Changes in triathletes' hearts that allow them to pump blood more efficiently suggest a link between exercise choices and cardiac adaptations, according to a new study.

Athletes who participate in triathlons must swim, bike and run in succession, which means they undertake an excellent combination of resistance and endurance exercises, the researchers say.

The researchers found that, as a result of this combination, triathletes' hearts showed physical changes. Compared to the hearts of a control group of men who were active no more than three hours per week, the hearts of male triathletes had larger left atria and right and left ventricles, and their ventricles also had greater muscle mass and wall thickness.

The triathletes' resting heart rates were also 17 percent lower than those of the control group, meaning their bodies needed to expend less energy during normal rest conditions.

The study "tells us how the heart basically adapts to different stimuli it receives or may not receive," said study researcher Dr. U. Joseph Schoepf, director of cardiovascular imaging at Medical University of South Carolina. "If someone is a couch potato and never exercises, that has certain consequences for the adaptation of the heart [compared to] someone who is a professional triathlete."

The study also brings scientists one step closer to finding differences between normal adaptations in the heart and those that may be dangerous, such as hypertrophic cardiomyopathy, a thickening of the heart muscle that can lead to cardiac death, Schoepf told MyHealthNewsDaily.

The researchers conducted cardiac magnetic resonance imaging, known as MRI, on 26 male triathletes from around the world, whose average age was 27.9 and who had six or more years of professional training. The results will be published in the October issue of the journal Radiology.

While not everyone is cut out to be a professional triathlete, the results show that getting the best exercise possible for your body makes a difference, Schoepf said. Running and swimming are examples of endurance sports, weight-lifting is an example of a resistance sport and cycling is a mixture of the two.

"The body successfully responds to whatever is thrown at it," he said. "Some people just aren't born to be endurance athletes, and some people aren't born to be weight lifters. But one basically has to be whatever suits one's own lifestyle" when it comes to exercise.