Changes in corticospinal excitability in the reactions of forearm muscles in humans to vibration.

Transcranial magnetic stimulation was used to study changes in corticospinal excitability during vibration of the flexor and extensor muscles of the wrist in healthy humans. The ratios of muscle stimulation responses to activity levels in these muscles on contraction associated with vibration (the tonic vibratory reflex, TVR) and after vibration of antagonist muscles in isometric conditions (the antagonist vibratory reflex, AVR) were analyzed. The normalized muscle response in the wrist flexors was found to increase by 66% compared with threshold values in the direct vibratory response (TVR), by 75% in the relayed vibratory response (AVR), and by 18% on voluntary contraction. However, increases in the motor response in vibratory responses as compared with those on voluntary contraction did not reach significance, which contrasted with the responses in the forearm flexors. These results are discussed from the point of view that the motor cortex plays different roles in vibratory responses in the distal and proximal muscles.