Stimulus-response characteristics of motor evoked potentials and silent periods in proximal and distal upper-extremity muscles.

To compare stimulus-response characteristics of both motor evoked potentials (MEP) and silent periods (SP) induced by transcranial magnetic stimulation (TMS) in proximal and distal upper-extremity muscles.Stimulus-response curves of MEPs and SPs were obtained from the biceps brachii (BB) and abductor digiti minimi (ADM) muscles in 15 healthy participants. A nonlinear mixed model was used to fit the stimulus-response curves to a sigmoid Boltzmann function.Small residuals of the function were found for MEPs and SPs in both muscles. Higher maximal MEP amplitudes were found for the BB compared to the ADM (p<0.01). The active motor threshold to obtain a SP was less for the ADM compared to the BB (p<0.01). The slope parameter of the function of the SP duration was steeper and more variable in the ADM than in the BB (p<0.01). For the MEP amplitude no difference in active motor threshold and slope of the function was found between both muscles.Excitatory (MEP) and inhibitory (SP) effects of TMS differ between proximal arm and distal hand muscles in healthy participants. The adequate fit of our model suggests that this model can be used to study between and within subject changes in future studies.