Somatosensory Inputs Induced by Passive Movement Facilitate Primary Motor Cortex Excitability Depending on the Interstimulus Interval, Movement Velocity, and Joint Angle.

Somatosensory inputs affect primary motor cortex (M1) excitability; however, the effect of movement-induced somatosensory inputs on M1 excitability is unknown. This study examined whether M1 excitability is modulated by somatosensory inputs with passive movement in 29 healthy subjects. Motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation (TMS) were recorded from the first dorsal interosseous (FDI) muscle (Experiment 1). M- and F-waves were measured from the FDI muscle (Experiment 2). Passive movements of the index finger were performed in the adduction direction. TMS pulses were preceded by starting passive movements with interstimulus intervals (ISIs) of 30, 60, 90, 120, 150, 180, and 210 ms. TMS or electrical stimulation was performed in the midrange of the metacarpophalangeal joint during passive movements. MEPs were significantly facilitated at 90, 120, and 150 ms (p < 0.05). No M- or F-wave changes were observed for any ISI. In addition, we investigated whether MEP changes were dependent on passive movement velocity and joint angle. Passive movement was performed at two movement velocities (Experiment 3) or joint angles (Experiment 4). MEP facilitation was observed depending on the movement velocities or joint angles. These experiments demonstrated that somatosensory inputs induced by passive movements facilitated M1 excitability depending on the ISIs, passive movement velocity, and joint angle.