Estimation of voluntary elicited motor neuron discharge using a peripheral nerve collision technique at different contraction strengths

Motor evoked potentials (MEP) are facilitated when the cortical stimulus is given during an ongoing voluntary contraction of the target muscle (Arányi et al., 1998; Hess et al., 1987). During voluntary contractions, a greater pool of spinal motor neurons (MN) are closer to their firing threshold, and are therefore brought to discharge by the cortical magnetic stimulus. The pool of MN just below firing threshold has been termed "subliminal fringe" (Denny-Brown and Sherrington, 1928). A motor evoked potential obtained during a voluntary contraction is therefore composed of those motor units discharging by voluntary activation plus those near-threshold "fringe" motor units that were additionally recruited by the brain stimulus.

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