Control of Ia afferent input to triceps surae (soleus) locomotor nucleus precedes agonist muscle activation during gait

Modulation of the triceps surae (soleus) H-reflex was investigated during treadmill walking to elucidate the temporal relationship between reflex facilitation and initiation of agonist muscle activation. Constant effective stimulation of the tibial nerve evoked the H-reflex. Contact foot switches monitored phases of the gait cycle. H-reflex changes were assessed at predetermined intervals across a window that anticipated the onset of soleus muscle electromyographic (EMG) activation. In five of six healthy subjects, multiphase statistical models revealed 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 muscle. In the sixth subject, no breakpoint was identified although the H-reflex amplitude gradually increased across the analysis window. These findings suggest that facilitation of the H-reflex is established well in advance of the descending corticomotoneuronal command to recruit spinal motoneurons in the motor act (i.e. prior to threshold spinal motoneuron depolarization). It is argued that a central programme controlling afferent input to the locomotor nuclei is part of the ‘preparatory set’ which precedes execution of the motor act.