It is no surprise that exercise takes a toll on the heart. The heart is key to the cardiovascular system as it is responsible for pumping blood throughout the body. During exercise, the heart is exerted since the muscles being used are hungry for oxygen and nutrients, demanding more blood. However, a recent study by Phillips et al. in the Clinical Journal of Sport Medicine suggests that prolonged strenuous exercise (PSE) such as marathon running may do more than just condition the heart itself. By impairing the heart’s ability to contract and relax, PSE is thought to induce cardiac fatigue by causing a reduction in left ventricular (LV) systolic and diastolic function.
A few trends motivate current research on the relationship between PSE and cardiac functioning: one is that ultramarathons have become increasingly popular over recent years, and another is the increase in female participation. Ultramarathons are races that consist of any distance longer than a typical marathon, 42.2 kilometers (26.2 miles). In addition, ultramarathon winners tend to be older than marathon winners on average. Without a doubt, studies have proven that those who exercise live longer, healthier lives than those who do not; but is more always better? Data has also shown that there are costly effects on the heart, leading researchers to question the long-term effects of cumulative high-volume endurance training, such as that associated with ultramarathons, over a lifetime.
Previous data collected by a team of researchers on half-ironman triathlon participants revealed that men experience greater reductions in Left Ventricular (LV) systolic function than women. This current study conducted by the same researchers from the University of British Columbia’s Cardiovascular Physiology and Rehabilitation Lab further investigated whether sex influences cardiac function in response to significant physiological stress. A second objective was to investigate the relationship between training experience and acute cardiac response to PSE. Based on the prior study, the researchers hypothesized that men would demonstrate greater evidence of cardiac fatigue post-exercise than women.
The field study took place at the Fat Dog 100 Ultramarathon Mountain Trail Running Race in Canada, examining 13 women and 21 men. Each participant underwent baseline testing the day before the race. It included height, mass, blood pressure (BP), heart rate (HR), arterial compliance-the heart’s ability to stretch under pressure, cardiovagal baroreflex sensitivity (BRS)—how the heart regulates acute changes in blood pressure, and echocardiography— a sonogram of the heart that provided information such as size, shape, and amount of pumping. Only 25 individuals (8 women and 17 men) completed the study, surpassing the minimum running distance of 50 km. Immediately after the race HR, BP, body mass, and echocardiography were re-measured. In order to assess LV function, a portable ultrasound system with simultaneous electrocardiogram (ECG) was used to evaluate regional myocardial function.
Contrary to the original hypothesis, which predicted that men and women would differ in heart functioning following exercise, the only differences found between sexes were in baseline cardiac structures and global function. Men had larger LV diameters and volumes, cardiac output (CO), and lower total peripheral resistance than women. However, females’ peak late transmitral flow velocity and estimated LV filling pressures were much greater than those of males.
Although males and females differ in heart structure and standard function, this seemed to make no significant impact on the effects of exercise-induced cardiac fatigue, implying that sex does not influence cardiac function under prolonged stress. The data revealed that there were no sex differences among the changes in BP, HR, and BRS after the race; for males and females alike, BP was not significantly reduced, HR was significantly higher, while BRS was reduced. Cardiac dimensions and volumes were also altered, while ejection fraction (EF) and CO were not significantly different. A multitude of other effects on the heart did not differ when comparing men and women.
While gender did not impact cardiac fatigue, running experience did. Researchers found that the greater baseline arterial compliance, the greater the elasticity and changes in septal and lateral tissue velocities, resulting in less cardiac strain and detriment in the individual’s cardiac function after exercise. More experienced runners— those who have ran more ultramarathons and had faster race paces— had increased arterial compliance because of more endurance training, and thus, had lower reductions in longitudinal strain and systolic function than less experienced runners.
Acute cardiac response following the ultramarathon was not impacted by gender, differing from what was observed in triathletes. However, this difference could possibly be attributed to the differences in intensity and duration of the two competitions, making the results incomparable. Running intensity was estimated to be higher, 80%, for the triathlon with a duration of 6 hours, while intensity during the ultramarathon, lower, at about 50%, but for a much longer duration of 29 hours. It is also hypothesized that the extreme mountain terrain of the Fat Dog 100 may have reduced cardiovascular fatigue by instead causing intense neuromuscular fatigue, perhaps lowering exercise intensity. Differing experimental conditions must be considered in attempting to explain exercise-induced cardiac fatigue, or conversely, the lack of such occurrence.
However, there are limitations to this study. Regardless of the recent increase in female ultramarathon participation, there were still only a small number of women that could be assessed. Also, the study could not control fitness level in comparing men and women. This poses a problem since it was shown that more experienced runners undergo less longitudinal heart strain, affecting the extent of cardiac fatigue. In addition, sodium supplementation, which increases plasma volume at rest in order to reduce physiological strain in heat, was not controlled for either, and perhaps improved exercise capacity.
The acute effects on cardiac health and LV function were observed immediately following PSE. It is clear that the heart’s functioning is impaired after it has been worked during cardiovascular exercise, but it is still uncertain if sex has any influence on the extent of such damage. With increased participation in ultraendurance events regardless of sex, further research is necessary to investigate the risks associated with endurance training over the course of a lifetime.
1 Du Toit, G., Roberts, G., Sayre, P. H., Bahnson, H. T., Radulovic, S., Santos, A. F., … & Lack, G. (2015). Randomized trial of peanut consumption in infants at risk for peanut allergy. New England Journal of Medicine, 372(9), 803‐813.