Modulation of triceps surae H-reflexes as a function of the reflex activation history during standing and stepping

The facilitatory effectiveness of spindle afferent feedback is controlled b y modulation of segmental reflex excitability such that the level of muscle activation is appropriate for the task; Phase-dependent modes of reflex mo dulation have been well-characterized. We hypothesized that segmental refle x excitability of the triceps surae was also modulated in a manner associat ed with the activation history of the spindle afferents and the segmental r eflex pathway during isometric contractions, standing and stepping. In the first experiment, pairs of soleus (S) H-reflexes were evoked 80 ms apart wi th equal strength stimuli at rest and while subjects isometrically contract ed their S against loads of 10%, 20%, and 50% of their maximum voluntary ef forts. The percent depression of the second H-reflex relative to the first was used as a measure of the effect of reflex activation history. At rest, the second H-reflexes were depressed an average of 73% relative to the firs t. The degree of depression was progressively reduced as the plantarflexion torque increased. In the second experiment, paired H-reflexes were obtaine d from the S and medial (MG) and lateral gastrocnemii (LG) muscles while su bjects were standing and during the stance phase of step initiation. The de gree of depression of the second H-reflex during standing (> 78%) was simil ar in magnitude to that produced at rest in Experiment 1. At the end if the stance phase of stepping, depression of the second H-reflex of all three m uscles was reduced to less than 25%. We conclude that the segmental reflex excitability is modulated as a function of the reflex activation history du ring these tasks. (C) 2000 Elsevier Science B.V. All rights reserved.