title: Sex differences in fatigue resistance are muscle group dependent.
abstract: OBJECTIVE
  Women are often reported to be generally more resistant to fatigue than men for relative-intensity tasks. This has been observed repeatedly for elbow flexors, whereas at the ankle, sex differences appear less robust, suggesting localized rather than systemic influences. Thus, the purpose of this study was to examine sex differences in fatigue resistance at muscle groups in a single cohort and which factors, if any, predict endurance time.
  
  
  METHODS
  Thirty-two young adults (age = 19-44 yr, 16 women) performed sustained isometric contractions at 50% maximum voluntary isometric contraction to failure for elbow flexion and ankle dorsiflexion. Pain, exertion, and muscle EMG were assessed throughout. Self-reported baseline activity was measured using the International Physical Activity Questionnaire.
  
  
  RESULTS
  Women were significantly more resistant to fatigue than men at the elbow (112.3 ± 6.2 vs 80.3 ± 5.8 s, P = 0.001) but not at the ankle (140.6 ± 10.7 vs 129.2 ± 10.5 s, P = 0.45). Peak torque was greater in men than that in women (P < 0.0001) at the ankle (45.0 ± 1.7 vs 30.1 ± 1.0 N·m) and at the elbow (75.7 ± 3.1 vs 34.4 ± 2.2 N·m). Peak torque was significantly related to endurance time at the elbow (R2= 0.30) but not at the ankle (R2 = 0.03). Peak pain, rate of pain increase, peak exertion, EMG, and baseline physical activity did not differ between sexes.
  
  
  CONCLUSIONS
  Sex differences in fatigue resistance are muscle group specific. Women were more fatigue resistant at the elbow but not at the ankle during a sustained isometric contraction. Further, factors that may contribute to fatigue resistance for one muscle group (e.g., sex, peak torque) may not be critical at another.
authors.affiliation: Graduate Program in Physical Therapy and Rehabilitation Science, The University of Iowa, Iowa City, IA 52242-1190, USA.
authors.forename: Keith G
authors.initials: KG
authors.lastname: Avin
authors.forename: Maureen R
authors.initials: MR
authors.lastname: Naughton
authors.forename: Brett W
authors.initials: BW
authors.lastname: Ford
authors.forename: Haley E
authors.initials: HE
authors.lastname: Moore
authors.forename: Maya N
authors.initials: MN
authors.lastname: Monitto-Webber
authors.forename: Amy M
authors.initials: AM
authors.lastname: Stark
authors.forename: A John
authors.initials: AJ
authors.lastname: Gentile
authors.forename: Laura A Frey
authors.initials: LA
authors.lastname: Law
mesh_terms: D000328:Adult; D000842:Ankle / Q000502:physiology; D015331:Cohort Studies; D004550:Elbow / Q000502:physiology; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D009043:Motor Activity / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D010146:Pain / Q000503:physiopathology; D010807:Physical Endurance / Q000502:physiology*; D012737:Sex Factors; D019415:Torque; D055815:Young Adult