The purpose of this study was to determine the effect of ankle joint position and muscle contraction intensity on soleus (SOL) H-reflex modulation in young and older adults. Twenty young (25±5 years) and twenty older (74±5 years) volunteers participated in the study. The recruitment curve of H-wave and M-wave was established for each participant. H-reflex was tested when the participants performed plantarflexions at 10, 30, and 50% isometric maximal voluntary contractions (MVC) and at ankle joint positions of neutral (0 degree), plantarflexion (20 degree), and dorsiflexion (-20 degree), respectively in a sitting position. The results showed that the maximum amplitude of SOL H-reflex of the older group was significantly smaller than that of the young group under all testing conditions, for example, it was 57% smaller during the 10%MVC in plantarflexion position, and 77% smaller during rest in dorsiflexion position. During voluntary contractions in the plantarflexion position, the young group demonstrated a fluctuation of the SOL H-reflex modulation, whereas the older group showed a facilitation of the SOL H-reflex at 10% and 30% MVC, and a decrease in the SOL H-reflex at 50% MVC. The latency of H-reflex was not joint angle-dependent in the older group but it was significantly influenced by ankle position in the young group. In addition, the young group demonstrated a shorter duration of the H-reflex than the older group. The significant differences in SOL H-reflex modulation found between young and older adults in relation to the joint position and intensity of muscle contraction might be an indication of the age-related changes in motor control strategies
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