Modulation of soleus H-Reflex during shortening and lengthening muscle actions in young and older adults

The Hoffmann reflex (H-reflex) is dependently modulated during isometric and anisometric muscle actions. However, the manner of the H-reflex modulation during dynamic muscle movements in relation to ageing is less stated in the literature. This study was designed to investigate the effects of ageing on soleus (SOL) H-reflex modulation during dynamic muscle actions. Twenty young (24±4 years of age) and 20 older adults (73±5 years of age) voluntarily participated in the study. The SOL H-reflex was measured during passive and active shortening and lengthening muscle actions in a sitting position. The older group showed a lower ratio of the maximal amplitude of H-reflex to M-wave (SOL Hmax/Mmax) during the passive lengthening than that during the passive shortening (shortening: 0.40±0.22 vs. lengthening: 0.15±0.10, P＜0.05), whereas the SOL Hmax/Mmax ratio of the young group was significantly higher during the shortening than that during the lengthening contractions at maximal effort (shortening: 0.51±0.26 vs. lengthening: 0.37±0.18, P＜0.05). These results suggested different modulations of group Ia afferent inputs to the SOL motoneurons during passive and active dynamic muscle actions between young and older adults.