Swimming and Running May Shape the Heart Differently

People select cardio exercise according to what they enjoy. Some run to improve speed and cover distance, whereas others choose swimming for a whole-body session. New research indicates that these options may affect the heart differently.

The findings indicate that swimming could produce more pronounced and complex changes within the heart muscle, even when the level of effort is equivalent.

“Swimming and running are two excellent ways to improve cardiorespiratory health and protect the heart muscle, but we wanted to know if one could be even more beneficial than the other,” said Andrey Jorge Serra, a professor at the Federal University of São Paulo and coordinator of the study.

“We found that, although both increase respiratory capacity, swimming goes a step further by combining functional and molecular adaptations that make the heart stronger and more efficient.”

The continuing cardio debate

Sports affect the body in distinct ways, and this is not a new concept. Endurance athletes commonly display noticeable variations in heart size and structure according to the activity they pursue.

Cyclists, rowers and skiers each develop distinctive cardiac patterns.

However, researchers have had few direct, controlled comparisons of individual activities. This has made it difficult to account for the reasons behind those differences.

“Although several studies had already examined the expression of microRNAs regulated by aerobic training in general, little was known about expression patterns when swimming and running were compared in the same experimental setting,” Serra noted.

“Therefore, this study reveals that there’s a distinction in cardiovascular effects between these two modalities.”

Creating an equal test

To investigate the issue, the researchers studied male Wistar rats, dividing them into three groups. One group stayed inactive, a second trained on a treadmill, and the third swam in a controlled tank.

The two exercise groups trained for eight weeks, completing 60-minute sessions five days per week.

Both forms of exercise were prescribed at roughly 75 percent of maximum oxygen capacity, ensuring the intensity was matched.

That point is important, as earlier studies have frequently compared unequal workloads.

Fitness gains in both groups

Both groups that exercised made clear improvements in fitness. Their oxygen capacity rose, whereas it fell in the sedentary rats. The trained animals also achieved better results in endurance testing.

At this point, the effects of swimming and running appeared alike. Each improved broad fitness measures, but more detailed analysis showed a difference.

Swimming increased heart size

The clearest contrast emerged in heart structure. Noticeable growth in heart size occurred only in the swimming group.

Their hearts were heavier in relation to body mass, while the left ventricle, which sends blood around the body, grew thicker and wider.

On a cellular scale, their heart muscle cells were bigger and had more developed internal structures.

The runners showed none of these changes. Their hearts stayed comparable with those of the inactive rats.

More powerful contraction in swimmers

Heart size alone is not a measure of performance, so the researchers assessed the ability of heart muscles to contract.

Muscles taken from swimmer hearts produced greater force, and they both contracted and relaxed more quickly. Together, these changes improve the efficiency with which the heart pumps blood.

Running brought only slight improvements in contraction speed. Swimming generated a considerably stronger response.

Swimming, running and signals within the heart

The team then examined what may cause these effects. Their focus was a recognised pathway associated with healthy heart growth, involving proteins including PI3K, AKT, PTEN, mTOR and S6K1.

Each form of exercise activated elements of this system, but swimming stimulated it more strongly.

Among swimmers, PTEN levels fell more markedly, removing a brake on growth. AKT activity rose at the same time. S6K1, another important protein, was activated only in the swimming group.

“Although we don’t yet know why this change occurs at the molecular level, of the microRNA, we were able to delve deeply into and investigate the molecular pathways that control physiological hypertrophy,” Serra noted.

The microRNAs involved

The researchers also assessed microRNAs, small molecules that regulate the way genes are expressed within cells.

Five important microRNAs increased more strongly in swimmer hearts: miR-1, miR-21, miR-27a, miR-124 and miR-144.

These molecules affect blood-vessel growth, responses to stress and the regulation of inflammation. Their greater activity indicates that swimming causes wider changes within the heart.

Why water may make a difference

The study does not identify a single explanation for the differences, although several possible factors stand out.

Swimming recruits more muscle groups simultaneously. The upper and lower limbs both work against resistance, while the horizontal body position alters the return of blood to the heart.

That posture raises the amount of blood processed during each heartbeat. Water temperature could also contribute by changing cardiovascular load.

In combination, these factors may prompt the heart to adapt more extensively.

Study limitations

The research involved rats rather than people. Because rats run on four limbs, their running mechanics differ from those of humans.

Their swimming behaviour is different too. Laboratory rats paddle in a simple manner, whereas human swimming differs greatly by stroke and intensity.

The research ran for eight weeks, and longer studies might reveal different patterns over time. The researchers also did not measure long-term health outcomes.

What it could mean for human health

Running continues to provide well-established benefits: it boosts fitness and lowers cardiovascular risk. Human research spanning decades supports this evidence.

The findings add further detail to that picture. For people seeking particular improvements in heart structure and function, swimming may have an advantage.

“People’s choice of sport depends largely on personal preference, aptitude, and enjoyment. But our results show that swimming may have a special impact in situations involving myocardial recovery, cardiac rehabilitation, and above all, scientific research,” Serra explained.

“This is also relevant because studies on aerobic exercise often use running and swimming interchangeably, and we now know that the effects aren’t the same.”

Swimming already benefits people with joint problems because it is low impact. This research suggests that it may also deliver more extensive benefits for the heart.