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1 Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin, United States
2 Prince of Wales Medical Research Institute, Sydney, New South Wales, Australia
3 Prince of Wales Medical Research Institute, Sydney, New South Wales, Australia; Discipline of Physiology, The University of Adelaide, Adelaide, Australia
* To whom correspondence should be addressed. E-mail: sandra.hunter{at}marquette.edu.
Cortical stimulation was used to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women and 9 men performed 6 sustained MVCs (22-s duration each, separated by 10 s). Men were stronger than women (75.9 ± 9.2 Nm vs 42.7 ± 8.0 Nm) in control MVCs. Voluntary activation using cortical stimulation was similar for the men and women during the final control MVC (95.7 ± 3.0 % vs 93.3 ± 3.6%) and at the start of the fatiguing task. By the end of the 6 sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 ± 3 % of initial MVC) than the women (52 ± 9 %, P < 0.05). The superimposed twitch generated by motor cortex stimulation increased with fatigue but was similar for men and women during the fatiguing task and recovery MVCs. Voluntary activation, the area of the motor evoked potential (MEP, % Mmax) and silent period duration were similar for the men and women throughout the sustained MVCs. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions during the fatiguing task for the men than the women. Thus, the sex difference in fatigue is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.
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