SPEED OF MUSCULAR MOVEMENT AND ITS EFFECTS ON POST-ACTIVATION DEPRESSION OF THE SOLEUS H-REFLEX

Abstract

Post-activation depression of the monosynaptic reflex is attributed to the reduction in the probability of neurotransmitter release from the Ia afferent fiber terminals. This intrinsic mechanism is mediated by the activity of the afferent fibers and the type of motoneurons coupled to them. The depression is restricted to the group I afferents that are activated by the conditioning stimulus. The large Ia afferents are crucial in determining the relationship between muscular activity and post-activation depression, because of their connection to low-threshold motor units, in which depression is dominant. PURPOSE: To examine the effects of afferent activity on the amplitude of the soleus H-reflex after a muscular movement. METHODS: Six young subjects (mean age = 22.2 ± 6.3 yr) participated in this study. Each subject was positioned prone on an athletic trainers table with the right ankle strapped to a motorized footplate as a battery of H-reflex tests was performed. Stimulus intensity was set at 25% of M-max. Soleus H-reflex amplitude (peak-to-peak EMG) was measured prior to and after a passive dorsiflexion about the ankle (from 120o to 110o plantarflexion) and randomly designated at three speeds (2o/s, 37o/s, and 80o/s). These speeds reflect the minimum, average, and maximum dorsiflexion movements of the ankle during normal gait. All H-reflex measurements were recorded at an ankle joint angle of 110o at random delay intervals of zero, two, and five seconds. After reflex testing, subjects were asked to stand on a Kistler force platform for 20 seconds to measure their postural sway. RESULTS: A two-way repeated measures ANOVA (Degree x Interval) indicated significant (p < 0.05) differences in delay intervals with a greater H-reflex depression at 0-seconds. Further, simple main effects identified significant (p < 0.05) differences in H-reflex depression at the 5-second delay interval with 2o/s having more depression. CONCLUSION: It is concluded that as the speed of movement is decreased there is prevalence for more H-reflex depression, which may be attributed to an increased frequency of firing of the active group I afferents. This finding may implicate functional regulation of neural pathways in relation to servomechanistic reflexes. This research was supported in part by a grant from the National Institutes on Aging (R29 AG/OD 13660-01) to D. M. Koceja.