scheduled for next week.

Are there any lingering concerns about the water quality in the Seine? And what risks could subpar water pose to the competitors? Live Science spoke with experts to find out.

Related: Will Olympic athletes ever stop breaking records?

Poop pollution

Paris invested the equivalent of $1.5 billion in making the Seine swimmable in time for the Olympics. The renovation involved connecting thousands of residences to municipal sewer systems and constructing a huge underground cistern. This massive tank, which holds 20 Olympic swimming pools' worth of water, is intended to prevent untreated sewage from flowing into the river.

Paris has a combined sewer network that captures stormwater and wastewater, funneling both to treatment plants. But in times of heavy rain, some untreated water is instead diverted into the Seine and its tributaries to prevent disastrous backups. The new basin beneath Paris helps curb such outflows by capturing excess water and then slowly releasing it back into the sewers so it can go through the normal treatment process.

This cistern was completed in May. But even so, when it rains in Paris, the quantity of bacteria-carrying poop in the Seine still spikes. In particular, rain comes with an uptick in Escherichia coli, a bacterium found in both human and animal feces.

"Elevated E. coli levels are to be expected following rain events," Sarah Lowry, a doctoral candidate in environmental engineering at Stanford University, told Live Science in an email. E. coli acts as a general indicator of fecal contamination, so its presence implies that other common poop-borne pathogens, such as norovirus and Salmonella, are likely also in the water. In addition to the germs from human wastewater, some of these pathogens come from animal feces that get washed into the river, Lowry noted.

After such an influx of poop, one can expect the related pathogens to then decline due to sun exposure and fluctuating temperatures in the water. In terms of how long that takes, "we are talking about days, not weeks in general," Patrick Gurian, a professor and the head of the Environmental Engineering Program at Drexel University, told Live Science in an email.

"In general, if levels of E. coli rise in a natural, moving body of water because of runoff from rain, those levels should start to drop when the rain stops," said Mia Mattioli, the team lead for the Environmental Microbiology and Engineering Laboratory within the Centers for Disease Control and Prevention's (CDC) Waterborne Disease Prevention Branch.

Related: What are the most polluted cities in the world?

"However, the time it takes for the levels to drop will depend on upstream runoff, surrounding land use, and drainage systems," Mattioli told Live Science in a written statement. Additional factors, such as the cloudiness of a given body of water, can also affect how quickly germs die off, Lowry added. "It's hard to say without knowing the specifics of a particular site," she said, but it typically takes one to two days.

Following the weekend's rain, another rainstorm hit Paris in the early morning hours Wednesday, The Washington Post reported. A few hours later, organizers met to discuss the results of the latest water-quality tests and deemed the water safe for racing. The rain let up around when the women's race started at 8 a.m.

"The results of the latest water analyses, received at 3:20 a.m., have been assessed as compliant by World Triathlon allowing for the triathlon competitions to take place," the organizers said in a statement sent to news outlets Wednesday. However, the Olympic organizers previously said the water samples used to make these decisions are taken 21.5 hours prior — which means they would have been taken before Wednesday's rainstorm. The test results from Wednesday have yet to be released.

What's the risk?

World Triathlon, the international governing body for triathlons, bases its water safety guidelines on the European Union's Bathing Waters Directive. These guidelines set a threshold for the E. coli level that's considered safe enough for swimming in inland waters: 900 colony-forming units per 100 milliliters (CFU/100mL).

That number came from epidemiological studies that looked at rates of illness among swimmers and then correlated them with the levels of an indicator bacterium, such as E. coli, in the body of water where they swam, Lowry explained. Such thresholds are specific to a given geography and type of water — salt versus fresh — so "there is no one number that is 'best' to set a threshold," she noted. That's why standards may vary among locations.

Paris Mayor Anne Hidalgo and Paris 2024 Chief Tony Estanguet took a dip in the Seine two weeks prior to the Games to demonstrate their confidence in its cleanliness. But notably, data later showed that E. coli levels were 985 CFU/100mL that day, The Associated Press reported. That's slightly above the "sufficient" threshold set forth in the guidelines.

So, what's the potential risk of swimming in poop-contaminated water?

In general, the various disease-causing germs found in feces can cause stomach cramps and diarrhea, which can lead to dehydration, Mattioli said. They can also cause ear, eye, respiratory and wound infections. "CDC cannot speak to the status of the Seine at present and defer to French authorities who are monitoring the water quality," she added.

E. coli, specifically, is not always a threat to human health — in this context, the microbe is being used as a benchmark to see if there's likely a concerning amount of poop in the water, Lowry emphasized. Most E. coli strains are harmless, but some can cause gastrointestinal problems and urinary tract infections. One type, called Shiga toxin-producing E. coli, can spark more-serious illnesses, involving bloody diarrhea and, in some cases, blood clots and kidney damage.

An individual's likelihood of getting sick after swimming in poop-contaminated waters varies depending on their existing immunity and their degree of exposure to the germs, Lowry said.

"The amount you ingest is usually the problem," Gurian said. Skin exposure can be an issue in some cases, but "by and large, the less you drink, the safer you are." In addition to avoiding swallowing the water, swimmers can protect themselves by wearing earplugs, nose plugs and goggles; covering any open wounds as much as possible with waterproof bandages and changing those bandages after swimming; and showering after swimming, Mattioli said.

Current weather forecasts suggest that more rain could befall Paris in the coming days, so the Seine's quality may continue to fluctuate. Only time will tell if the upcoming scheduled races will be postponed, downgraded to exclude swimming or moved to alternate arenas — or if anyone will wind up ill after braving the river's waters.

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At the 2024 Paris Olympics, the youngest competitor is 11-year-old Chinese skateboarder Zheng Haohao. She's just one year older than the youngest Olympian in history: 10-year-old Greek gymnast Dimitrios Loundras, who won a bronze at the 1896 Summer Olympics in Athens.

The oldest athlete at the Paris Games is Australian Mary Hanna, 69, who competes in equestrian. She's three years shy of the oldest-ever Olympic athlete, Oscar Swahn of Sweden, who clinched that record at the 1920 Antwerp Games. 

These athletes are outliers. According to a 2021 study by the Arc Centre of Excellence in Population Ageing Research (CEPAR), two-thirds of athletes at the Tokyo Olympics, held in 2021, were in their 20s. Overall, 90% of the competitors were under 30.

This tendency for the Olympics to be dominated by young adults raises a question: At what age does an athlete's performance generally peak? Given that athletic demands vary among sports, there are likely a range of answers to this question. Live Science spoke with experts to find out more.

Related: Which Olympic sport burns the most calories?

Endurance

At the Tokyo Olympics, 28-year-old Richard Carapaz of Ecuador won the men's Olympic cycling road race, and 27-year-old Peres Jepchirchir of Kenya won the women's marathon.

"From an endurance point of view, you often peak in your mid-to-late 20s and into your 30s," said sports physiologist Garry Palmer, who runs Sportstest, a sports performance center in Cannock, England. "One of the key reasons behind this is an athlete's aerobic capacity, which is the maximum amount of oxygen that an individual can use each minute during strenuous exercise."

This capacity, also known as VO2 max, is measured as the maximum amount of milliliters of oxygen the body can use per minute per kilogram of bodyweight (mL/min/kg). Top endurance athletes reach well into the 80s and even the 90s, compared with an average nonathlete's 30 mL/min/kg.  

Multiple factors drive aerobic capacity down with age. One 2016 study, published in the journal PLOS One, chalks it up to a decrease in maximal heart rate and the amount of blood pumped out per beat. This is accompanied by a decline in the efficiency of valves and muscles that push blood back to the heart, as well as stiffening of the heart's muscle fibers and the walls of arteries.

Related: What's the most dangerous sport in the world?

Speed

In 2009, Usain Bolt completed a 100-meter race in a remarkable 9.58 seconds, winning gold at the world championship in Berlin while also smashing his own world record. He was five days shy of turning 23. At this year's Olympics, 24-year-old American Sha'Carri Richardson stands out as a favorite for the women's 100-m gold. 

These athletes' sprinting superiority aligns with research by kinesiology professor Edward Merritt and his former student Corban Ruiz of Southwestern University. In 2021, they published research examining age-related performance differences between aerobic and anaerobic events. 

In aerobic events, such as marathons, the muscles get enough oxygen to use fats and carbohydrates as their primary fuel. In anaerobic events, like the 110-m hurdle, there's a lack of oxygen and the body instead burns glucose stored locally in muscles. This supply is exhausted quickly.

The researchers used data from the Olympics spanning 1960 to 2016. They defined anaerobic events as those lasting less than two minutes and aerobic events as over five minutes, while events in-between were "mixed." The average age for peak anaerobic performance was around 23, compared with 26 for aerobic events, the study found.

Under-30s often prevail in the sprint thanks to their muscles. Sprinters have an abundance of fast-twitch muscle fibers, which generate short, forceful muscle contractions but fatigue swiftly. Slow-twitch muscle fibers aren't as action-packed but are far more fatigue-resistant. The quantity of slow-twitch fibers remains pretty constant throughout life, but the fast-twitch fibers start to decay with age, starting around your 30s. 

Explosive sports like the 100-m sprint are also brutal on the body and can often lead to career-limiting injuries, especially in the Achilles tendon, on the lower back of the leg, and hamstring muscles, on the upper back of the leg. 

Meanwhile, young sprinters are falcon-like in their reflexes, Palmer noted. "Your reaction times are faster when you're young," he said. "If you're looking at a short event, the start is vital for a strong finish."

Research suggests that reaction time peaks at age 24 and declines by about four to 10 milliseconds each year, at least in nonathletes. A 34-year-old's reactions could be up to 100 milliseconds slower than they were a decade before — in racing, that could be the difference between victory and defeat. That decay stems from changes in nerve fibers that slow the speed at which they conduct signals.

Related: What's the heaviest weight a person can lift?

Dynamic events

"In dynamic sports like gymnastics, the younger athletes often excel because as you age, you lose flexibility," Palmer told Live Science. "You might strengthen your body, but you're just not quite as supple." This decline in flexibility is partly related to a loss of elasticity and water in the tendons, as well as increased stiffness in joints. 

Research in The Journal of Human Sport and Exercise (JHSE) looked into the age of peak performance at the 2012 London Olympics. It suggested that the average age of a successful male artistic gymnast was 24 years old, compared with 19 years old for female gymnasts. According to CEPAR, this disparity between women and men may reflect differences in physiology and rates of maturation, as well as differences in the types of events they compete in.

When it comes to women's gymnastics, though, times are changing. Historically, the Olympic gym featured the likes of Nadia Comaneci and Dominique Moceanu, both of whom were age 14 when they won their respective golds. These gymnasts symbolized the "younger, the better" dogma of women's gymnastics at the time — the thought was that young girls were more flexible than older athletes and unrestrained by the physical changes brought on by puberty. They were also deemed more fearless, needed in a sport where injury risk is high.

Then, at the Tokyo Olympics, the average age of the U.S. women's gymnastics team tipped over 21. This year, Simone Biles hits Paris at age 27. 

The reasons for this age increase are varied, and include that the minimum age to compete is now 16. In addition, gymnasts' training and recovery regimens have become more scientific. For instance, Great Britain's team employs a scientist who tracks gymnasts' maturation rates, calculating their predicted adult height, in part, so coaches can help guide athletes safely through growth spurts. In general, athletes are being better managed than in the past, resulting in more sustainable careers. 

Related: Will Olympic athletes ever stop breaking records?

Target events

"Sports where there's a high-skill element with less emphasis on physical attributes means older competitors can excel," Palmer said. "Take shooting and archery — the older competitors have years of experience to draw on; plus they might be calmer and more easily control their breathing than their younger opposition."

In both of these sports, athletes often win in their late 30s and well into their 40s. The JHSE study on the 2012 London Olympics showed that the average age for a gold-medalist men's shooter was 33. The Czech shooter Jiri Liptak is a more recent example, in that he won Tokyo gold in the men's trap event at 39. 

Liptak's victory also highlights that differences between shooters are fueled by experience. CEPAR cites evidence that shooting events that involve stationary targets, such as air rifle, "tend to have lower median and average ages" than shotgun events, which involve the more-difficult, moving clay targets. As long as you still have good eyesight — or otherwise, good glasses — there's no reason you can't continue to excel into older ages. 

In some sports, athletes can even hit their stride at older ages: Great Britain's Nick Skelton won two gold medals in equestrian at 54 and 58 years old, respectively. Age, certainly in some events, is just a number.

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With 11 Olympic medals and 30 World Championship medals under her belt — and potentially more to come soon — U.S. gymnast Simone Biles is considered one of the greatest gymnasts of all time. 

So what's the science behind her success? 

There's a certain "X factor" that Gina Pongetti, a former gymnast and physical therapist who is treating athletes at the 2024 Paris Olympics, calls "Simone being Simone." Beyond that, the answer lies in Biles' dedicated training of her neuromuscular system — everything from the ribbon of tissue that traverses the frontal lobe in the brain and controls movement to the sensory receptors that provide feedback to the muscles themselves. 

"To be an elite athlete is one level," Pongetti told Live Science. "To step up to Simone's level involves an absolute keen understanding of her body and the input signals that come to it." 

Biles stands out from other gymnasts both in terms of how she senses her body in space and how she hones her muscle memory, "as well as [her] having one of the most phenomenal bases of strength and conditioning that I've ever seen in an athlete," Pongetti added.

Related: Will Olympic athletes ever stop breaking records?

Proprioception to perfection 

Biles started gymnastics at 6 years old. A youthful start is common among elite gymnasts; growing brains are adept at taking in information, said Dr. David Neuman, a fellow of the American Academy of Orthopedic Surgery and clinical director NY Sportscare in Manhattan. And that early practice, he told Live Science, often provides a basis for later adeptness. 

Particularly important to gymnasts is proprioception, the sense of where one's body is in space. The sense depends on signals from specialized cells within muscles, tendons and joints. In muscles, for example, bundles of nerves called muscle spindles tell the brain how stretched or contracted a muscle is. In the skin and joints, receptors known as Pacinian corpuscles react rapidly to sensations of pressure. 

When Biles competes on the vault, these corpuscles are one of her key informants as to how squarely her palms hit the apparatus and if she's effectively transferred the force she needs to launch into and twist through the air. 

None of those decisions are conscious. The movements happen so quickly that the motor planning has to be preordered in advance, Pongetti explained. She compared it to a fireworks show, in which fuses of different lengths are lit at the same time. The programmed sequence is triggered when the gymnast takes their first step. In motor learning, linking together movements in this way is called "chunking," said Gregory Youdan, an independent movement scientist and dancer in New York City.  

"Because it's chunked, it frees up a little bit more cognitive load for other things and allows for that automaticity," Youdan told Live Science.

Then, throughout the sequence, the proprioception system tells the athlete if all is going well. The muscles and fascia — the nerve-rich casing that covers the muscles and organs — respond accordingly with the required movements to make tiny corrections. 

And that's where Biles' famous work ethic comes in. To reach that level of automaticity, gymnasts have to practice perfectly, Pongetti said. It's a stepwise process that begins in a gym's loose-foam pit, a swimming-pool-like structure filled with foam cubes where gymnasts can fall without hurting themselves. From there, the athletes slowly build up to doing the routine on the gymnastics floor, without padding. 

"As each step progresses, your level of confidence should increase, thereby allowing you to take more risks of a less-forgiving landing," Pongetti said. 

In a 2004 study, scientists used a sequence of finger movements — rather than gymnastic routines — to investigate how this type of memorized motor learning becomes automatic. The same brain areas lit up when people were first learning the finger movements and after they learned to do them automatically. However, brain activity declined slightly in many regions once the movement became automatic, suggesting that automaticity stems from an increase in singaling efficiency. 

In 2016, researchers proposed a new theory dubbed the Optimizing Performance Through Intrinsic Motivation and Attention for Learning (OPTIMAL) theory of motor control. This suggests that honing an external focus of attention — on the outcome of the movement, rather than the precise focus on a single body part — helps to smooth this learning process. 

"Athletes focus relentlessly on process and technique," said Dr. Lorenzo Norris, a psychiatrist and medical director of the George Washington University School of Medicine Resiliency & Well-being Center. Practice doesn't only make perfect; it also makes poise: By drilling skills over and over, elite athletes can stay cool and focused in stressful competitions, Norris told Live Science. 

Related: 'Muscle memories' get 'zipped and unzipped' in the brain, like computer files

Muscles and mental health

Biles carries serious muscle on her 4-foot, 8-inch (1.4 meter) frame, a body type that gives her a lot of efficiency in her movements, Pongetti said. Her quickness, bestowed by fast-twitch muscle fibers that contract quickly with a burst of force, is just as important as her power. 

Gymnasts also need isometric strength, which is the ability to produce force with muscle without shortening its length. This involves stiffening a muscle without moving its associated joint. That stiffness enables gymnasts to rebound off the floor or vault table, getting the air they need to execute a twist or flip. 

And finally, there's the landing: At that critical moment, Pongetti said, the muscles need to absorb the force generated by all those spins and flips with an eccentric contraction, which is when the muscle contracts while lengthening. (Think of slowly lowering a dumbbell in a bicep curl, during which the bicep muscle is working hard as it's extending.) 

Biles is excellent at absorbing those forces without wobbling or hopping, Pongetti said, which enables her to stick her landings neatly. 

Beyond practicing the skills themselves and doing the strength and conditioning needed to execute them, elite gymnasts have to focus on flexibility, nutrition, recovery time, mental wellness, rehab for injuries, and scheduling events and competitions so as not to overtax themselves, Pongetti said. A lower-level gymnast may spend only 10% of their time thinking about those non-skills factors, she estimated, while someone like Biles might devote at least a third of their time to them. 

This focus on the foundations of performance becomes increasingly important in a long career like Biles has, now competing at age 27, Pongetti added. 

Another key to high performance, Norris said, is knowing when to step back. Biles famously withdrew from most events in the Tokyo Olympics in 2021 after developing the "twisties," a psychological phenomenon in which gymnasts lose their sense of where they are during flips and spins. The twisties are not only disorienting — they're dangerous, especially for gymnasts attempting particularly daring skills. 

"Look at the strength it took her to step away when she knew that she needed to for her own self, and look at the strength that it took to come back again," Youdan said. 

That inner strength is tough to quantify and even tougher to replicate. 

"Simone is a very, very rare bird," Neuman said. "For her from such a young age to be so awesome and so elite and then go through her mental health struggles and then to come back and be one of the oldest people on the team … That's endurance and resilience right there, for her to be able to do that. That's why people love watching her."

Editor's note: This story was updated on Aug. 5, 2024, to update Biles' Olympic medal count to 11, following events at the Paris Olympics. 

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The world was in awe when British strongman Eddie Hall deadlifted 1,102 pounds (500 kilograms) at the World Deadlift Championships in 2016. Hall was the first person to break the half-ton barrier — and then, in 2020, Icelandic strongman Hafþór Júlíus Björnsson took it up a notch by deadlifting 1,104.5 pounds (501 kg).

Meanwhile, the record for heaviest weight ever lifted by a human belongs to Canadian strongman Gregg Ernst, who in 1993 backlifted two cars with drivers that together totalled 5,340 pounds (2,422 kg).

These mind-blowing feats of strength raise a question: What is the heaviest weight possible for a person to lift?

Experts told Live Science that it's likely athletes are still operating below their muscles' maximum capacities, and it's unclear what the limit of human strength could be. But that said, it's difficult to measure a given person's maximum muscle capacity.

Related: Which Olympic sport burns the most calories?

Bradley Schoenfeld, a professor of exercise science at Lehman College, said that muscle strength can be measured using an electromyography (EMG) machine. EMG works by recording the electrical activity generated within a muscle, both by nerve cells and by contracting muscle fibers. Such tests can only be performed in the lab, and the EMG only monitors a localized set of muscles, so it can't assess a person's body-wide muscle capacity.

"It's hard to define that limit," said E. Todd Schroeder, a professor of clinical physical therapy at the University of Southern California who studies how strength and muscle mass can be increased in older adults. The only way to really determine your muscle capacity is through continuous training — by establishing new personal records and then seeing if you can break them, he told Live Science.

"If someone says, 'Oh, I can lift 200 kilos [441 lbs],' I'm like, that's great, but I know you can lift more," he said anecdotally. "We just don't know how much more."

Physically, a person's capacity to bear weight depends on actin and myosin, the two proteins that enable muscles to contract. These proteins are arranged within different types of muscle fibers, including "fast-twitch" and "slow-twitch." A person's muscle mass and ratio of these fibers depends on their training regimen, as well as biological factors like genetics and sex. In general, the greater your muscle mass, the more force you can generate.

Elite powerlifters push themselves to the limits by continuously increasing their muscle mass — however, the return on force diminishes as muscle mass grows ever larger, and eventually, the muscles reach their limit.

And sometimes, simply accumulating muscle mass is not enough, Schroeder told Live Science. Paradoxically, sometimes people with less body mass lift more weight than people with more.

One factor powerlifters must overcome is "neuromuscular inhibition," which puts a cap on the force with which a muscle can contract, to help prevent injury. Studies find this cap can be raised with resistance training.

Related: What's the most dangerous sport in the world?

Another part of the puzzle is that, in addition to physical training, powerlifters must push past mental blocks to lift heavier and heavier weights, Schroeder said. The best weightlifters are those who can upend these inhibitions and get themselves in the right mental state to recruit all their muscle fibers, he said.

"If I tried to lift the maximum force that I could, say 200 pounds [90 kg], and then I am able to go into a state where I remove that neural inhibition, I might be able to lift 300 pounds [136 kg]," Schroeder said, as an example.

This effect was demonstrated in a 2020 study published in the journal Impulse. The researchers tried to determine whether positive visualization — a technique that involves mentally rehearsing positive outcomes — impacted strength training. They recruited 133 student athletes at a university and divided them into two groups. The first was asked to envision themselves lifting 110% of their lift capacity for at least five minutes a day while listening to motivational music. The second group did not do this.

After three weeks, the athletes returned to the lab. Those who had practiced positive visualization each increased their lift capacity by at least 10 to 15 pounds (4.5 to 6.8 kg), while those in the comparison group averaged only a 5-pound (2.2 kg) increase.

"To a certain extent, you can lift a lot more than you think if you have the right mindset," Schoenfeld said.

"It always seems that someone can be a little bit stronger and perform a little bit better," he said. In the weightlifting world, this manifests in athletes who repeatedly break their own records at competition. "I try not to be surprised," Schroeder said of these repeat record-breaking achievements.

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Sports offer a variety of benefits, from lowering stress and boosting self-esteem to improving heart and bone health. But they can also pose risks.

In extreme cases, elite athletes complete seemingly death-defying stunts at major sporting events, such as the Olympics. But which sports are the most dangerous to participants? And which pose the highest risk of death?

Which sports cause the most injuries?

These are tricky questions to answer because sports aren't always studied in the same way, making them difficult to compare. Let's start by looking at nonfatal-injury statistics.

In the U.S., the sports and recreational activities most frequently associated with injuries are "exercise," cycling and basketball, according to the National Safety Council. These statistics come from injuries that resulted in emergency department visits in 2023. Here, exercise isn't defined, but there's no separate category for running and the category covers injuries related to exercise equipment, so it appears to be broad.

Related: Extreme workouts: The nutritional needs of elite athletes

American football comes in fourth on the National Safety Council's list, behind basketball. That said, football produces the most injuries of any sport per 1,000 hours of participation, according to Ohio State University (OSU). Alongside basketball, it's also a strong contender for the most injury-riddled sport.

Football players' risk of concussions and subsequent brain damage has also been highlighted in recent years. Similar attention has fallen on other high-impact, collision sports, such as rugby, in which players routinely collide at all levels of the game.

Carolyn Emery, a professor and chair of the Sport Injury Prevention Research Centre at the University of Calgary, studies collision sports as part of her work in trying to prevent injuries in Canadian youth sports. "Collision sports are our highest risk for concussion, as well as all injury that leads to time loss," meaning time spent away from sports to heal, she said.

Statistically, there's a higher rate of concussions in girls' sports than in boys' sports, Emery noted. At the youth level in Canada, female rugby produces the highest rate of concussions, followed by ice hockey and football. It's unclear why girls seem to get more concussions than boys, but Emery said she suspects it may be related to both biological factors and the fact that "girls are more likely to report baseline symptoms."

There's a link between sports with a high incidence of head impacts and long-term neurological impairment and neurodegenerative diseases, such as Alzheimer's, according to a 2024 study in the journal Sports Medicine - Open. Such sports include football, rugby and boxing — up to 20% of professional boxers experience a chronic traumatic brain injury. One example is chronic traumatic encephalopathy (CTE), a progressive disease with no available treatment.

It's not just collision sports that pose a risk of head injury. Equestrian sports — such as horse racing, show jumping and polo — are the most common cause of sports-related traumatic brain injury in U.S. adults, according to OSU. A 2024 study in JAMA Network Open also found that horseback riding was the most common cause of sports-related concussion in Europe.

Related: Why is exercise important, according to science?

What's the deadliest sport?

Fatalities in sports are rare, but they seem more likely in activities that pose a risk to an athlete's head and neck.

For instance, a 2021 study documented 320 horse-related fatalities in the U.S. over a decade, or an average of 32 per year. In 2023, PBS News reported that about two jockeys die, and 60 become paralyzed, from horse racing each year.

Boxing is another contender for the world's deadliest sport. A 2010 study found that there are about 10 boxing deaths every year, worldwide. However, the authors noted that this figure is likely higher, due to incomplete record-keeping. A 2011 survey estimated an average of 13 boxing deaths per year, CNN reported. These figures include amateur and professional boxing, but most boxing fatalities occur at the professional level.

The National Center for Catastrophic Sport Injury Research documents football fatalities at all levels of the game with its Annual Survey of Football Injury Research, and in 2023, three people died from traumatic injuries. In addition, there were 10 indirect exertional or medical fatalities, which included sudden cardiac arrests and conditions like heat stroke.

This survey highlights that not all sports-related deaths stem from collisions. Notably, only two athletes — a marathon runner and a cyclist — have ever died during competition at the Olympic Games. In both cases, the deaths were attributed to heat stroke or heat exhaustion, Quartz reported.

The death of a cyclist during competition at the 2016 Paralympics was attributed to cardiac arrest following a crash. Other Olympic athletes, including those competing in luge and downhill skiing, have died during practice sessions held in the lead-up to the games.

Sports and recreational activities that take place in water come with a risk of drowning, even at elite levels. Drowning is a leading cause of death for children in the U.S., where there are an average of 11 fatal drownings per day. And although it's rare, adult athletes can sometimes faint underwater, leading to near-fatalities and the occasional death.

Studies and reports often present sports fatalities as total numbers across several years or per year rather than rates based on the number of participants. Most figures don't account for the long-term impact of sports with high-impact collisions, which can shorten life spans. Thus, it's challenging to directly compare fatalities in what appear to be the deadliest sports, such as boxing and horse racing.

However, if you also consider animal health, then horse racing is by far the deadliest sport, given that hundreds of horses die or are euthanized due to racing-related illnesses and injuries in the U.S. each year.

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The 2024 Summer Olympics have just kicked off in Paris.

You may be rooting for your favorite elite athletes, hoping to witness them shatter records. But as performances improve and records get increasingly harder to surpass, will athletes ever reach the limits of what is possible?

The hammer throw may have already met this fate — no one has broken its world record since 1986. Yet, athletes frequently outdo themselves in other sports, such as the 400-meter sprint.

Biophysical constraints on muscles — as well as other factors, such as reaction time — could set a limit on human achievement, experts told Live Science. However, innovative techniques and improved sportswear could help athletes continue to unlock new levels of performance.

Related: Why is it harder for some people to build muscle than others?

The limitations of muscles

Olympians often boast impressive muscles, but bigger gains don't necessarily lead to improvements in athleticism. This is partly because muscles lose efficiency as they grow.

Kevin Murach, a muscle researcher at the University of Arkansas, told Live Science that the most-efficient muscles contract along a line between two tendons. However, as they expand, muscles contract at a less-efficient, oblique angle. Overall, more muscle mass produces extra force, but the returns diminish as muscle mass continues to grow — and muscles will eventually approach a hard limit on what they can achieve.

What's more, huge muscles could even curtail athletes in some sports, such as sprinting. On one hand, enlarged muscles endow a sprinter with more force and thus help them overcome inertia — the tendency of an object at rest to stay at rest — upon starting. However, "there's probably a point at which you don't want your muscles to be super large," Murach said. "You wouldn't want to have so much that it became an anchor."

Besides muscle size, an athlete's dominant type of muscle fiber can affect their performance. There are two main types of muscle fibers: fast-twitch and slow-twitch.

"If you look at a sprinter, they have a high proportion of fast-twitch fibers as opposed to a marathon runner, who has a high proportion of slow-twitch fibers," John Hawley, an exercise physiologist who heads the Exercise & Nutrition Research Program at the Australian Catholic University, told Live Science. Fast-twitch fibers are richer in energy stores, produce more explosive movements and fatigue more quickly than the slow-twitch variety, he said.

This imposes a speed limit on high-endurance sports, such as long-distance running. Because of this, "I don't know if we will ever be at a place where you can run a marathon at a sprinting pace," Murach said.

The influence of technique and gear

Muscles set a ceiling on how hard athletes can work, but performance also depends on what athletes do with their muscles.

In the 1968 Olympics, Dick Fosbury used an innovative strategy for the high jump that literally raised the bar. Instead of soaring forward over the pole, he rotated his body midflight. The backward jump enabled him to arc his body and lower his center of mass so he could leap over the pole with less effort. Five years later, Dwight Stones perfected the "Fosbury flop" and surpassed the high-jump world record using this method.

Related: 'Muscle memories' get 'zipped and unzipped' in the brain, like computer files

"Now you won't find a high jumper in the world — you may find one out of 100 — who doesn't use that technique," Hawley said. This means that a physical peak doesn't necessarily prevent records from being broken in the future.

Improvements in sportswear can also boost performance significantly. For example, athletes wearing polyurethane-coated swimsuits broke 25 world records in the 2008 Olympics. One hypothesis is that "the polyurethane panel laminated over the fabric could prevent the air in the fabric from escaping from the costume [swimsuit] when the athlete entered the water," Ada Ferri, a textile engineer at the Polytechnic University of Turin in Italy, told Live Science in an email.

A tiny amount of trapped air might have given the contestants an edge by producing some buoyancy, she suggested. "As races are won by a few hundredths of a second, even buoyancy of a few millimetres can be decisive," Ferri said. Polyurethane swimsuits were later banned from competition, in 2010. However, this hasn’t stopped athletes from surpassing records, suggesting swimmers' actual performance still hasn't hit a ceiling.

Since 2016, shoes equipped with carbon-plated insoles can give runners an advantage, Ferri said. This type of footwear limits energy loss through the shoe's shock-absorbing midsole, meaning the runner has to put in less work to run faster. World Athletics, an organization of track-and-field federations, allows athletes to compete with these shoes as long as the foam base is no higher than about 1.6 inches (4 centimeters), so it's possible this sportswear could lead to new running records.

More broken records to come?

Other factors can influence performance, such as an athlete's diet, their psychological state or their mode of training.

For example, cyclists and runners train at high altitudes, driving their bodies to produce more oxygen-carrying red blood cells. However, Ferri argued that these additional factors produce gradual advances in performance. Huge leaps tend to be linked to breakthroughs in designing materials and equipment, she said.

Historically, more men than women have competed in sports and Olympic events. So, most of the research about athletic achievement has been based on male anatomy and there is a shortage of data on female physiology. Learning better training methods geared specifically toward female athletes could bring about the greatest performance gains, Ferri hypothesized.

"For the first time at the next Olympics in Paris, the percentage of female and male athletes will be [closer to] equal," Ferri said. The more female athletes who compete, the more likely that some will surpass previous milestones, so an even split between the sexes might lead to more broken records.

Hawley noted that historically, most advances in athletic performance were breached through trial and error, and scientists only later began studying the reasons why they occurred. But science now has a growing presence in sports.

With more data monitoring, "we will have a goldmine of information that athletes and coaches can use to make informed decisions about strategies in training and competition, Ferri said. This might streamline new breakthroughs that push athletes to set new all-time bests.

This article is for informational purposes only and is not meant to offer medical or athletic training advice.

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During the 2024 Summer Olympics, athletes from more than 200 countries will compete in over 300 events. As you watch athletes sprint, leap, swim and flip in pursuit of a gold medal, you might wonder: which Olympic event burns the most calories?

There are two ways to answer this question: looking at the total energy spent across a whole event or judging the biggest burst of energy required in the course of a given challenge.

Researchers usually define the energy cost of an activity as a measure of calories per kilogram of body weight per hour (kcal/kg/hour). So, if an exercise uses 2 kcal/kg/hour, you could expect an average human weighing about 185 pounds (84 kg) to burn 168 calories, if they worked out for a full hour.

There are individual differences between people; some will use less energy to perform the same activity, while others whose body weight is made up of more fat cells typically burn more. On average, an adult at rest will use 1 kcal/kg/hour, and any activity that requires more than 6 kcal/kg/hour is classified as "vigorous-intensive" in the Physical Activity Guidelines for Americans.

Related: How calories are calculated: The science behind your food

Running demands more

Of all the sports at this year's Olympics, the running events will result in the most calories burned per kilogram of body weight per hour.

Accelerating is much more energetically demanding than running at a constant speed, so events such as the 100 meters — in which runners accelerate from 0 to 5 meters per second (16 feet per second) in under a second — will use a large number of calories in a short period. Calculations by Pietro di Prampero, a professor of physiology at the University of Udine in Italy, revealed that during the first 0.85 seconds of Usain Bolt's world-record 100-m sprint, he was burning 91.2 kcal/kg/hour.

"In terms of metabolic power, of course, the most demanding [of the running events] are the 100-meter sprints," di Prampero told Live Science. "But in terms of overall energy, if you run a marathon, the amount of energy spent is much more."

For every kilometer an athlete runs at a steady speed, they typically burn 1 kcal per kilogram of body weight. This figure is independent of the runner's speed, di Prampero said — so running a marathon burns 42 kcal/kg. The 2020 Tokyo men's marathon gold medalist, Eliud Kipchoge of Kenya, weighs about 115 pounds (52 kg) and burned 2,339 kcal by the time he reached the finish line. The race took him 2 hours, 8 minutes, which comes out to around 21 kcal/kg/hour.

Related: Best running shoes for supination 2024: Find the perfect fit for your feet

Calorific cost of other Olympic sports

According to the Compendium of Physical Activities, a comprehensive review that compiled data from hundreds of studies to estimate the calorific cost of activity, cycling events above 20 mph (32 km/h) burn 16.8 kcal/kg/hour. Rowing at racing speeds burns 15.5 kcal/kg/hour. Martial arts are thought to use 10.5 kcal/kg/hour, and competitive trampolining uses 10.3 kcal/kg/hour.

When it comes to swimming events, the energy demand varies depending on the stroke, the athlete's skill level and whether the event takes place in a pool or in open water.

"In elite swimmers, the most demanding stroke is breaststroke," said Tiago Barbosa, a professor of sport science at the Polytechnic Institute of Bragança in Portugal. "After that comes butterfly, then backstroke. The most economical is freestyle."

Barbosa's earlier research, published in the International Journal of Sports Medicine in 2006, calculated that the cost of swimming breaststroke at 1.6 meters per second — slightly slower than Adam Peaty of England swam to win the gold in the 100-m breaststroke at the 2020 Olympics — was around 30.4 kcal/kg/hour.

The open-water events may take more energy, due to the athletes' potential exposure to waves, wind and cooler temperatures. However, Barbosa said swimmers can also save more energy by drafting — positioning themselves behind other swimmers to reduce drag — much like cyclists do at the Tour de France.

However, both di Prampero and Barbosa stressed that these calorific figures can vary greatly among individuals.

"We must be honest — these are all estimates," di Prampero said.

"I said the average Olympic runner consumes about 1 kcal/kg per kilometer. But if you are a good runner, you may spend 0.95, and if you have good shoes, maybe 0.83," he said. "If the terrain is wet, the cost goes up … at the Olympic level, even a 0.01 difference could be the difference between the gold medal or not."

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We've all been there — you're just starting to find your running rhythm on the treadmill, or maybe you're hustling to catch a train, when you're suddenly struck by a sharp pain in your side, just under your rib cage.

A side stitch — medically known as "exercise-related transient abdominal pain" (ETAP) — is annoying and uncomfortable, albeit not medically serious. But what causes side stitches? 

Although side stitches are common, no one knows for sure what causes them, and there are competing theories as to what body part the pain comes from. 

For decades, after this theory was first proposed in 1941, scientists and sports doctors believed that a side stitch is caused by inadequate blood flow to the diaphragm, the large muscle that sits under the ribs and draws air in and out of the lungs. This lack of blood could cause a painful cramp or spasm. This idea was based on the theory that blood travels away from the diaphragm and concentrates in the muscles of the limbs when we exercise. 

Related: How long should you wait to exercise after eating?

However, "this idea has lost popularity because the diaphragm also works harder during exercise which would redirect blood to it not away from it," Andrew Lavender, a senior lecturer in exercise and sports science at Federation University in Australia, told Live Science in an email.

Another idea is that the "constant jolting" that the body endures during activities like running puts strain on the ligaments that secure abdominal organs to the diaphragm, Lavender said. This theory possibly explains why runners often suffer from stitches, but it doesn't explain why a stitch can occur during swimming, rowing or cycling, he said.

Some have suggested that having a poor fitness level may raise the risk of side stitches, but Lavender questions this idea because highly trained athletes also get these pains. Still other scientists have proposed that the pain of a stitch stems from the intestines, as a result of  poor blood flow to the gut during exercise. Another theory attributes the pain to the compression of nerves that branch off the spinal cord and run between the ribs. 

However, the most widely accepted theory to date pinpoints a membrane called the peritoneum as the likely source of the pain. The peritoneum is a double-layered, balloon-like sac that encases the organs inside the abdomen. Its inner layer wraps around the organs, such as the stomach, and the outer layer sticks to the walls of the abdomen and pelvis. 

In a study published in 2000 in the journal Medicine & Science in Sports & Exercise, researchers proposed that the most likely cause of side stitches is that the inner and outer layers of the peritoneum rub together during exercise, causing irritation. The researchers reached this conclusion after interviewing more than 900 regular sports participants about their experiences of ETAP and then comparing their descriptions to theories of how side stitches arise.

Nowadays, peritoneum irritation is thought to be the source of the pain we think of as a side stitch, Lavender said. This theory is supported by the fact that side stitches become more likely if you eat or drink right before exercise. When your stomach is full, it pushes against the peritoneum, forcing its two layers closer together and increasing friction between them, the authors of the 2000 study wrote. 

Related: Is it dangerous to eat right before you swim?

A 2004 study published in the International Journal of Sport Nutrition and Exercise Metabolism also found that fruit juice and high-sugar sports drinks were much more likely to cause a stitch than water or no fluid. This may be because such "hypertonic" drinks draw water out of cells in the peritoneum, leading to more friction, or it may be that these drinks hang around in the gut longer than water would, putting pressure on the peritoneum.  

With that in mind, being mindful of how much you eat before exercising and when you time your snack might help prevent side stitches, Lavender said.

"Have a small meal before training and make sure you give yourself plenty of time between your meal and beginning your workout," he advised. Foods high in fiber or fat take longer to digest than low-fiber, low-fat foods, so if eating the former type of food, it's best to wait at least 60 to 90 minutes before exercising, he added. 

Note that a typical side stitch is a pain on one side of the abdomen that comes on with exercise and goes away when you rest. If you feel persistent pain in your chest or abdomen, experts recommend seeing a doctor, as it could be something more serious. 

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

The 111th edition of the world-famous Tour de France starts June 29 in Florence, Italy. This year, the grueling 21-stage bike race takes place over 2,175 miles (3,500 kilometers) across Italy and France.

Slovenian rider Tadej Pogačar of UAE Team Emirates is widely considered the favorite to win, having won the yellow jersey in 2020 and 2021 and finished second the past two years. Part of Pogačar's success comes down to Iñigo San Millán, who coached the champion cyclist for years as UAE Team Emirates' head of performance.

But in addition to being an expert in sports performance, San Millán has a second passion: He's hunting for new ways to beat cancer.  

As a professor of medicine at the University of Colorado School of Medicine, San Millán has studied the interactions of cancer and lactate, a byproduct released by cells during regular metabolism — and in particular, by muscle cells during intense exercise like cycling. While elite athletes' bodies easily flush that lactate out of their systems, in people with cancer, tumors release huge amounts of lactate that then sticks around, spurring on the cancer. San Millán is investigating how to disrupt this vicious cycle. 

Live Science spoke with San Millán about his double life as sport performance coach and cancer researcher, discussing his work in both spheres.

Related: Exercise may reverse sign of aging by 'flushing' fat from muscle 

James Witts: What does the role of head of performance in elite sport involve?

Iñigo San Millán: Years ago, you'd have to do a bit of everything. Now, you have the resources to recruit excellent coaches, nutritionists, biomechanics, psychologists … and then it's about coordinating and integrating them as seamlessly and productively as possible.

I've been very fortunate over the last 25-plus years to work in multiple sports. ... If you focus on one sport, you might get blindsided by developments elsewhere. That said, cycling's arguably the most advanced sport when it comes to understanding a sportsman.

JW: What does that understanding comprise of?

ISM: You must understand the physiology and metabolism of the rider, before moving into the fields of nutrition, biomechanics and training. You then establish season goals and monitor the riders during competition. 

Everyone responds differently to training. Someone like Pogačar can absorb a high workload. But if we give his training plan to someone else, it might kill them. You must understand who you are working with. 

JW: Why do you head to altitude to train?

ISM: There are two key adaptations to training at altitude that then transcend to racing at both sea level and in the mountains. The first one is around boosting your hematological values, especially your red blood cells. Red blood cells are the taxi of oxygen, while hemoglobin [acts as the] the seats in which the oxygen sits. At altitude, the rarefied air forces your body to produce more red blood cells. 

Related: What's the highest place on Earth that humans live?

As one gram of hemoglobin carries around 1.36 milliliters [0.05 fluid ounces] of oxygen, you can then generate stronger performances at the next race. You also have an angiogenic response, which is the formation of new blood vessels and again helps the transportation of oxygen to the working muscles.

JW: What do you measure physiologically when it comes to riders like Pogačar?

ISM: Firstly, you establish physiological and metabolic parameters of each athlete in order to prescribe the most accurate and progressive training program. You need to evaluate the athlete. Do we have a Ferrari or a Fiat? You identify their weak and strong points. In my system, factors like lactate variances, lactate metabolizing, fat metabolism and carbohydrate metabolism are very important. With the evaluation of these parameters, it's possible to establish an appropriate individual training program for an athlete. And all of this is achieved thanks to metabolomics [the study of chemical processes involving metabolites, small molecules made during or used for cell metabolism].

JW: Tell us about your metabolomic work in cycling.

ISM: With a few drops of blood, we can analyze between 1,000 and 2,000 parameters of the body and so understand how the body functions at a level that we've never seen before in cycling. Mitochondrial function, cell oxidation, glycolysis, anaerobic capacity, catabolic capacity … We can identify differences between cyclists and see what makes a truly elite athlete.

We had Pogačar and 20 other riders at UAE Team Emirates undertake a 15-minute warm-up at a low intensity of 2 watts per kilogram [w/kg] of bodyweight. Intensity cranked up by 0.5 w/kg every 10 minutes. Power output, heart rate and lactate were measured throughout the test, including at the end when the riders were exhausted. We discovered that [the] lactate-clearance capacity of the stronger riders was incredible.

JW: What does "lactate-clearance capacity" mean? 

ISM: When a rider's sprinting or climbing, they generate a huge amount of lactate. We call this "highly glycolytic." The problem is, the more your muscles use glucose, the more lactate is produced because it's a byproduct of glucose utilization. This lactate builds up, and hydrogen ions associated with lactate increase acidosis [a build-up of acid] of the muscle microenvironment, decreasing contraction capacity and power output. That's why it's critical to clear that lactate. 

One of the measurements I take is for lactate to see how quickly the lactate levels recover after a hard effort. With Pogačar, I noticed that his lactate recovery capacity was huge. His levels would return to normal after 2 minutes, while some riders took 20 minutes. That's a great advantage when you're attacking a climb as you can keep on attacking. It's been shown that world-class athletes produce more because they have a higher glycolytic capacity and can also clear it more proficiently. Well, Pogačar has one of the greatest glycolytic capacities I've ever seen.

Related: Workout in a pill: Scientists move one step closer to an exercise-mimicking drug 

JW: When did you start to see your metabolomic work pay off at races?

ISM: I remember the Vuelta a España of 2019. Many thought we were crazy to have a 20-year-old [Pogačar] lead the team in his first three-week stage race. Many said he was there for experience and that after 10 or 12 days he'd go home. 

But we knew different. We knew from his blood parameters that he could not only recover quickly between in-stage efforts but between stages, too. No one improves during a race of that length, but it's about reducing the decrement in performance. And that's what Pogačar managed, and why the three stage wins he bagged all came in the second and third weeks.

JW: Your work in sports is just one hat you wear. You're also involved in cancer research. Tell us about this, please.

ISM: We can see with riders like Pogačar how well they clear out lactate. Well, through my clinical work we know that cancer burns through glucose, creating huge amounts of lactate. But unlike when exercising, that lactate isn't cleared. This is bad for the cell and is a key driver of carcinogenesis [the formation of cancer].

With our work we're looking to reprogram a cancer sufferer's metabolism by, among many options, blocking the enzyme that produces lactate. We have a new study under review where we've silenced the enzyme that produces lactate, LDHA, and we can see that in 6 hours, EGFR is not expressed [made by the cells]. EGFR is a protein mutated in many cancers, and also the "mother" of the key signaling pathways that are mutated in most cancers, which increase cellular growth and proliferation of cancer cells. By inhibiting lactate production, we can inhibit EGFR production and so should be able to inhibit the signaling pathway cascade initiated by EGFR. 

We're proving that this is a new door to understand cancer better and cornering it someday.

Related: What happens to cancer cells when they die? 

JW: Tell us about the importance of mitochondria to both sporting performance and general health.

ISM: When you train a rider, you're looking for both central and local adaptations. The central adaptations are cardiorespiratory adaptations, so things like increasing blood volume, reduction in heart rate, enhanced sweating response … It's important but, at this level, it's the local adaptations at a cellular level that are the gamechangers. Things like mitochondria efficiency. [Mitochondria are small structures in cells that make chemical energy.] 

It's not about how much oxygen you can deliver to the cells, it's about how well that oxygen is utilized by the cell and how the different fuels and substrates are utilized by mitochondria. Someone like Pogačar can burn fat at a high intensity in his furnace of mitochondria, which is a great advantage as you have huge stores of fat. The really hard efforts demand greater carbohydrate use but their stores are sparing in the body — so if you can ride hard using fat stores, well, to my mind this is where the great cyclists differ from the good. 

When talking about general health … Cardiometabolic diseases, like cardiovascular diseases, type 2 diabetes and even Alzheimer's, which will soon be called "type 3 diabetes," shows one thing in common: mitochondria dysfunction. They can't burn glucose efficiently, meaning glucose builds up, giving rise to insulin resistance and eventually type 2 diabetes. The same things happen with fat. You can't burn it properly, it builds up, leads to inflammatory responses and ultimately cardiovascular disease, weight gain.

JW: Finally, beyond your wish to win the 2024 Tour de France, what does the future hold?

ISM: Sadly, I've recently closed my lab in the U.S. It's been heartbreaking as I haven't been able to finish all the experiments that I'd started, experiments where we'd made important findings with the hope to open new doors in terms of treatment against cancer or at least a better understanding of its metabolism.

Thankfully, a reputed cancer researcher in the top research institute in the Basque Country has the technology and resources to help me conclude my experiments. Now with time and no great rush, I should be able to finalize my experiments and contribute to opening new doors.

Editor's note: This interview has been condensed and edited for clarity.

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The science of why we age is a hot topic, with studies pointing to changes in our chromosomes, cellular stress and epigenetics as culprits. These changes are difficult to reverse with treatments — but what if there were a molecular cause of aging that we could change more easily?

In a study published Friday (April 12) in the journal Nature Aging, researchers may have identified a particular kind of fat molecule, or lipid, that plays a major role in the aging process. This lipid, called bis(monoacylglycero)phosphate (BMP), was found at consistently higher levels in the muscles of older people than in those of younger people. And notably, those high levels fell with short periods of exercise. The researchers also studied this effect in more detail in mice.

Aging research lacks detailed studies of fat molecules, "so it was great to see this study doing such a comprehensive analysis of the changes in many different tissues in mice and humans," Dr. Alexandra Stolzing, a professor of biogerontological engineering at Loughborough University who was not involved in the study, told Live Science in an email.

Related: 'Biological aging' speeds up in times of great stress, but it can be reversed during recovery

Creating a map of fat

Scientists generally understand how simple fats, such as cholesterol, contribute to aging and diseases like coronary artery disease. However, "little is known on how 'complex' lipids contribute," said Dr. George Janssens, an assistant professor of genetic metabolic disease at Amsterdam UMC and first author of the study.

The researchers started exploring this relationship by analyzing the fats of young and old mice. Using "lipidomics" — a technology that quantifies the many different fats in the same tissue simultaneously — they looked at 10 different tissues and identified more than 1,200 unique types of lipids.

What jumped out at them was the consistent increase of BMP in old mice, which they saw across most of the tissues analyzed.

The researchers then analyzed muscle tissue biopsied from human volunteers of different ages; younger participants were 20 to 30 years old while older participants were ages 65 to 80. Again, they found that BMP accumulated in the aging tissue.

"The nearly tissue-wide accumulation of one lipid in mice, and the same change conserved in human muscle … was astonishing," Janssens told Live Science in an email. The results suggest that BMP lipids may be a driving force in the aging process — however, more research is needed to determine whether they actually drive aging or rather increase as a result of it.

An hour a day keeps the lipids away

Evidence suggests that even short bouts of daily exercise reduce people's risk of early death. And in general, exercise has been closely linked with improvements in longevity, with one explanation being that it tends to change how the body breaks down lipids.

Related: Extreme longevity: The secret to living longer may be hiding with nuns ... and jellyfish

In a second phase of their study, the researchers analyzed the lipid content in people's muscles once before and once after they exercised one hour a day for four days. They compared these numbers with those of people who remained seated for most of that time period.

Surprisingly, even within such a short time frame, BMP levels were significantly reduced in those who exercised versus those who were sedentary. This suggests this lipid could be a key player in the benefits of exercise on longevity.

"This study is intriguing" because the researchers discovered that this fat increases with age in both mice and humans, said Adiv Johnson, director of research and innovation at the longevity company Tally Health, who was not involved in the study. But the findings also show that exercise — "a powerful pro-longevity intervention" — can deplete this age-related fat in humans, he told Live Science in an email.

"The idea that we could reverse aging is something that was long considered science fiction," senior study author Riekelt Houtkooper, a professor of genetic metabolic diseases at Amsterdam UMC, said in a statement. "But these findings do allow us to understand a lot more about the aging process."

The scientists plan to conduct further studies to learn why BMP accumulates in the first place and whether methods other than exercising could deplete BMP. 

Until then, the study provides one hint as to why going for a walk or run around the neighborhood might help us live just a little longer. For those who are physically unable to exercise, there could be other solutions on the horizon, as some scientists are working on drugs to mimic the helpful effects of physical activity. 

This article is for informational purposes only and is not meant to offer medical advice.

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Flexibility is important for everyday movements, from bending down to put on your shoes to turning your head and even walking upstairs. Without adequate flexibility, these motions become far more difficult to execute. Meanwhile, athletes like ballet dancers and gymnasts push their flexibility to the limits in order to access a more extreme range of motion.

Given that people exhibit different degrees of flexibility, that raises a question: What factors determine how flexible you are? 

Flexibility is an intrinsic property of body tissues that determines the range of motion achievable without injury at a joint or group of joints. That's according to a definition developed by kinesiologists at Dalhousie University and presented at the 13th International Conference on Biomechanics in Sports. They note that, beyond muscles and connective tissue, one key factor that determines flexibility is how a person's skeletal structures all fit together. 

"Individual differences in your innate flexibility can be quite large, with genetics playing a big effect," said Stephen Dunn, a physical therapist and co-founder of Core Therapy and Pilates in Austin Westlake Hills, Texas. "Your flexibility is influenced by a variety of genetically inherited traits, including the structure of your joints, the length of your muscle fibers and the suppleness of your connective tissues," Dunn told Live Science in an email.

Related: How to improve your flexibility, according to physiotherapists 

We naturally lose some flexibility with age, said Dunn. "This decrease is brought on by alterations in the tendons and ligaments that make up connective tissues, which lose their elasticity as we age," he explained. Tendons connect the bones of a joint to the muscles that manipulate it, while ligaments surround and support that joint, limiting the ways it can move. 

"Collagen and elastin are two proteins that are essential for preserving the elasticity and flexibility of our connective tissues," Dunn said. "Collagen formation declines with age, and the existing collagen stiffen[s] and becomes brittle."

Other age-related joint changes can impact flexibility, "such as cartilage wear and tear, and sarcopenia or age-related muscle loss," Dunn added. This is why young people are generally more flexible than older adults.

Other factors, including sex, also affect flexibility. The female sex hormone estrogen increases the amount of collagen in connective tissue, which generally makes females' tendons and ligaments more elastic and flexible than males', according to a 2019 study in the journal Revista Paulista de Pediatria. Hypermobility disorders, which typically have a genetic cause and alter the connective tissues, also affect flexibility, in that people with the conditions show an excessive range of motion in their joints. 

How does stretching improve flexibility? 

Of course, in addition to one's biology, physical activity also greatly impacts flexibility. A sedentary lifestyle can result in stiffness and a reduced range of motion, Dunn said. 

Flexibility can be improved through changes in physical activity — for example by adopting a consistent stretching routine. The American College of Sports Medicine recommends engaging in some type of stretching at least two to three days a week, or preferably daily.

Related: Best yoga mats 2023: The best designs for hot yoga, travel and general home use

In the short-term, meaning during a stretch and shortly thereafter, stretching lengthens the muscle-tendon unit attached to a joint and helps counteract stiffness that may have arisen from sitting in a static position. In the longer term, on the order of weeks, regular stretching is thought to improve range of motion by increasing one's tolerance levels to stretching, according to a commentary published in the International Journal of Sports Physical Therapy (IJSPT).  

Also known as the sensory theory for increasing muscle extensibility, this idea suggests that the increased extensibility, or stretchiness, of muscles observed immediately after a stretching session or after several weeks of regular stretching is solely due to changes in sensation. 

A muscle contracts when it is first stretched because the muscle spindles, sensors that detect changes in muscle length, send signals to the spinal cord, which then communicate back to the muscle, Dunn said. This is due to the stretch reflex, an involuntary defense against overstretching and potential muscle damage. 

Through repeated stretching, however, "our neural system becomes acclimated to the stretch stimulus," he explained, allowing the joint to move through a greater range of motion before the stretch reflex activates. That said, people show differences in how much they can adjust their stretch tolerance using the same stretches. 

The IJSPT commentary says all types of stretching can increase flexibility. This includes three different types of stretching: Static stretching involves extending a muscle to its limit and holding that position in stillness. Dynamic stretching requires active movement while extending a joint through its full range of motion. And finally, pre-contraction stretching involves contracting the muscle being stretched or its antagonist (the one that opposes its action) before stretching.

There's also evidence that eccentric contractions — during which a muscle contracts as it's lengthened, like during the lowering of a bicep curl — can help increase flexibility. Low-impact exercises like yoga and pilates also engage these different modes of stretching and thus improve one's range of motion. 

This article is for informational purposes only and is not meant to offer medical advice.

If you're looking to gain muscle mass, a quick web search will tell you to "bulk" on a high-calorie, high-protein diet, progressively challenge yourself with heavier weights and rest in between workouts. But have you ever wondered how these measures actually boost the size of your guns? Whether you're a novice gymgoer or an experienced bodybuilder, learning the biology of skeletal muscle growth can help you understand the process at the cellular level. 

Everyone can benefit from weight training, even if getting ripped isn't your end goal. You lose muscle mass with age, and it becomes exceedingly difficult to reverse course the frailer you become, Kevin Murach, who researches muscle growth at the University of Arkansas, told Live Science.

Growth occurs when cells in the muscle make, or synthesize, new proteins faster than existing proteins are broken down. Tension from weightlifting and other exercises in which you lift or pull against resistance triggers synthesis. Muscle damage that occurs during exercise and leads to soreness can also trigger growth, lending credence to the phrase "no pain, no gain." 

But if you're not used to working out or if you complete high-intensity training, muscles can incur too much damage and a significant amount of synthesis will be used to repair broken muscle fibers and to clear away inflammation during rest rather than to pump up muscles, Murach explained. Focusing on concentric contractions, in which the muscle shortens as resistance is applied, more so than eccentric ones, in which the muscle lengthens, can maximize tension and minimize damage, he suggested. 

Related: Stunning images show how muscles heal themselves after a workout 

Within individual muscle fibers, some proteins, like the mammalian target of rapamycin (mTOR), are activated by tension and support growth by promoting muscle protein synthesis. Other proteins, like myostatin, keep muscle growth in check. 

Some animals genetically suppress myostatin and thus maximize their gains, including some incredibly buff racing dogs called "bully" whippets. "The breaks are taken off of muscle growth, so the muscle becomes very large," Murach said. In people, regular resistance training lowers the amount of myostatin secreted by muscle cells over time, he added, which could lead to faster gains.

Muscle fibers are tubular muscle cells. Unlike other body cell types, they contain multiple nuclei, none of which can divide. Although protein synthesis can boost muscle mass, these non-dividing nuclei limit the potential for muscle growth. Muscle stem cells called satellite cells compensate for this by donating their nuclei into growing muscle fibers. You lose satellite cells as you age, which could be one reason why it's harder to recover muscle mass in your later years. But this loss can be partially offset by exercise, which causes satellite cells to proliferate. 

How do genetics affect muscle growth?

Have you ever wondered why some people struggle to see results, even if they bulk on protein and pump iron to their limits? Muscle growth is controlled by genetic factors, as well. 

Everyone has a variable mixture of two muscle fiber types that are specialized for different exercises. Fast-twitch fibers are geared toward powerful, explosive movements, whereas slow-twitch fibers support sustained, endurance exercises and joint stabilization. Fast-twitch fibers grow more readily than slow-twitch ones, so people who struggle to put on muscle mass may have a smaller proportion of these. 

Although the ratio is largely genetically predetermined, Murach said "it can shift — you're not stuck with what you have." Focusing on weightlifting over endurance training could make fast-twitch fibers dominant, but these changes are localized to the muscles you train, he explained.

Related: Best adjustable dumbbells 2023: Space saving weights for your home gym

Sex differences can also affect muscle growth. The "male" hormone testosterone boosts protein synthesis and activates satellite cells. The higher levels of testosterone that members of the male sex experience during puberty could explain why males typically have more muscle mass than females at baseline. But when adults are put on the same weightlifting programs, their gains relative to their size tend to be equivalent between the sexes, Murach said, because the transient bursts of testosterone that occur with exercise don't contribute massively to growth. 

Ribosomes — the molecular machines that synthesize proteins — have recently been identified as another potential genetic factor behind muscle gains. Due to genetic differences, people possess different types and quantities of ribosomes, which could affect levels of protein synthesis and the types of muscle proteins produced.

These genetic factors can unfairly burden some gymgoers, making it more difficult to put on muscle mass. The encouraging news is that several lines of evidence — including that you can lower myostatin, proliferate satellite cells and tune your muscle fiber ratio — suggest everyone's muscles can be adapted to grow. 

This article is for informational purposes only and is not meant to offer medical advice.

Tapping into your "muscle memory" to tie your shoes or play an instrument may feel automatic — but to execute these learned motions, the brain erupts into a flurry of activity, rapidly "unzipping" and "zipping" all the key information about the movement being performed, a new study suggests.

The study, published Feb. 1 in the Journal of Neuroscience, used a brain scanning technique called functional magnetic resonance imaging (fMRI) to collect snapshots of people's brains as they played simple melodies on a keyboard. fMRI tracks the flow of oxygenated blood through the brain, and because active brain cells require more oxygen than inactive ones do, the scans provide an indirect measure of brain activity. 

The 24 study participants — none of them trained musicians — learned simple, one-handed keyboard melodies over several days and were then asked to play these sequences from memory while in the fMRI scanner. In each trial in the scanner, the participant would receive a visual cue to prepare to perform one of the melodies and then a second cue to execute it. 

In some of the trials, the participants weren't given the second cue, so the researchers got snapshots of the brain both planning and executing movements. 

Related: How does the brain store memories? 

These scans revealed that movement-related regions of the brain's wrinkled outer surface, the cerebral cortex, lit up during the planning stage, and this activity reflected the order and timing of the notes to come. In other words, specific patterns of brain activity reliably translated to particular sequences of notes, and separately, other activity patterns reflected the durations of those notes. 

"This happens very rapidly and automatically each time in the hundreds of milliseconds before the action starts," Katja Kornysheva, the study's senior author and co-director of the Centre for Human Brain Health at the University of Birmingham in the U.K., told Live Science in an email. 

Then, when it comes time to actually play the notes, these separate patterns representing note order and timing become integrated, or "zipped," resulting in a new, unique pattern of brain activity. 

"The integrated patterns were those that were unique for a particular combination of key-press order and timing, not something that transferred across these combinations," Kornysheva said. So the brain went from handling each element of the movement separately, like paint and a canvas, to considering them a single, integrated unit, like a completed painting. 

An established theory suggests that the parts of the cortex that control movement are in a kind of hierarchy, but this study runs counter to that idea, said Tanuj Gulati, an assistant professor of biomedical sciences at Cedars-Sinai Medical Center in Los Angeles who was not involved in the new research. 

Two regions, known as the premotor and parietal areas, are thought to store "high-level" information about movements — in this case, the order and timing of keystrokes. The primary motor cortex, which communicates with muscles via the spinal cord, handles only "low-level" information — what muscles in the fingers and forearms actually need to activate to make the keystrokes happen.

"This notion is challenged in this study," Gulati told Live Science in an email. "The areas thought to be 'low-level' that can only communicate fixed commands to downstream muscles were instead found to be constantly updating based on order and timing challenges of a movement," and so they were dynamically involved in movement planning and execution.

Kornysheva and her team are currently studying muscle memory in the context of disorders such as dyspraxia, a neurological disorder that affects the ability to plan and coordinate movements. Their work could also be useful for helping people regain motor skills after they've had a stroke, Kornysheva added. 

The team is also starting to study motor learning in trained musicians, in addition to novices, she said. 

"Musicians with seasoned finger proficiency and their sequence/timing control are akin to elite athletes, say a gymnast with excellent postural control," Gulati said. It may be that, in highly trained individuals, certain movement sequences become "hardwired" in the motor cortex and the rapid adjustments to high-level features of those movements may unfold differently than they do in the brains of novices, he said.

Exercise leaves muscles riddled with microscopic tears, so after a rigorous workout, the control centers of muscle cells — called nuclei — scoot toward these tiny injuries to help patch them up, scientists recently discovered.

In the new study, published Oct. 14 in the journal Science, researchers uncovered a previously unknown repair mechanism that kicks in after a run on the treadmill. Striking images show how, shortly after the exercise concludes, nuclei scuttle toward tears in the muscle fibers and issue commands for new proteins to be built, in order to seal the wounds. That same process likely unfolds in your own cells in the hours after you return home from the gym.

The study authors discovered that "nuclei moved toward the injury site within 5 hours of injury," Dr. Elizabeth McNally and Alexis Demonbreun, of the Northwestern University Feinberg School of Medicine, wrote in a commentary, also published in Science. And within only 24 hours of the injury, the repair process was "nearly complete."

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Skeletal muscles, which enable voluntary movements like walking, are made up of many thin, tubular cells; these cells are also called "muscle fibers," due to their thread-like appearance. A single muscle can contain hundreds to thousands of muscle fibers, according to the National Cancer Institute. And each fiber contains units of contractile machinery, known as sarcomeres, that contract and lengthen during exercise.    

Eccentric contraction, where your muscles are forcibly lengthened as they contract, can cause these sarcomeres to overstretch. (The second half of a bicep curl, where you slowly lower a dumbbell from shoulder-height to your side, and running downhill are examples of this type of exercise.) When sarcomeres overstretch during eccentric exercise, they can pull at the membrane surrounding them, causing damage, according to a 2001 review published in the Journal of Physiology.   

In these situations, muscle cells rely on a skilled cellular pit crew to help fix them up. Previous studies have shown that, seconds after an exercise-induced injury occurs, various proteins form a "cap" over the  damaged region of the membrane, and nearby mitochondria, the so-called powerhouses of the cell, help sop up any excess calcium that entered the cell through the tear, since the amount of calcium in muscle cells must be kept in check for them to function properly. 

And now, the new study suggests that the nuclei in muscle cells rush over to help, too.

For the study, the researchers placed adult mice on a downward-tilted treadmill and then sampled muscle fibers from the animals following their jogging sessions. In addition, they asked 15 healthy human volunteers to run on a (person-sized) treadmill and then biopsied muscle fibers from the vastus lateralis, a part of the quadriceps.

Related: Does running build muscle? 

They found that, in both the mouse muscle fibers and the human muscle fibers, proteins accumulated around tears in the fibers and formed "scars" within 5 hours after exercise. And in muscle fibers sampled 24 hours after exercise, clusters of nuclei had drawn close to the tears, whereas nuclei appeared farther away in the 5-hour samples. To see exactly how the nuclei had migrated toward the injury sites, the team grew mouse muscle cells in lab dishes and zapped them with lasers, to mimic exercise-induced injury.

In the lab-grown cells, the nuclei assembled around the laser injuries within 5 hours and soon generated "hotspots" of protein construction nearby. Specifically, the migration of nuclei was followed by a sudden explosion of mRNA molecules, a kind of genetic instruction manual built in the nucleus; mRNA essentially copies down the blueprints encoded in DNA and carries them out into the cell, where new proteins can be constructed. The newly built proteins then help to seal and reconstruct the injured muscle cells.

In the future, medical treatments could potentially be devised to target the molecular pathways that allow the nuclei to migrate and start this repair process. That could help speed patients' recovery after muscular injuries, McNally and Demonbreun wrote in their commentary.

Interestingly, the authors also found that mice that trained on the treadmill prior to the study developed fewer scars on their muscle fibers than mice that didn't undergo any prior practice. This aligns with previous evidence that, with consistent training, muscles become stronger and less prone to tearing during trained movements, according to The New York Times.

Originally published on Live Science.

Major sporting events like the Olympic Games always raise the topic of "doping" — the use of banned substances capable of boosting athletic performance.

Athletes may dope for a variety of reasons, including the media pressure to win and the financial rewards that are tied to success. Very competitive athletes can be vulnerable to the idea that drugs may help them gain a "secret edge," Thomas Hildebrandt, a clinical psychologist at the Mount Sinai Health System in New York, told Live Science.

Athletes can also hold a belief that "everyone's doing it," whether they are or aren't, which can make doping seem normal, he said.

The World Anti-Doping Agency (WADA), an independent agency that coordinates global anti-doping policy, maintains an extensive list of banned substances and methods thought to increase an athletes' performance. Many items on the list are banned during competitions, specifically, and others are also banned during training.

WADA rules apply not only at the Olympics but also during other high-stake competitions, such as the FIFA World Cup, the Tour de France and international tennis competitions.

Here is a look at what is currently banned by WADA as of January 2024, and how these substances and methods are thought to work in the body to give athletes an edge.

Related: What's the most dangerous sport in the world?

Substances that are banned at all times

The following substances are banned at all times, both in competition and during training. There are a few exceptions to this, including if an athlete has an illness or medical condition that requires them to take a specific medication.

Anabolic agents

The term "anabolic agents" broadly refers to drugs that stimulate muscle growth and development. One of the primary examples of anabolic agents are androgenic steroids. These are synthetic versions of androgens, or male sex hormones, such as testosterone and dihydrotestosterone.

Androgenic steroids are designed to bind to the same receptors in the body as the hormones they mimic, thus triggering similar effects. In skeletal muscle, the drugs can lead to artificial increases in muscle mass and strength. Androgenic steroids differ from testosterone replacement therapy, which is designed only to bring a person's testosterone levels back to normal if they are deficient.

Androgenic steroids drugs may increase people's performance in sports where having stronger muscles is particularly handy, such as weightlifting and boxing. However, they can't improve an athlete's agility or skill. If used improperly, these drugs can also cause serious side effects, including high blood pressure, heart disease, liver damage and stroke.

Growth factors and peptide hormones

Growth factors are substances that primarily act on muscle and bone to, as the name implies, promote growth. Specifically, these substances stimulate cells to replicate, Hildebrandt said. This can lead to bigger muscles. Examples of banned growth factors include fibroblast growth factors (FGF) and insulin-like growth factor 1 (IGF-1).

Another banned growth factor is erythropoietin (EPO). Rather than stimulating muscle growth, EPO triggers the production of red blood cells, which are the cells that deliver oxygen to tissues via the bloodstream. A greater number of red blood cells means that the blood can carry more oxygen to muscles, which enhances performance. Using EPO is sometimes referred to as "blood doping."

This group of drugs are particularly challenging to detect because they include naturally occuring hormones, such as IGF-1, said Rhonda Orr, an associate professor of exercise and sport science at the University of Sydney. It is unknown how prevalent blood doping is, but one study suggests it could be as high as 22% in endurance athletes.

Beta-2 agonists

Beta-2 agonists are a class of drugs typically found in asthma inhalers. During an asthma attack, they help relax the muscles of the airways. WADA places strict limits on the type and amount of beta-2 agonists that can be inhaled by athletes with asthma or similar conditions, and it bans the drugs for all other athletes.

When certain beta-2 agonists are taken by mouth, they can enhance an athlete's performance by stopping muscle from degrading, Hildebrandt said. Specifically, they exert their action on fast-twitch muscle fibers in skeletal muscle that are responsible for bursts of fast, powerful movement.

This effect can make it easier for athletes to build up their muscles and improve their endurance, which can help them to avoid muscle fatigue, he said.

Related: Which Olympic sport burns the most calories?

Hormone and metabolic modulators

Another category of drugs that are banned by WADA at all times are hormone and metabolic modulators.

One example of these drugs are aromatase inhibitors, such as anastrozole and letrozole, which impede the action of an enzyme called aromatase. This enzyme is responsible for converting androgen hormones into estrogen — one of the primary female sex hormones. Aromatase inhibitors are used to treat breast cancer, as estrogen is known to stimulate the growth of breast cancer cells.

However, these drugs can also be misused by athletes to keep testosterone levels in their body at a very high level, which could potentially improve their performance in a similar way to taking synthetic testosterone.

Diuretics and masking agents

Diuretics are substances that make a person produce more urine. This means that they can help athletes shed unwanted water weight before a competition. In addition, athletes sometimes use these compounds to flush out other drugs from the body, so that they don't show up in urine tests, Hildebrandt said.

In this way, diuretics can act as a so-called masking agent — in other words, a substance that's used to cover up, or mask, the presence of other doping drugs in a lab test, he said. For this reason, diuretics are banned at all times, unless an athlete needs them for therapeutic purposes.

WADA also bans other substances that act as masking agents, such as desmopressin.

Non-approved substances

The name "non-approved substances" may appear redundant as none of the substances that are banned by WADA are approved for use by athletes to enhance performance. However, this category represents a broader class of substances that are not approved for use in humans for any reason at all, Orr said.

This includes drugs that are still in clinical development or that have been discontinued; so-called designer drugs, which are synthetic versions of legally restricted or prohibited drugs; and substances that are only approved for use in animals.

The list of substances that athletes are willing to use in order to gain an edge over competitors keeps on expanding, which makes it harder for Olympic officials to keep up with testing, Orr said.

Related: At what age does athleticism peak in different sports?

Banned methods

In addition to banning drugs, WADA also prohibits several non-pharmaceutical, performance-enhancing techniques.

For instance, athletes sometimes train at a high elevation, where the lower levels of oxygen in the air trigger the body to produce more red blood cells. Such training isn't banned, but sometimes athletes collect and store some of their blood that contains extra red blood cells. Then, just before a competition, they re-infuse it into the body, Hildebrandt said. This practice is banned because it means that these athletes have extra red blood cells in their blood to carry more oxygen to their muscles.

Other practices that WADA forbids include tampering with samples collected during drug testing and using gene-editing technologies that may enhance performance.

Substances banned only in competitions

Stimulants

Stimulants are drugs that can increase an athletes' energy, attention and focus, while potentially giving them a "psychological edge" over competitors, Hildebrandt said. For instance, the drugs could provide a boost for a tennis player in a five-hour match in which every stroke counts, he said.

Examples of stimulants include cocaine and amphetamines. These drugs share similar effects but work differently in the body and last different lengths of time.

Narcotics & cannabinoids

Narcotic drugs which include opioids like morphine and heroin, may seem like odd choices for people wanting to enhance their athletic prowess. Similarly, cannabinoids, the active ingredients found in cannabis, aren't known for boosting athletic ability. However, these substances are instead banned because of their pain-blocking abilities.

The concern is that athletes who take these drugs can compete through pain that would otherwise force other people to give out, Hildebrandt said. For sports where the amount of pain an athlete has to endure really matters — like boxing or wrestling — blocking pain could give an athlete a competitive advantage, he said.

Related: Runners enjoyed their workouts more after using cannabis, but physically floundered

Glucocorticoids

Glucocorticoids, such as the stress hormone cortisol, are steroid hormones that are produced by the adrenal glands on top of the kidneys. They play a role in what's known as the "fight or flight" response, which is the body's automatic, physiological reaction to perceived threats. This response can cause a person's heart rate and blood pressure to go up, making them feel "amped up," Hildebrandt said. It wouldn't be unusual for an athlete's fight-or-flight system to kick in during a high-stake competition, but the idea is that the drug can kick this into high gear.

For athletes, this could mean that they become more primed to act and can respond to things more quickly, Hildebrandt said.

Substances banned in particular sports

Beta-blockers

Some substances are banned by WADA in only certain sports. One example are beta-blockers, which are drugs that are typically used to treat heart disease and other conditions. They work by blocking the release of stress hormones — such as epinephrine, also known as adrenaline — and thus slowing a person's heart rate and reducing their blood pressure.

Beta-blockers inhibit the sympathetic nervous system, which governs the fight-or-flight response, so they can make people feel more relaxed and less anxious. They can also reduce tremors, which may be beneficial for athletes competing in specific events that require high-precision movements.

CThus, these drugs are banned by WADA in sports like darts and golf during competitions, and in sports such as archery and shooting at all times.

The WADA updates its list of banned substances and practices every year. Since the initial list's release in 2004, substances like the narcotic tramadol and the anabolic agent Epiandrosterone have been added, and more are sure to come in the future.

Editor's note: This story was last updated on August 2, 2024. An earlier version of the article was published August 10, 2016.

This article is for informational purposes only and is not meant to offer medical advice.

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