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

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 (Hmax ) 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). Hmax 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 Hmax greater during the unloading than the loading phase (p < 0.001). In Protocol-2, no difference in Hmax was observed between phases. In Protocol-3, PT was lesser and Hmax 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 Hmax (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 Hmax 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.

Descending Inputs to Spinal Circuits Facilitating and Inhibiting Human Wrist Flexors.

Recently we reported in humans that electrical stimulation of the wrist extensor muscle extensor carpi radialis (ECR) could facilitate or suppress the H reflex elicited in flexor carpi radialis (FCR), for inter-stimulus intervals (ISIs) of 30 ms or 70 ms, respectively. The facilitation at 30 ms may be produced by both flexor afferents and extensor Ib afferents acting on a spinal circuit; the origin of the suppression at 70 ms is less certain. In this study, we investigated possible descending inputs to these systems. We used magnetic stimulation of the contralateral primary motor cortex, and click sound stimulation, to activate the corticospinal and the reticulospinal tracts respectively, and measured the effects on the H reflex conditioned by ECR stimulation. Corticospinal inputs reduced both the 30 ms facilitation and 70 ms suppression, indicating corticospinal inhibition of both circuits. By contrast, we failed to show any effect of clicks, either on the H reflex or on its modulation by ECR stimulation. This suggests that click-activated reticulospinal inputs to these circuits may be weak or absent.