Modulating the soleus Hoffmann reflex (H-reflex) during gait initiation

The purpose of this series of three studies was to investigate how the soleus (SOL) H-reflex was modulated during human gait initiation (GI) under various experimental situations.; Study 1 examined the role of peripheral influences from contracting tibialis anterior (TA) in modulation the SOL H-reflex during early phase of GI. To assess the H-reflex modulation when TA activation was eliminated, the stance limb of a person with complete peroneal nerve injury was tested during GI. Normal control subjects were also tested when they initiated walking after swaying forward without initial TA activation. The H-reflex was considerably depressed during initial GI without initial TA activation. This finding supports the view that reciprocal inhibition of the SOL H-reflex during GI, which normally involves TA activation, is due to a centrally mediated process; and not due to peripheral influences from TA.; In Study 2, patterns of SOL H-reflex modulation as a function of posture, task, and reflex activation history were assessed with three experimental paradigms: lying prone compared with standing unsupported, standing compared with the initiation of walking, and standing compared with the (mid) stance phase of walking. Paired H-reflexes were evoked under each condition. When different postures and tasks were compared, paired reflex depression (a measure of the relative influence of the reflex activation history on reflex excitability) was modulated independently of the first H-reflex (a representation of the task or phase dependent modulation). These experiments demonstrated divergent patterns of segmental reflex modulation.; Study 3 was done to investigate SOL H-reflex for swing limb and stance limb during the reaction time (RT) period and during the early phase of GI relative to the kinetic events in the process of GI. Both stance and swing limb H-reflexes were significantly and gradually depressed during RT, however, the stance limb H-reflexes were significantly lower and their depression started earlier on than the swing limb. Although there was some inhibition of the H-reflex due to TA contraction, the H-reflexes for both limbs after the TA EMG onset were more closely modulated with the ground reaction forces.