Selective effect of repetitive hand movements on intracortical excitability.

Repetitive training of isolated movements induces local reorganization of the motor cortex. To investigate the effect of repetitive movements on intracortical inhibition and facilitation shortly after the movements, we performed paired-pulse transcranial magnetic stimulation in 12 healthy subjects by using interstimulus intervals of 2 and 8 ms, respectively. Motor evoked potentials (MEPs) were recorded simultaneously from the extensor carpi radialis (ECR) and the flexor carpi ulnaris muscle (FCU) following paired stimuli at rest or 1 s after repetitive (every 6 s) active or passive wrist extensions and compared with MEPs produced by single stimuli at rest. The active movements produced significantly larger MEPs in the ECR but not the FCU muscle. Passive movements had no effect. Intracortical facilitation increased significantly after movements in the ECR only. Intracortical inhibition, however, did not change significantly after repetitive movements in either muscle. These findings support the notion that intracortical inhibition and facilitation may be controlled independently. Further, these data suggest that shortly after repetitive movements the excitability of the primary motor cortex representing the agonist muscle is increased by interaction with intracortical facilitatory circuits rather than by inactivating inhibitory circuits.