Effects of Task and Task Persistence on Magnetic Motor-Evoked Potentials

William J. Triggs, Sanjay Yathiraj, Michael S. Young, and Fabian Rossi

Published: 11 March 1998

Some cortical motoneurons may be more active during independent finger movement than during synergistic flexion of the fingers in a power grip. This possibility has been tested in humans using transcranial magnetic stimulation (TMS), but results have been conflicting. We attempted to resolve this controversy by testing the hypothesis that magnetic motor-evoked potentials (MEPs) are affected by whether an isometric contraction task has been recently initiated, or is simply sustained. Repeated-measures analysis of variance of mean MEP amplitude using task (finger abduction vs. power grip) and task persistence (repetitive vs. sustained contraction) as grouping variables showed a significant main effect of task persistence, such that MEPs were larger when the finger-abduction and power-grip tasks were repeated rather than sustained. Furthermore, there was a significant interaction between task and task persistence, such that the effect of task persistence was greater for the finger-abduction task. These results indicate that repetitive isometric contraction may be associated with higher levels of cortical motoneuron activity than sustained isometric contraction. This effect is greater for finger-abduction than for power-grip tasks, consistent with a greater corticospinal role in independent finger movement.

Keywords: transcranial magnetic stimulation, motor evoked potentials, task persistence, independent finger movement