Contact forces evoked by transcranial magnetic stimulation of the motor cortex in a multi-finger grasp.

Transcranial magnetic stimulation (TMS) is a tool of choice to study the functionality of the corticospinal pathway. In this study, we used single-pulse TMS at different intensities and during different levels of grasping force, to stimulate the hand area of the left primary motor cortex (M1). We measured, the TMS-evoked forces, or motor evoked forces (MEFs) in a multi-fingered three-point grasp in addition of the conventional motor evoked potentials (MEPs) from the right forearm and intrinsic hand muscles. This paper aims at presenting the viability of this innovative approach and some preliminary results. The timing (i.e., latencies and peak times), amplitudes and directions of the MEF were analyzed. We found that the TMS evoked synergistic increases of the force magnitudes, akin to those observed when participants voluntarily increased the grip force. The MEF sizes and MEP amplitudes increased with TMS intensity in most cases. The grip force (which measures the overall force involved in the grasp) and the net force (which measures the net effect of all contact forces exerted on the object) seem to be differently affected by single TMS pulses of the motor cortex.