Active Voice: Men and Women Recover Differently from Fatiguing Exercise - Let’s Stop Pretending They Don’t!
By Sandra K. Hunter, Ph.D., FACSM, and Jonathon Senefeld, Ph.D.
Sandra Hunter, Ph.D., FACSM, is a professor of exercise science in the Department of Physical Therapy, Marquette University in Milwaukee, Wisconsin. Dr. Hunter’s research is focused on understanding the mechanisms for sex and age differences in fatigability.
Jonathon Senefeld, Ph.D., is a postdoctoral research assistant in exercise science in the Department of Physical Therapy at Marquette University.
In the May 2018 issue of Medicine & Science in Sports & Exercise® (MSSE), Hunter, Senefeld and co-authors published a related research paper entitled “Sex Differences in Mechanisms of Recovery after Isometric and Dynamic Fatiguing Tasks.”
The limits of human performance in healthy men and women during athletic events and activities of daily living generally can be defined by fatigability of skeletal muscles due to mechanisms that originate along the motor pathway from the brain to the active muscles. Although men are typically stronger and more powerful, women are typically less fatigable than men during submaximal static and slow dynamic contractions of limb muscles performed at the same relative intensity (see the review by S.K. Hunter in November 2016 issue of MSSE). Emerging evidence, however, suggests that this difference in fatigability between men and women varies with the type of task performed, although the causes are relatively undefined. Fatiguing contractions of muscles followed by adequate recovery are necessary to induce appropriate physiological adaptations along the neuromuscular system. Hence, any sex-based differences in fatigability and recovery must be considered in training and rehabilitation regimes. Unfortunately, there is minimal information about potential differences between men and women in recovery from fatiguing exercise, leading to potentially unfounded assumptions by scientists and practitioners that recovery is similar between the sexes.
In our recent study, as reported in the May 2018 issue of MSSE, we highlighted the sex difference in fatigability and recovery of the quadriceps muscles for a one-minute, maximal-effort static contraction and repeated fast-dynamic contractions over 6 minutes in young, healthy men and women. We also determined the site(s) along the motor pathway that contributed to any sex differences in fatigability or recovery of the two tasks by evoking involuntary contractions with stimulation of the motor area of the brain and the quadriceps muscles. Both decreased neural drive from the brain and fatigue in the muscle contributed to fatigability of the quadriceps muscles after the static exercise, but fatigability after the dynamic exercise was due exclusively to fatigue within the quadriceps muscles.
An interesting finding was that maximal force of the quadriceps muscles of the women recovered more quickly than was the case for men. This was true regardless of whether the fatigue had been induced with our static or dynamic exercise protocols. Women had faster recovery than men in both the neural drive from the motor cortex and fatigue within the quadriceps muscles after the isometric fatiguing exercise. The faster recovery of the women after the dynamic fatiguing exercise was primarily due to factors within the quadriceps muscles. Our work highlights that there are fundamental differences in the recovery of muscle force and the involved mechanisms in men and women after fatiguing exercise of the lower limb.
This study serves as a reminder that the assumption that men and women respond similarly to exercise perturbations is erroneous—so let’s not pretend they do! Despite on-going efforts over the past 25 years to increase the inclusion of women in biomedical research by the National Institutes of Health (NIH) (see NIH policy in 2015, which makes explicit the need for scientists to address sex as a biological variable) there remains inadequate inclusion of women in exercise physiology and rehabilitation studies. Advances in the understanding of human performance during athletic events and activities of daily living in healthy and clinical populations are dependent on increasing the inclusion of women in research studies.