EFFECT OF CONTRACTION STRENGTH ON RESPONSES IN BICEPS BRACHII AND ADDUCTOR POLLICIS TO TRANSCRANIAL MAGNETIC STIMULATION

The sizes of the motor-evoked potentials (MEPs) and the durations of t he silent periods after transcranial magnetic stimulation were examine d in biceps brachii, brachioradialis and adductor pollicis in human su bjects. Stimuli of a wide range of intensities were given during volun tary contractions producing 0-75% of maximal force (maximal voluntary contraction, MVC). In adductor pollicis, MEPs increased in size with s timulus intensity and with weak voluntary contractions (5% MVC), but d id not grow larger with stronger contractions. In the elbow flexors, M EPs grew Little with stimulus intensity, but increased in size with co ntractions of up to 50% of maximal. In contrast, the duration of the s ilent period showed similar changes in the three muscles. In each musc le it increased with stimulus intensity but was unaffected by changes in contraction strength. Comparison of the responses evoked in biceps brachii by focal stimulation over the contralateral motor cortex with those evoked by stimulation with a round magnetic coil over the vertex suggests an excitatory contribution from the ipsilateral cortex durin g strong voluntary contractions.