ABSTRACT
Skeletal muscle fatigue has been a topic of interest to physiologists since the turn of the twentieth century (Di Giulio et al., 2006). Specific examination of the role of sex in fatigue however, has occurred only in comparatively recent years, with the first study designed to systematically address this factor published in the 1970s (Petrofsky et al., 1975). This initial study evaluated fatigue of submaximal, isometric (40% of maximum volitional contraction (MVC)), handgrip exercise in a fairly large number of women (n = 83) of ages 19-65 years, and compared their responses to those
recorded in a similar population of men during an earlier study (Petrofsky and Lind, 1975). These investigators found that although the women were weaker than the men, they were less fatigable, a finding they reported as “unexpected”. Despite the novel findings of this seminal paper, evaluation of the role of sex in muscle fatigue was sporadic and limited over the ensuing years. This might have been due, in part, to the persistence of baseless societal attitudes that women were unable to tolerate the stress of heavy physical exertion and were unfit for participation in sports (Thein and Thein, 1996). Beginning in the mid-1980s, study of differences in physical performance between the sexes began to increase. Coincident with the increased focus on female performance were the debut of the women’s marathon in the Olympic Games and the National Collegiate Athletic Association taking over the administration of intercollegiate athletics for both men and women in the United States. As the number of investigations comparing skeletal muscle fatigue in men and women has increased, so have the attempts to isolate the mechanisms that could potentially contribute to any sex-related differences in fatigability. Despite the proliferation of research in this area, the role of sex in fatigue remains unclear. Many studies have confirmed Petrofsky et al.’s findings of greater fatigue resistance in women, but others have not (see Table 6.1). Furthermore, a consistent mechanism for observed differences in fatigue between men and women has not been established. These discrepancies are likely the result of the different experimental protocols used to study fatigue, combined with the complex, multi-factorial nature of muscle fatigue itself. A requirement for any study of muscle fatigue is that the phenomenon be defined so that it can be measured. A myriad of operational definitions of fatigue have been used (for recent reviews, cf. Barry and Enoka, 2007; Enoka and Duchateau, 2008), with components ranging from the perceived effort to spectral changes in electrical activity of the contracting muscles. Two different definitions have been employed in the majority of the studies of sex and muscle fatigue. The first is the transient decline in maximum force-generating capacity of the muscle, associated with recent muscle activity (Bigland-Ritchie and Woods, 1984; Kent-Braun et al., 2002; Pincivero et al., 2003; Russ et al., 2008). Typically, the MVC of the muscle or muscle group being studied is used as the measure of maximum force-generating capacity. Sometimes, electrical stimulation is superimposed during the MVC to assess central activation (see the section, “Central drive”). In these studies, the principal measure is the decline in force, relative to the pre-fatigue state, during and after the fatiguing activity (Bigland-Ritchie and Woods, 1984). The second frequently used definition is the “failure to achieve or maintain the expected force” (Edwards et al., 1977). In such instances, the main outcome is the time at which such failure occurs, often referred to as the time to task failure. In
some instances, this definition of fatigue is also referred to as “muscular endurance”, with greater endurance associated with a greater time to task failure. In each case, the typical experiment involves measuring various parameters thought to represent potential mechanisms of fatigue in men and women before and after a specific exercise task. Baseline differences and exercise-associated changes in the different parameters between men and women are then related to any observed sex-related differences in fatigue. Some studies combine both definitions by periodically assessing MVC during a submaximal exercise task (Cheng et al., 2003). The study of fatigue is made more difficult by the fact that the extent of fatigue and the mechanisms driving it vary with the nature of the fatiguing task or exercise. Thus, the type of fatiguing exercise chosen, the selected method of measuring fatigue and the physical fitness of the subjects tested are likely to influence the outcome of any study aimed at determining the influence of sex on muscle fatigue. This task-specific nature of fatigue, combined with continued use of differing definitions of fatigue, clearly contributes to the discrepancies in the literature. Recently, it has been suggested that studies using the second definition of fatigue use the term “task failure” rather than “fatigue” (Barry and Enoka, 2007). The same group advocates the use of task failure studies, rather than fatigue, as most functional tasks do not involve maximal contractions.
