Suppression of motor evoked potentials in biceps brachii preceding pronator contraction.

Reciprocal control of antagonists is essential for coordinated limb movement. While Ia afferent dependent reciprocal inhibition has been extensively studied, reports of the control of antagonists during preparation for a motor action are limited. It has been demonstrated that corticomotor (CM) excitability of antagonists is suppressed prior to wrist extension/flexion suggesting the existence of a pre-contraction cortical control mechanism for distal upper limb antagonists. It is unknown whether pre-contraction suppression is evident in the control of proximal upper limb antagonists. Here we used transcranial magnetic stimulation and a rhythmic motor task to assess pre-contraction changes in excitability of corticospinal pathways projecting to biceps brachii (BB), when BB was an agonist (forearm supinator) or an antagonist. We found a suppression of motor evoked potential (MEP) amplitude in BB prior to pronator contraction and facilitation prior to BB contracting as a supinator. The extent of modulation was more profound as the agonist contraction approached. In contrast, there was no suppression evident in brachioradialis and triceps brachii under similar conditions indicating that pre-contraction suppression was specific to the antagonist BB. Our data in combination with published data from wrist muscles suggest that pre-contraction suppression of CM excitability may be a centrally induced mechanism to prevent antagonistic activity before Ia afferent dependent reciprocal inhibition is imposed. The importance of assessment of this inhibitory mechanism in neurologically impaired populations is discussed.