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Sex Differences In Neuromuscular Fatigue Effects On Intermuscular Control Patterns In Leg Extensors

Muscles in females tend to be less fatigable than in males. This may be because females have a higher proportion of Type I muscle fibers. It is also possible that females could employ different synergistic activation patterns during sustained fatiguing contractions. Sex differences in intermuscular control pattern changes following fatigue have not yet been investigated. PURPOSE: To investigate differences in VMO-VL activation level changes pre-, during and post-fatigue between females and males. METHOD: Five healthy females and 5 healthy males performed 5 trials of step-up before and after a sustained fatiguing isometric leg extension task of 20% maximal voluntary contraction (MVC). VMO and VL EMG signals were recorded and normalized to percent maximum. RESULTS: Males took longer to perform the step-up than females (P< 0.05), but the speeds did not change with fatigue. With sexes pooled, the activation levels of both the VMO and the VL during step-up were lowered following fatigue (prevs. post-fatigue, VMO: 21.2% vs. 16.7% EMGmax, VL: 28.3% vs. 21.7% EMGmax; both P<0.05). Normalized pre-fatigue VL activation levels were lower in males than in females (11.6% vs. 45.0% respectively, P<0.05), whereas pre-fatigue VMO levels were similar between the sexes (males: 13.1% vs. females: 29.4% EMGmax). After fatigue, there was no difference in VMO and VL activation levels between the sexes, indicating greater fatigue in the VL relative to the VMO in females (activation %Post %Pre, female VL vs. VMO: -11% vs. -5%, P<0.05). In males, the VMO and VL fatigued to a similar degree (VL vs. VMO: -2.5% vs. -3.3%). During the sustained isometric fatiguing contraction, the female VL tended to be activated to a greater degree than the VMO, whereas males activated both muscles more equally (VL/VMO activation ratio, 1.55 vs. 0.86, females vs. males, p=0.084). CONCLUSION: Our results show that males and females exhibit different relative VMO-VL neuromuscular fatigue patterns. Females tend to rely more on the VL than the VMO during fatigue while males tend to use both muscles equally.