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1 Discipline of Physiology, School of Molecular and Biomedical Science, University of Adelaide, Adelaide, South Australia, Australia
2 Prince of Wales Medical Research Institute, Sydney, New South Wales, Australia
3 Prince of Wales Medical Research Institute, Randwick, New South Wales, Australia
4 Prince of Wales Medical Research Institute, Sydney, Australia
5 Prince of Wales Medical Research Institute, Randwick, Sydney, New South Wales, Australia
* To whom correspondence should be addressed. E-mail: s.gandevia.jap{at}unsw.edu.au.
Force responses to transcranial magnetic stimulation of motor cortex (TMS) during exercise provide information about voluntary activation and contractile properties of the muscle. Here, TMS generated twitches and muscle relaxation during the TMS-evoked silent period were measured in fresh, heated, and fatigued muscle. Subjects performed isometric contractions of elbow flexors in two studies. Torque and EMG were recorded from elbow flexor and extensor muscles. One study (n=6) measured muscle contraction times and relaxation rates during brief maximal and submaximal contractions in fresh and fatigued muscle. Another study (n=7) aimed to 1) assess the reproducibility of muscle contractile properties during brief voluntary contractions in fresh muscle; 2) validate the technique for contractile properties in passively heated muscle; 3) apply the technique to study contractile properties during sustained maximal voluntary contractions. In both studies, muscle contractile properties during voluntary contractions were compared to the resting twitch evoked by motor nerve stimulation. Measurement of muscle contractile properties during voluntary contractions is reproducible in fresh muscle and reveals faster and slower muscle relaxation rates in heated and fatigued muscle, respectively. The technique is more sensitive to altered muscle state than the traditional motor nerve resting twitch. Use of TMS during sustained maximal contractions reveals slowing of muscle contraction and relaxation with different time courses and a decline in voluntary activation. Voluntary output from the motor cortex becomes insufficient to maintain complete activation of muscle although slowing of muscle contraction and relaxation indicates that lower motor unit firing rates are required for fusion of force.
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