Similarity in the dynamics of contralateral motor overflow through increasing frequency of movement in a single limb.

This study was designed to investigate the effects of increasing movement frequency of a single limb on the degree of similarity and coherence of the motor outflow in the non-active limb. Twelve young adults performed a series of unilateral hand-clapping tasks (horizontal and vertical in 25-s trials) while seated. Individuals began the movements at a frequency of 1 Hz for 5 s and were required to increase the movement frequency so as to reach their maximum movement frequency during the latter parts of the trial. Hand and finger kinematics and surface EMG of each arm were recorded. The results showed a progressive emergence of overflow muscle activity and involuntary motion in the non-active arm as the movement frequency of the unilateral action increased toward the upper frequency limits of voluntary movement. This ceiling occurred within the range of 6-7 Hz. Activity in the non-active limb emerged as the movement frequency requirements increased, irrespective of the direction of motion for the task (vertical, horizontal), hand used (preferred, non-preferred) or the auditory timing stimulus provided (metronome, no-metronome). The dynamics of the motor overflow in the non-active limb exhibited time- and frequency-dependent patterns similar to those of the active arm. Together, these results demonstrate that the high-frequency unilateral movements of one limb drives the emergence of motor outflow to the opposite limb with the motor output dynamics being produced across both limbs being progressively similar as movement speed increases.