title: Modulation of triceps surae H-reflexes as a function of the reflex activation history during standing and stepping.
abstract: The facilitatory effectiveness of spindle afferent feedback is controlled by modulation of segmental reflex excitability such that the level of muscle activation is appropriate for the task. Phase-dependent modes of reflex modulation have been well-characterized. We hypothesized that segmental reflex excitability of the triceps surae was also modulated in a manner associated with the activation history of the spindle afferents and the segmental reflex pathway during isometric contractions, standing and stepping. In the first experiment. pairs of soleus (S) H-reflexes were evoked 80 ms apart with equal strength stimuli at rest and while subjects isometrically contracted their S against loads of 10%. 20%. and 50% of their maximum voluntary efforts. 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 first. The degree of depression was progressively reduced as the plantarflexion torque increased. In the second experiment, paired H-reflexes were obtained from the S and medial (MG) and lateral gastrocnemii (LG) muscles while subjects were standing and during the stance phase of step initiation. The degree of depression of the second H-reflex during standing ( > 78%) was similar in magnitude to that produced at rest in Experiment I. At the end of the stance phase of stepping. depression of the second H-reflex of all three muscles was reduced to less than 25%. We conclude that the segmental reflex excitability is modulated as a function of the reflex activation history during these tasks.
authors.affiliation: Department of Physical Therapy, University of Florida, Gainesville, FL 32610-0154, USA. mtrimble@hp.ufl.edu
authors.forename: M H
authors.initials: MH
authors.lastname: Trimble
authors.forename: P
authors.initials: P
authors.lastname: Du
authors.forename: D
authors.initials: D
authors.lastname: Brunt
authors.forename: F J
authors.initials: FJ
authors.lastname: Thompson
mesh_terms: D000328:Adult; D005260:Female; D005684:Gait / Q000502:physiology*; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology; D011187:Posture / Q000502:physiology; D017729:Presynaptic Terminals / Q000502:physiology; D016474:Weight-Bearing / Q000502:physiology