Can supraspinal/central fatigue explain the lesser muscle endurance of men compared with women?

Can supraspinal/central fatigue explain the lesser muscle endurance of men compared with women? Several studies have shown that women can present with greater muscle endurance (less fatigability) compared with men (3, 5, 10, 12), although this is not always the case (4, 8). Most of these studies have focused on peripheral physiological mechanisms, mostly those within the muscle, to explain this effect of sex on fatigue. For example, reported differences in muscle fiber-type composition between men and women have been suggested to explain the lesser changes in electromyo-graphic (EMG) signal variables of women compared with men when performing a sustained contraction (11). Because the effect of sex can be eliminated under ischemic conditions (5, 12), a predominant role for muscle metabolism or perfusion in explaining the sex-related differences in muscle fatigue has been suggested. This is also consistent with studies showing no difference in muscle fatigue when men and women are matched for strength (6, 8), which implies similar levels of perfusion. However, sex differences in muscle fatigability can still exist even when subjects are matched for strength (9). Although the mechanism(s) responsible for the sex differences could still involve peripheral structures, differences in the descending drive from higher centers of the brain to motoneu-rons (9) could also influence how well men and women are able to sustain a required force output. The ability to maximally activate a muscle or muscle group under volition, also termed " voluntary activation, " can be assessed by delivering a pulse of electrical stimulation to a muscle (or the motor cortex via magnetic stimulation) during a maximal contraction. Any additional force produced by the stimulation above the voluntary force implies a submaximal level of voluntary activation. A decrease in the level of voluntary activation with sustained activity is referred to as " central fatigue. " The presence of significant central fatigue could lead to the early termination of a sustained contraction, even when the muscle(s) can still generate the required force. Central fatigue has been a research topic of increased interest in the past 10 yr. It appears to be task and muscle dependent (1, 2), which means that results of individual studies are difficult to generalize, and underlines the importance of testing hypotheses using multiple experimental protocols and muscle groups. Nevertheless, the presence of a possible impairment in the ability of the central nervous system to drive a muscle to produce maximal force …