Resetting voluntary movement using peripheral nerve stimulation: influence of loading conditions and relative effectiveness.

The effect of peripheral nerve stimulation on voluntary rhythmic flexion-extension movements at the wrist was studied in nine normal volunteers, and the results compared with the effect of cortical stimulation on the same task. In the first part of the study, magnetic stimulation was given over the inner aspect of the right arm at levels which, at rest, resulted in a wrist flexion twitch of at least 10 degrees. We were able to confirm that this form of (peripheral-nerve) stimulation is an effective means of phase-resetting voluntary wrist movements. In addition, and unlike magnetic stimulation applied over the contralateral motor cortex, changes in the standing torque load, against which the subjects moved, had little influence on the effectiveness of this form of stimulation. Similarly, the amplitude and direction of the averaged first post-stimulus position peak ("P1"), previously identified as important determinants of the resetting induced by a cortical stimulus, were largely independent of the loading torque. In a second part to the study, we directly compared, for a constant loading torque, the resetting induced by magnetic cortical stimulation with that following magnetic stimulation of peripheral nerves. The relationship between the amplitude of P1 and the associated resetting index was identical for both forms of stimulation. Our observations indicate that magnetic stimulation of peripheral nerves is an effective means of resetting voluntary movement. It differs from magnetic cortical stimulation in that the effects of peripheral nerve stimulation are little altered by changes in loading torque. When differences in the size of P1 are allowed for, both peripheral nerve and cortical stimulation are equally effective means of resetting voluntary rhythmical movement.