People tend to pick their cardio based on what they enjoy. Some opt for running to chase pace and mileage, while others choose swimming for a workout that recruits the whole body. New findings indicate that these preferences may remodel the heart in distinct ways.
The data point to swimming producing more powerful and more wide-ranging changes within heart muscle, even when the training effort is matched.
“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
That different sports shape the body in different ways is a familiar idea. Among endurance athletes, the heart’s size and structure often differ in visible ways depending on the discipline.
Cyclists, rowers and skiers, for instance, can develop their own characteristic cardiac patterns.
Even so, researchers have had few head-to-head comparisons of specific activities carried out under tightly controlled conditions. That lack of direct testing has made it difficult to clarify where the differences come from.
“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 scientists worked with male Wistar rats and split them into three groups: one stayed sedentary, one trained on a treadmill, and one swam in a controlled tank.
The two training groups exercised for eight weeks, completing sessions of 60 minutes per day, five days per week.
Both programmes were set at roughly 75 percent of maximum oxygen capacity, so the intensity was kept comparable.
That point is important, because earlier research has often contrasted programmes with mismatched workloads.
Fitness improved in both groups
Clear improvements in fitness were seen in both exercise groups. Oxygen capacity rose in the trained rats, whereas it fell in the inactive group. The trained animals also achieved better results in endurance testing.
Up to this point, running and swimming appeared broadly alike, as both boosted general markers of fitness. However, more detailed assessment showed the picture was not the same.
Swimming enlarged the heart
The clearest split emerged in cardiac structure. Noticeable enlargement of the heart occurred only in the swimming group.
Compared with body mass, their hearts were heavier. The left ventricle-the chamber that pumps blood around the body-became both thicker and wider.
At the level of individual cells, heart muscle cells were bigger and showed more developed internal architecture.
These shifts were not observed in the runners. Their hearts looked much like those of the sedentary animals.
Stronger contraction in swimmers
Bigger is not automatically better, so the team examined how effectively the heart muscle could contract.
Muscle taken from swimmer hearts produced greater force, and it also contracted and relaxed more quickly. Together, those changes support more efficient pumping.
By contrast, running led to only modest gains in contraction speed, while swimming produced a far more pronounced effect.
Signals inside the heart
The researchers then looked for mechanisms that could explain the differences. They targeted a recognised pathway associated with healthy cardiac growth, involving proteins including PI3K, AKT, PTEN, mTOR and S6K1.
Elements of this signalling network were activated by both forms of training, but swimming drove a stronger response.
In the swimming group, PTEN levels fell more markedly, which removes a brake on growth. At the same time, AKT activity rose. Another crucial protein, S6K1, was activated only among the swimmers.
“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 team also analysed microRNAs-small molecules that influence how genes are expressed within cells.
Five specific microRNAs increased more strongly in swimmer hearts: miR-1, miR-21, miR-27a, miR-124 and miR-144.
These molecules are linked to blood vessel formation, responses to stress and regulation of inflammation. Their heightened activity suggests swimming prompts broader internal changes in the heart.
Why water changes things
The study does not identify a single definitive explanation for the gap between swimming and running, but it offers several plausible cues.
Swimming engages more muscle groups simultaneously, with both upper and lower limbs working against resistance. The body also stays horizontal, which alters venous return to the heart.
That posture increases how much blood is handled with each heartbeat. Water temperature could also contribute by changing cardiovascular load.
Combined, these factors may provide a stronger signal for the heart to adapt.
Limitations of the study
This work examined rats rather than humans. Because rats run using four limbs, the mechanics and physiological demands of running are not identical to those in people.
Swimming differs too: laboratory rats paddle in a simple, uniform manner, whereas human swimming can vary greatly by stroke, technique and intensity.
The intervention ran for eight weeks, and longer studies might reveal different patterns over time. The researchers also did not measure long-term health outcomes.
Implications for human health
Running remains clearly beneficial. It improves fitness and lowers cardiovascular risk, supported by decades of studies in humans.
The new results add detail to that picture. For those aiming to influence heart structure and function in particular ways, swimming may confer an edge.
“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 is already widely used by people with joint problems because it is low impact. This study suggests it may also deliver more substantial benefits for the heart itself.
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