title: Rates of performance loss and neuromuscular activity in men and women during cycling: evidence for a common metabolic basis of muscle fatigue.
abstract: The durations that muscular force and power outputs can be sustained until failure fall predictably on an exponential decline between an individual's 3-s burst maximum to the maximum performance they can sustain aerobically. The exponential time constants describing these rates of performance loss are similar across individuals, suggesting that a common metabolically based mechanism governs muscle fatigue; however, these conclusions come from studies mainly on men. To test whether the same physiological understanding can be applied to women, we compared the performance-duration relationships and neuromuscular activity between seven men [23.3 ± 1.9 (SD) yr] and seven women (21.7 ± 1.8 yr) from multiple exhaustive bouts of cycle ergometry. Each subject performed trials to obtain the peak 3-s power output (Pmax), the mechanical power at the aerobic maximum (Paer), and 11-14 constant-load bouts eliciting failure between 3 and 300 s. Collectively, men and women performed 180 exhaustive bouts spanning an ~6-fold range of power outputs (118-1116 W) and an ~35-fold range of trial durations (8-283 s). Men generated 66% greater Pmax(956 ± 109 W vs. 632 ± 74 W) and 68% greater Paer(310 ± 47 W vs. 212 ± 15 W) than women. However, the metabolically based time constants describing the time course of performance loss were similar between men (0.020 ± 0.003/s) and women (0.021 ± 0.003/s). Additionally, the fatigue-induced increases in neuromuscular activity did not differ between the sexes when compared relative to the pedal forces at PaerThese data suggest that muscle fatigue during short-duration dynamic exercise has a common metabolically based mechanism determined by the extent that ATP is resynthesized by anaerobic metabolism.
  
  
  
  Although men and women differed considerably in their absolute cycling performances, there was no sex difference in the metabolically based exponential time constant that described the performance-duration relationship. Similarly, the fatigue-induced increases in neuromuscular activity were not different between the sexes when compared from a metabolic perspective. These data suggest that men and women have similar rate-limiting mechanisms for short-duration dynamic exercise that are determined by the extent the exercise is supported by anaerobic metabolism.
authors.affiliation: Biomechanics Laboratory, Departments of Health and Human Performance and Organismal Biology and Ecology, University of Montana, Missoula, Montana; and christopher.sundberg@marquette.edu.|Neuromuscular Physiology Laboratory, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin.
authors.forename: Christopher W
authors.identifier: 0000-0002-6709-4078
authors.initials: CW
authors.lastname: Sundberg
authors.affiliation: Neuromuscular Physiology Laboratory, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin.
authors.forename: Sandra K
authors.initials: SK
authors.lastname: Hunter
authors.affiliation: Biomechanics Laboratory, Departments of Health and Human Performance and Organismal Biology and Ecology, University of Montana, Missoula, Montana; and.
authors.forename: Matthew W
authors.initials: MW
authors.lastname: Bundle
mesh_terms: D000328:Adult; D001642:Bicycling / Q000502:physiology*; D004576:Electromyography / Q000379:methods; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010807:Physical Endurance / Q000502:physiology; D011597:Psychomotor Performance / Q000502:physiology*; D013997:Time Factors; D055815:Young Adult
keywords: critical power; metabolism; performance-duration relationship; sex differences; skeletal muscle