During intense or prolonged exercise, muscles can feel like they’re burning the longer and harder someone pushes.
This occurs because the muscles produce energy as quickly as possible by dipping into the body’s stores of glucose, producing an acidic substance called lactate as a byproduct. This is what gives people that "burn".
A person’s lactate threshold is when the production of lactate exceeds their body's ability to clear it from the system. Research suggests training around one’s lactate threshold could be beneficial for exercise performance — here's how.
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What is lactate?
Lactate is the electrically charged form of lactic acid produced by muscle cells and other tissues during anaerobic activity — or activity that uses energy produced without oxygen.
The body's preferred energy production process is aerobic, which uses oxygen. This supports lower intensity activity such as a gentle run. If there is an adequate supply of oxygen, the energy occurs in mitochondria (the powerhouse of a cell), which use glucose and oxygen to produce adenosine triphosphate (ATP) to create muscular contractions.
During higher intensity exercise the body uses more anaerobic energy because the process to create energy with oxygen takes too long and the body needs it more quickly. This is where lactate comes into play.
During fast sprints, weight lifting and more explosive exercise, the body turns to anaerobic energy production to metabolise glucose and produce ATP. The by-product of this process is lactic acid which can build up faster than the liver can break it down. This can cause weak muscles, shaking and nausea.
What is lactate threshold?
As lactate is produced, it needs to be cleared from the system. The presence of lactate brings with it hydrogen ions which drop the PH of the blood, making it more acidic.
"This is where the 'burn' in your muscles comes from,” said Jesse Grund, an exercise scientist in Florida. “The threshold is when the production of lactate exceeds the body's ability to clear it from the system. This means the body can no longer produce adequate energy and needs to switch to aerobic production of energy, requiring a lower intensity level to clear it from the system," he told Live Science.
A higher lactate threshold means a person can exercise for longer at more intense levels and their body is more efficient at clearing the lactic acid.
Novice exercisers tend to have a lactate threshold at 50% to 60% of their maximum heart rate, Grund said, while more experienced athletes don't reach lactate threshold until 70% to 90% of their maximum heart rate.
High intensity training around the lactate threshold is a fundamental part of an endurance athlete's preparation and helps to improve both aerobic and anaerobic capacity, according to a 2014 study in the International Journal of Exercise Science (opens in new tab).
Lactate threshold training is also an effective way to predict race performance, according to a 2018 study of male cyclists in the same journal (opens in new tab).
Lactate threshold is typically tested in a laboratory where researchers take blood from athletes at set work intervals to see their blood lactate level. It can also be estimated via a treadmill test, but this is just an estimate, Grund said.
What is lactate threshold training?
Lactate threshold training works to push an athlete to the heart rate associated with their individual lactate threshold. This can be done with interval training, or steady state work at or near their threshold pace.
For performance benefits, research indicates that lactate threshold is the greatest performance indicator for endurance activities. The body is constantly using both aerobic and anaerobic metabolism simultaneously.
"The better we are at creating energy at low intensities of effort, the more efficient we can be without the aerobic system having to increase ventilation and heart rate to make up for it,” Grund said. “For example, the people who get really winded walking up multiple flights of stairs usually have very low lactate thresholds.”