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Competitive swimming is a demanding sport that requires maintaining a high level of activity—while partially submerged in the water—without being able to breathe naturally. It’s a sport—like any form of exercise—that may not be suited to everyone.

Years of research have shown that competitive swimmers have larger lungs and better lung function than non-swimmers. In fact, swimmers have larger lungs and better lung function than most other people of the same age and body size. But from the time these data began to surface, there’s been a roiling debate about how these differences came about. Researchers have wondered if there is something about swim training that increases lung size and function, or maybe it’s a case of swimmers choosing a sport they’re ideally built for.

In a recent study published in Physiological Reports, researchers explored this decades-old debate with two groups of healthy girls between the ages of 11 and 14 years old. The researchers chose this age range because it’s a key time for lung development. Both groups were athletic: The swimmers had several years of competitive swim experience, and the non-swimmers participated in other sports like dance, gymnastics and team sports. The girls’ lung size (volume) and function were measured before and after a swim season, which lasted about seven and a half months.

Even before the swim season began, the swimmers had larger and stronger lungs than the non-swimmers. Not surprisingly, this difference between the groups continued after the season of swim training. What might be surprising is that lung volume and function increased in both groups after seven and a half months of exercise. However, the researchers also acknowledge that for preteens and young adolescents, these changes are typical of normal growth patterns. There wasn’t any evidence that a season of training enhanced lung size or performance in the swimmers. The study suggests that swimmers naturally have larger lungs and, for that reason, may gravitate toward the sport.

While there may not be enough data yet to fully squash this “nature versus nurture” debate, the findings stretch beyond the study of training and physical development. Exercise is challenging, but people should listen to their bodies. If swimming—or any type of exercise—is more difficult for some people than others, perhaps there’s a physiological reason behind it. Another sport, like running, rock climbing or boxing may be easier—and more fun.

Casey A. Gilman, MS, is a PhD candidate in the Organismic and Evolutionary Biology graduate program at the University of Massachusetts Amherst. She was the 2016 American Physiological Society-sponsored AAAS Mass Media Science and Engineering Fellow at The Philadelphia Inquirer. Gilman’s research focuses on the effects of social and physical interactions on lizard morphology, behavior and ecology. She is also a freelance writer.