Researchers found that different activities affect the heart in different ways. What could this mean for athletes who routinely train in three sports?

Runners and triathletes like to debate which is harder: Ironman or a marathon. Cyclists and triathletes debate things like aero bars and riding sockless. But a recent article in the New York Times brings a new comparison set: Which athlete has the strongest heart?

The Times article summarizes a study published in the journal Frontiers in Physiology titled “Left Ventricular Structure and Function in Elite Swimmers and Runners.” The tl;dr: Both runners and swimmers have stronger and healthier hearts than a person who doesn’t exercise—no surprise there. But the research did uncover one surprising finding: A runner’s left ventricle functions differently from that of a swimmer’s, suggesting that the type of exercise one does yields sport-specific physiological adaptations in the heart.

Though it’s long been known that different types of exercise create changes in the structure and function of the heart, most of the studies done previously have compared the hearts of athletes in land-based sports, such as runners to weight lifters. Given that the compared sports are vastly different stimuli on the heart as far as heart rate, blood pressure, and demands on the heart muscles, it’s not completely extraordinary to find the two groups have different heart functions. This new study, however, compares two fairly similar sports as far as cardiovascular demand: swimming and running. It also adds a previously uninvestigated element of heart health known as left ventricular mechanics, a fancy way of saying how well and how fast the heart contracts and relaxes as it fills with and ejects (or “pumps”) blood.

The major finding was that runners had improved early diastolic filling. The researchers posit a number of potential explanations for why this is, most of which relate to the ability of the body to return blood to the heart—which is easier for swimmers due to their horizontal body posture, higher blood pressure, and pressure from the surrounding water. As such, it is perhaps not too much of a stretch to think that runners may develop their ability to return blood to the heart to a greater degree owing to the typical demands on their body during training.

Naturally, we were curious: What does this mean for triathletes, who do both swimming and running? Study authors Dr. Jamie Burr of the University of Guelph and Dr. Katharine Currie from Michigan State University tell Triathlete that’s on their list of questions, too:

“We can’t yet say conclusively, but this is a really interesting question—not only because triathletes both swim and run, but most non-draft legal triathletes also cycle in the aero position, which means some of the typical land-based stresses during exercise (such as blood having to overcome the full forces of gravity to return back up to the heart) are lessened by the horizontal posture of the upper body. It remains to be seen if triathletes thus get the “best of both worlds” from being exposed to these different training stresses, but it is an intriguing thought and something we would like to follow up on in the future.”

Burr and Currie also say they’re interested in comparing training loads, as athletes can only log so many miles before they risk injury or over-training: “Because of the multiple disciplines and the ability to train across modalities, triathletes may offer a unique group to study in this regard considering both the overall and discipline-specific training loads.”