The history of contraction of the wrist flexors can change cortical excitability

Voluntary contractions induce thixotropic changes in intrafusal muscle fibres and hence, by induction or removal of "slack", the background discharge and sensitivity of spindle endings to stretch is altered. This study assessed whether such changes also altered the "excitability" of the motor cortex. Eleven subjects performed a series of voluntary conditioning contractions of the wrist flexors designed to remove slack in the intrafusal fibres (contract and test at intermediate length, termed "contract-test") or to introduce slack (contract at long length and test at intermediate length, termed "contract-long"). Surface electromyographic recordings were made from one wrist flexor, flexor carpi radialis. Subjects relaxed after each contraction, and 10 s later a test stimulus was applied to elicit a tendon tap response, H-reflex, or motor-evoked potential (MEP) to transcranial magnetic stimulation in the flexor carpi radialis. Each of the three test stimuli was applied during 15 consecutive pairs of contractions ("contract-long" and "contract-test"). Three subjects repeated the protocol using transmastoid electrical stimulation as the test stimulus to evoke a cervicomedullary motor-evoked potential (CMEP). For the group of subjects, after conditioning contractions designed to induce slack there was a significant reduction in the amplitude of the tendon reflex, no significant change in the H-reflex, and a small but significant reduction in the amplitude of the MEP. In one subject the CMEP was significantly reduced, while it was unchanged in two others. In the absence of corresponding changes in the H-reflex (or CMEP), changes in the size of the response to motor cortical stimulation suggest that the level of motor cortical "excitability" changes according to naturally induced variations in the discharge of muscle spindle afferents.