CONTROL OF IA AFFERENT INPUT TO TRICEPS SURAE (SOLEUS) LOCOMOTOR NUCLEUS PRECEDES AGONIST MUSCLE ACTIVATION DURING GAIT

Modulation of the triceps surae (soleus) II-reflex was investigated du ring treadmill walking to elucidate the temporal relationship between reflex facilitation and initiation of agonist muscle activation. Const ant effective stimulation of the tibial nerve evoked the II-reflex. Co ntact foot switches monitored phases of the gait cycle. H-reflex chang es were assessed at predetermined intervals across a window that antic ipated the onset of soleus muscle electromyographic (EMG) activation. In five of six healthy subjects, multiphase statistical models reveale d a 'breakpoint', reflecting a significant rise in H-reflex amplitude, beginning with stimuli delivered 144.1, 142.9, 148.8, 186.4 and 140.5 ms, respectively, prior to mean onset of EMG activity in the soleus m uscle. In the sixth subject, no breakpoint was identified although the H-reflex amplitude gradually increased across the analysis window. Th ese findings suggest that facilitation of the H-reflex is established well in advance of the descending corticomotoneuronal command to recru it spinal motoneurons in the motor act (i.e. prior to threshold spinal motoneuron depolarization). It is argued that a central programme con trolling afferent input to the locomotor nuclei is part of the 'prepar atory set' which precedes execution of the motor act.