Task-specific sex differences in muscle fatigue: is there a common underlying cause?

Dear Editor-in-Chief: We welcomed and much appreciated Sandra Hunter’s recent review, ‘‘Sex Differences and Mechanisms of TaskSpecific Muscle Fatigue’’ (8), that was published in the July 2009 issue of Exercise and Sport Sciences Reviews (ESSR). Not only has she shed more light on the general topic of muscle-functional sex differences but managed to drive home the message that women’s fatigue resistance is ‘‘task specific’’ rather than just a uniform generality. Hunter did not suggest an all-encompassing explanation for the various observed differences and concentrated on the specifics of each comparison. The review would be complete as such, but the author chose to take the definite position that ‘‘there is no one global cause for the sex difference in muscle fatigue. ’’ Although, indeed, there currently is no established ‘‘unified theory’’ to explain all observations under a single ‘‘umbrella,’’ we would like to propose a common underlying reason for the diverse observations. As pediatric exercise physiologists, we found it striking that, had the term women been replaced by children, the review would need very little editing to remain factually correct. This is most remarkably true for the Figure 3 schema. Children’s lower relative strength, explosive force, glycolytic capacity, and short-term power, along with higher muscular endurance and fatigue resistance, are but a partial list of child-adult differences that can provisionally be explained by a single underlying reason. Namely, children are more limited in their capacity to recruit and use fast-contracting, Type II motor units. We first introduced this notion in a 2006 ESSR review of ‘‘child-adult differences in the recovery from high-intensity exercise’’ (3). We showed that children’s seemingly faster recovery was primarily due to lesser fatigue stemming from compromised maximal exertion, presumably due to children’s relative inability to maximally use all their motor units. This topic is in its infancy, but others have previously suggested the same (1,6) based on boys’ lower electromyography meanpower-frequency and muscle-fiber conduction velocity (6) and their blunted rate of torque development after pretension (1). Initial data from our laboratory are consistent with this notion, showing boys as having dimensionally corrected lower peak torque and rate of torque development with longer electromechanical delay (4). Remarkable for the issue at hand is that no statistically significant differences could be shown when the same variables were compared between women and girls (5). Thus, motor-unit recruitment seems more similar between women and children than between either group and men and seems to be a common factor distinguishing respective muscular performance and fatigue. As boys’ strength is not much different than that of girls throughout preadolescence and markedly diverges only thereafter (2), we believe that the male hormonal changes associated with puberty also are responsible for the observed sexand age-related muscle activation differences. It is noteworthy that Hicks et al. (7), whom Hunter cites in her review, felt already in 2001 that ‘‘one cannot ignore the possibility of a sex difference in neuromuscular activation as a contributor to the apparent differences in fatigue resistance between males and females. ’’ We believe Hunter’s review would have benefited from that insight in explaining the reported data.