Potentiating and fatiguing cortical reactions in a voluntary fatigue test of a human hand muscle

Fatigue-associated changes in the excitability of central motor mechanisms were investigated using transcranial magnetic stimulation (TMS) of the motor cortex. Test stimuli were applied before, during and after a voluntary fatigue test of the first dorsal interosseus muscle (FDI). Subjects were required to maintain 50% of their maximum voluntary force (MVC) for at least 2 min (1/2-MVC test) and electromyographic (EMG) reactions of FDI were measured with surface electrodes. Prior to the test, TMS pulses of 70% maximum output (about 1.4 T) produced muscle-evoked potentials (MEPs) of widely different amplitudes in different subjects, ranging from 13% to 55% of the maximum compound action potential (M-wave) evoked by ulnar nerve stimulation. During the test, MEPs of all subjects showed a potentiation; this effect was markedly greater in subjects with a small initial MEP. After the test, the differential degrees of contraction-evoked potentiation still influenced the MEP amplitudes; small pre-test MEPs showed a post-test net potentiation and larger pre-test MEPs showed a net post-test depression. The results underline that the net outcome of motor activation on motor cortex excitability, as studied with TMS, depends on a complex balance of fatiguing and potentiating effects.