Postural instability enhances motor responses to transcranial magnetic stimulation in humans.

Does the state of postural instability require a high hierarchical level of posture control? Electromyographic (EMG) activity of leg muscles was recorded during transcranial magnetic stimulation (TMS) of the motor cortex and electrical stimulation of the tibial nerve (H-reflex) in healthy subjects standing on a rigid floor and on a rocking platform. In the soleus muscle, TMS-evoked EMG responses increased considerably (2.2+/-1.1 times) when balancing on the rocking platform, whilst the H-reflex tended to decrease. The effect of support instability was specific to the muscles participating in the posture control. The results suggest that postural instability might change the state and the role of the motor cortex in equilibrium maintenance.