Reduced H-reflex gain in soleus during isometric plantar flexor and dorsiflexor muscle contractions in old adults

The modulation of the Hoffmann (H) reflex amplitude during agonist and antagonist muscle contractions provides relevant information on the neural control of leg muscles but remains largely unknown in older adults. This study investigated the H-reflex gain in the soleus muscle at rest and during plantar flexor (PF) and dorsiflexor (DF) muscle contractions performed at 10%, 20%, and 30% of the maximal electromyographic (EMG) activity (EMG_MVC) in 12 young (23-35 years) and 12 old adults (61-76 years). The reflex gain was measured as the slope of the relation between H-reflex amplitude and background EMG activity. The stimulation intensity was set to evoke at rest an H reflex in the ascending phase of its recruitment curve preceded by an M wave (5%-10% of its maximal amplitude; M_max). During PF contractions, the H-reflex amplitude increased with the increase in contraction intensity in both groups with a greater reflex gain (p = 0.024) in young (1.2 (0.5)% M_max/% EMG_MVC) than older adults (0.7 (0.4)% M_max/% EMG_MVC). During DF contractions, the H-reflex amplitude decreased in both groups, with a more negative reflex gain (p = 0.009) in young (-6.8 (2.8)% M_max/% EMG_MVC) than older adults (-2.7 (4.0)% M_max/% EMG_MVC). The decrease in H-reflex amplitude was linearly associated with the increase in tibialis anterior EMG in both groups. The present study showed a smaller reflex gain in older adults in soleus during submaximal PF and DF contractions, suggesting a decreased ability to adjust the excitatory afferent inputs during PF contractions, and to modulate reciprocal inhibition during DF contractions.