Passive torque influences the Hoffmann reflex pathway during the loading and unloading phases of plantar flexor...

This study investigated the influence of passive tension on Hoffmann reflex during the loading (muscle stretched by passive joint movement) and unloading phase (joint returned to initial position) of muscle stretching. The maximal H-reflex amplitude (H_max ) was recorded in soleus in 19 young adults during the loading and unloading phases of a passive 30° dorsiflexion, from 90° ankle angle (reference position). H_max was evoked at similar angles (Protocol-1) or similar passive torque (PT; Protocol-2) during the loading and unloading phases, or during two loading phases separated by a 5-min stretch hold at 30° ankle dorsiflexion relative to the reference position (Protocol-3). Homosynaptic depression (HD) was assessed with paired H reflexes (0.5-s interstimulus interval) during the loading and unloading phases (Protocol-4; n=13). In Protocol-1, PT was lesser and H_max greater during the unloading than the loading phase (p < 0.001). In Protocol-2, no difference in H_max was observed between phases. In Protocol-3, PT was lesser and H_max greater during the second than the first loading phase (p < 0.001). Changes in PT during in these three protocols were associated with those in H_max (r²  ≥ 0.97). In Protocol-4, HD increased and decreased during the loading and unloading phases, respectively (p < 0.001), without differing between phases. Additional experiments (n=12) showed a similar modulation of H_max in gastrocnemius medialis during loading and unloading phases, while muscle fascicle length did not differ between phases. This study indicates that the H-reflex modulation during muscle stretching relies in part on mechanisms associated with the PT developed by the muscle-tendon unit.