Effect of Ankle Angles on the Soleus H-Reflex Excitability During Standing

Soleus H-reflex modulation during a double-legged drop landing task

Muscle spindle afferent feedback is modulated during different phases of locomotor tasks in a way that facilitates task goals. However, only a few studies have studied H-reflex modulation during landing. This study aimed to characterize soleus (SOL) H-reflex modulation during the flight and early landing period of drop landings. Since landing presumably involves a massive increase in spindle afferent firing due to rapid SOL muscle stretching, we hypothesized H-reflex size would decrease near landing reflecting neural modulation to prevent excessive motoneuron excitation. The soleus H-reflex was recorded during drop landings from a 30 cm height in nine healthy adults. Electromyography (SOL, tibialis anterior (TA), medial gastrocnemius, and vastus lateralis), ankle and knee joint motion and ground reaction force were recorded during landings. Tibial nerve stimulation was timed to elicit H-reflexes during the flight and early ground contact period (five 30 ms Bins from 90 ms before to 60 ms after landing). The H-reflexes recorded after landing (0-30 and 30-60 ms) were significantly smaller (21-36% less) than that recorded during the flight periods (90-0 ms before ground contact; P ≤ 0.004). The decrease in H-reflex size not occurring until after ground contact indicates a time-critical modulation of reflex gain during the last 30 ms of flight (i.e., time of tibial nerve stimulation). H-reflex size reduction after ground contact supports a probable neural strategy to prevent excessive reflex-mediated muscle activation and thereby facilitates appropriate musculotendon and joint stiffness.

Alteration of H-reflex amplitude modulation is a marker of impaired postural responses in individuals with incomplete spinal cord injury

Individuals with incomplete spinal cord injury (iSCI) show altered postural reactions leading to increased risk of falls. To investigate neural correlates underlying this deficit, we assessed the modulation pattern of the Soleus H-reflex in iSCI individuals following unexpected perturbations of a base of support. Ten men with iSCI (AIS D) and 8 age-matched controls (CTRL) stood on a force-platform randomly tilted forward or backward. The center of pressure (CoP) excursion, 95% confidence ellipse area and electromyographic (EMG) activity of the Soleus (SOL) and Tibialis Anterior (TA) muscles were analyzed. SOL H-reflex amplitude was assessed by stimulating the tibial nerve prior to and at 100, 150 and 200 ms following perturbation onset. Although SOL and TA short-latency EMG responses were comparable in both groups, long-latency EMG responses occurred later in the iSCI group for both directions: during backward tilt, a decrease in H-reflex amplitude was observed at all stimulus timings post-tilt in CTRL, but only at 200 ms in iSCI. The decrease in H-reflex amplitude was smaller in iSCI participants. During forward tilt, an increase in H-reflex amplitude was observed at 150 and 200 ms in the CTRL group, but no increase was observed in the iSCI group. Decreased and delayed SOL H-reflex amplitude modulation in the iSCI group accompanied impaired balance control as assessed clinically with the Berg Balance Scale and biomechanically through CoP displacement. Overall, delayed and reduced spinal reflex processing may contribute to impaired balance control in people with iSCI.