Reciprocal inhibition in the forearm during voluntary contraction and thinking about movement

The regulatory mechanism for the interaction of agonist and antagonist muscles is not still clear. In these studies we investigated reciprocal inhibition during voluntary contraction and thinking about movement by using transcranial magnetic stimulation (TMS) and the H reflex technique. Magnetic stimuli, with intensities 10% above threshold, were delivered at the scalp under 3 experimental conditions; at rest, during voluntary isometric weak contraction and during thinking about movement. Surface EMG responses were taken from extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles in 14 normal subjects. Inhibitions of antagonist muscles induced by thinking about agonist movement occurred in 8 ECR and 10 FCR muscles, but most of them were not statistically significant. H reflexes were obtained from the FCR muscles in 10 normal subjects under the same conditions. Voluntary wrist extension and thinking about wrist extension produced inhibition of the H reflex in the FCR muscles. Reciprocal inhibition of antagonist muscles may occur at the cortical and spinal cord levels. The excitability of motoneurons has effects on the H reflex amplitude. Motor evoked potential (MEP) response to TMS might be affected by the combination of input from the motor cortex and the spinal cord. TMS may evoke simultaneous facilitatory and inhibitory volleys to antagonist muscles. This may explain why the inhibitions in the TMS study were weaker than those in the H reflex study.