Crossed effects of muscle vibration on motor-evoked potentials

Objectives: Muscle vibration (MV) to a forearm muscle augments motor-evoked potentials (MEPs) following transcranial magnetic stimulation (TMS) and the underlying mechanism involves cortical structures. Although MV-induced cortical activation is bilateral, the effects of MV on MEPs in contralateral muscles have not been investigated. Methods: Low-amplitude MV (80 Hz, amplitude 0.5 mm, duration 4 s), subthreshold for the tonic vibration reflex, was applied to the right extensor carpi radialis muscle (ECR). MEPs were elicited (0.5, 3 and 5 s after MV onset) in the left and right ECR and flexor carpi radialis muscle (FCR) by TMS (120% of threshold at rest) to the left and right hemisphere, respectively. Results: During MV of right ECR the left ECR revealed a slight non-significant augmentation of MEPs. In contrast, the left FCR showed a gradual depression of MEPs with ongoing MV and at 3 s the reduction of MEPs was significant. The time course of MEP changes in left FCR correlated with the facilitation of the vibrated right ECR. Post-vibration MEPs at 1 s after the offset of MV were still significantly decreased. Conclusions: The study demonstrates crossed effects of MV on motor cortex excitability, suggesting transcallosal MEP modulation.