Swimming and Running May Shape the Heart Differently

People select cardio exercise according to their preferences. Some run to build speed and cover distance, whereas others choose swimming for a whole-body session. New research indicates that these options could affect the heart in distinct ways.

The findings indicate that swimming can produce more pronounced and complex changes within heart muscle, even where exercise intensity 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 cardio debate continues

Sport affects the body differently depending on the activity, and this is not a new concept. Endurance athletes frequently display noticeable variations in heart size and structure according to their chosen discipline.

Cyclists, rowers and skiers each develop distinctive cardiac patterns.

However, researchers have had few direct, controlled comparisons of individual activities. As a result, explaining the reasons for these differences has been difficult.

“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.”

Designing a fair test

To investigate, the researchers studied male Wistar rats, splitting them into three groups. One group stayed inactive, the second exercised 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.

Each workout was calibrated to roughly 75 percent of maximum oxygen capacity, ensuring that the training intensity was the same.

That control was important because earlier research has often compared unequal workloads.

Fitness improved in both groups

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

At this point, swimming and running appeared comparable, as both improved broad fitness measures. More detailed analysis, however, showed a difference.

Swimming enlarged the heart

The clearest contrast involved heart structure. Evident growth in heart size occurred only in the swimming group.

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

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

Rats that ran showed none of these changes. Their hearts stayed similar to those of the inactive animals.

Swimming and stronger heart contraction

Heart size alone does not determine how well it performs, so the researchers assessed the ability of heart muscle to contract.

Muscle taken from swimmers’ hearts produced greater force. It also contracted and relaxed more quickly. Together, these effects improve the efficiency with which the heart pumps blood.

Running led to only minor gains in contraction speed, while swimming had a considerably stronger effect.

Signals inside the heart

The team then examined the mechanisms behind these effects. Their focus was an established pathway associated with healthy heart growth, involving proteins including PI3K, AKT, PTEN, mTOR and S6K1.

Both forms of exercise activated elements of this system, but swimming took the response further.

Among swimmers, PTEN levels fell more substantially, removing a restraint on growth. AKT activity also rose. S6K1, another important protein, was activated solely 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 role of microRNAs

The researchers also examined microRNAs, small molecules that regulate gene expression within cells.

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

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

Why water changes things

The research does not identify one definitive explanation for the differences, although several clues emerged.

Swimming engages more muscle groups simultaneously. The upper and lower limbs both work against resistance. The horizontal position of the body also changes the flow of blood returning to the heart.

This posture raises the amount of blood handled during each heartbeat. Water temperature could contribute as well, by changing cardiovascular load.

Collectively, these factors may send a stronger adaptive signal to the heart.

Limitations of the study

The study examined rats rather than people. As rats run on four limbs, their running differs from that of humans.

Their swimming behaviour is also different. Laboratory rats paddle in a simple manner, whereas human swimming styles and intensities vary greatly.

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

Implications for human health

Running continues to provide well-established benefits, improving fitness and lowering cardiovascular risk. Decades of studies in people support this evidence.

These new results add further nuance. For people seeking particular improvements in heart structure and function, swimming could offer 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 supports people with joint problems because it is low impact. This research indicates that it could also offer more extensive cardiac benefits.