title: Variations in soleus H-reflexes as a function of plantarflexion torque in man.
abstract: The purpose of this study was to evaluate the effects of the levels of voluntary isometric contraction on the Hoffman reflex in human soleus and medial gastrocnemius (MG) muscles. H-reflexes were recorded in sixteen healthy adults at each of 16 isometric plantarflexion (pf) torque levels ranging from 0-100% of their maximum voluntary isometric contraction (MVC) and were elicited at two intensities of stimulation: (i) supramaximal for M-response and (ii) a submaximal stimulus that produced an H-reflex in soleus that was 50% of maximum H-reflex at rest. The H-reflex peak-to-peak amplitudes were linearly related to pf torque levels ranging from 0 to 50% MVC at both supramaximal and submaximal stimulus intensities. The slope of this relationship was higher for the submaximal stimulation. Beyond 60% of MVC, the soleus H-reflex amplitude showed no further increase with increasing pf torque for both stimulus intensities. Thus, beyond 50-60% of MVC the soleus H-reflex does not provide an accurate measure of soleus motor neuron pool excitability. Further experimental results showed that the H-reflex amplitude at a given torque level depended on whether torque was increasing or decreasing. When torque was increasing, the amplitude of the H-reflex was larger than when the same torque was maintained at a constant level. In contrast, if the torque was decreasing, amplitude of the reflex was lower than when torque was increasing. Therefore, variations in H-reflex amplitudes at a given torque level may be more closely correlated to the direction of the ongoing contraction than to the actual muscle force being produced at the time the H-reflex is elicited.
authors.affiliation: Department of Exercise Science, University of Iowa, Iowa City 52242.
authors.forename: A J
authors.initials: AJ
authors.lastname: Butler
authors.forename: G
authors.initials: G
authors.lastname: Yue
authors.forename: W G
authors.initials: WG
authors.lastname: Darling
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D005528:Foot; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D007537:Isometric Contraction; D007866:Leg; D008297:Male; D009132:Muscles / Q000502:physiology*; D012016:Reference Values