Active Voice: Battle of the Sexes: Are Females More Resistant to Extreme Neuromuscular Fatigue?
By John Temesi, Ph.D., and Guillaume Millet, Ph.D.
John Temesi, Ph.D., is a postdoctoral fellow in the faculty of kinesiology at the University of Calgary. His research focuses on neuromuscular fatigue, especially central and supraspinal fatigue, related to locomotor exercise in both athletic and clinical populations.
Guillaume Millet, Ph.D., is professor in the faculty of kinesiology at the University of Calgary. His general research area investigates the physiological, neurophysiological and biomechanical factors associated with fatigue, both in extreme exercise (ultra-endurance, hypoxia) and patients (neuromuscular diseases, cancer). Guillaume is also an ultra-marathon runner.
This commentary presents Dr. Temesiís and Dr. Milletís views on the topic of a research article which they and other colleagues had published in the July 2015 issue of Medicine & Science in Sports & Exerciseģ (MSSE).
Ultra-endurance sports such as running, cycling and adventure racing are rapidly increasing in popularity. The great distances and extreme conditions in these events are uniquely capable of being used to understand the limits of human performance. With the rapid growth of ultra-endurance sports there has also been a huge uptick in female participation. To date, ultra-endurance research has focused exclusively on male participants, resulting in a large knowledge gap pertaining to the physiology and performance of females in such demanding conditions.
Our recent study, published in MSSE, is the first to directly compare neuromuscular fatigue in the knee extensors and plantar flexors, both functionally important muscle groups for locomotion, in males and females. There are reports that females may be as good or even better performers than males at extremely long distances. In fact, women have won or placed in the top three overall in such extreme events as the Badwater ultramarathon and Western States Endurance Run. One possibility that has been suggested is that females demonstrate less fatigue than males, and thus can perform better.
In this study, the neuromuscular function of 10 male and 10 female ultra-trail runners, were evaluated before and after they completed a 110-km ultra-trail running race with almost 6000 m in positive elevation change. The subjects were matched by finishing time calculated as a percentage of the winning time of their sex. The major neuromuscular parameters that were assessed were the changes in maximal voluntary torque (i.e. indicating the amount of overall fatigue), torques evoked by nerve stimulation in the relaxed muscle state (i.e. indicating fatigue within the muscle) and maximal voluntary activation assessed by nerve and transcranial magnetic stimulations. We also assessed electromyography, including the excitability and inhibition of the corticospinal tract to the knee extensors.
After the trail running race, all subjects demonstrated substantial general neuromuscular fatigue in both the knee extensors and plantar flexors (i.e. decrease in maximal voluntary torque). Although overall fatigue in the knee extensors was greater in males than females, the source of this greater fatigue could not be clearly isolated. In the plantar flexors, males also demonstrated greater muscle fatigue than females. In both muscle groups, the magnitude of fatigue within the central nervous system and brain was similar for both sexes. Moreover, similar changes occurred after the event in males and females with regard to corticospinal excitability and inhibition of the knee extensors.
The greater general neuromuscular fatigue in the knee extensors and greater muscle fatigue in the plantar flexors for males suggests that differential fatigue between the sexes occurs solely at the muscle level. These differences may partly explain the reports of better performance by females in extremely long duration running races, particularly as the race distance increases.