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1 Prince of Wales Medical Research Institute, Sydney, New South Wales, Australia
* To whom correspondence should be addressed. E-mail: p.martin{at}student.unsw.edu.au.
This study investigated transmission of corticospinal output through motoneurons over a wide range of voluntary contraction strengths in humans. During voluntary contraction of biceps brachii, motor evoked potentials (MEPs) to transcranial magnetic stimulation of the motor cortex grow up to ~50% maximal force and then decrease. To determine whether the decrease reflects events at a cortical or spinal level, responses to stimulation of the cortex and corticospinal tract (cervicomedullary motor evoked potentials, CMEPs) as well as maximal M-waves (Mmax) were recorded during strong contractions at 50% to 100% maximum. In biceps and brachioradialis, MEPs and CMEPs (normalized to Mmax) evoked by strong stimuli decreased during strong elbow flexions. Responses were largest during contractions at 75% maximum and both potentials decreased by ~25% during maximal efforts (P<0.001). Reductions were smaller with weaker stimuli, but were again similar for MEPs and CMEPs. Thus, the reduction in MEPs during strong voluntary contractions can be accounted for by a reduced responsiveness of the motoneuron pool to stimulation. During strong contractions of the first dorsal interosseous, a muscle which increases voluntary force largely by frequency modulation, MEPs declined more than in either elbow flexor muscle (35%, P<0.001). This suggests that motoneuron firing rates are important determinants of evoked output from the motoneuron pool. However, motor cortical output does not appear to be limited at high contraction strengths.
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