Effects of transcranial magnetic stimulation on muscle activation patterns and joint kinematics within a two-joint motor synergy

We studied the effects of transcranial magnetic stimulation (TMS) on muscle activation patterns and joint kinematics during oscillatory movements of a joint within a two-joint (elbow-wrist) synergy. Healthy young adults (N=8) performed cyclic movements in one of the joints while the other joint was or was not involved into a postural task. TMS was applied unexpectedly at different phases of the movement cycle. TMS-induced motor evoked potentials (MEPs) showed a modulation within the movement cycle, partly related to changes in the background muscle activity. After normalization by the background activity, MEPs were higher in conditions involving maintenance of a posture by one of the joints. MEPs in a flexor-extensor muscle pair could show peaks at the same phase within the movement cycle while background EMG peaks were out of phase. TMS induced a phase shift in the ongoing movement, and the phase shift changed over the next cycles. It stabilized by the fourth cycle and could show both phase advances and phase delays, the latter dominated. The findings are interpreted as supporting two hypotheses: (1) TMS can activate cortical neurons interposed in a transcortical long-latency loop leading to pre-programmed reactions to perturbations; and (2) apparent effects of TMS on the timing of an ongoing cyclic movement are defined by an interaction between its effects on a central 'clock' and those induced by the peripheral mechanical response.