The Heart Wants What the Heart Needs: Study Offers New Insight into Cardiac Adaptations in Aquatic Athletes

By Joshua Micko Budd

16 January 2023

Synchronized swimmer's legs above the water surface of a swimming pool


The heart, like any other muscle in the body, adapts to exercise training. It is a longstanding assumption in sports science that endurance training leads to different changes in the heart than resistance training. But cardiac adaptations to training may not be quite that simple after all. A new study has found that cardiac adaptations in aquatic athletes depend not only the type of aquatic sport, but on sex of the athlete as well.

“It is important to understand how cardiac adaptations to training can differ among different individuals, so that it is easier to distinguish what is normal versus abnormal,” says Dr. Alexandra Coates, a recent PhD graduate from the Department of Human Health and Nutritional Sciences.  

Aquatic athletes are unique because most of their training takes place in the water, and Coates – who is herself a former Canadian triathlete – was curious if this would also result in unique cardiac adaptations. To find out, she and her advisor, Dr. Jamie Burr, ventured to the 2019 FINA world championships in Gwangju, South Korea, where they recruited three different types of aquatic athletes: elite swimmers, water polo players, and artistic swimmers.

Based on the characteristics of each sport, Coates made predictions about what each type of athlete’s heart would look like. These predictions were based on the longstanding “Morganroth hypothesis”, which states that endurance training increases the capacity of the heart (eccentric hypertrophy) whereas resistance training makes the heart walls thicker (concentric hypertrophy).

Accordingly, Coates expected that the swimmers would have the greatest eccentric adaptations, water polo players would have mixed adaptations, and the artistic swimmers would have the least eccentricity.

The results of the study came as a surprise, however. While Coates did indeed find that swimmers have mostly eccentric adaptations and water polo players show increased concentricity, this was only true in male athletes. In contrast, the cardiac structures of female swimmers and water polo players were similar to each other, with greater signs of eccentric adaptation than concentric for both types of athletes.

“These results support a newer line of thinking that women may be predisposed to eccentric adaptations, even if they are doing exclusively resistance exercise. But because most research has focused on men, this has only been recently discovered,” explains Coates.

The study emphasizes that sex differences are an important research front in sports science.  Future studies will be required to determine if the sex-based discrepancies observed by Coates are the result of training, or perhaps rooted in hormonal differences.

This research also provides the first insights into the heart of artistic swimmers, who are almost exclusively female.

“At first glance, their hearts look normal, but when you look closely, they have really pronounced wall thickening of the posterior left ventricle, and expansion of the left atrium.”

Coates believes that this unique adaptation occurs because artistic swimmers hold their breath for long periods underwater. This activates the human dive reflex, which in turn increases blood pressure and decreases the volume of blood pumped out from the heart.

This type of research has important clinical implications, adds Coates.

“When an athlete goes to a cardiologist, it is important for the practitioner to know what is normal for the particular sport so that they can distinguish between healthy and unhealthy adaptations. This is particularly important for artistic swimmers. We were the first to observe them and therefore more research needs to be completed to establish a norm.”

Perhaps, in the end, a generalized theory like the Morganroth hypothesis is far too broad of a stroke to paint an accurate picture of cardiac adaptations to exercise. Coates’ findings demonstrate the necessity of a more bespoke approach, and serve as an important stepping stone for future research in the sex-dependency of cardiac adaptations.


Read the full study in the Clinical Journal of Sport Medicine.

Read about other CBS Research Highlights.