Velocity-dependent suppression of the soleus H-reflex during robot-assisted passive stepping

Modulation of corticospinal excitability related to the forearm muscle during robot-assisted stepping in humans

In recent years, the neural control mechanisms of the arms and legs during human bipedal walking have been clarified. Rhythmic leg stepping leads to suppression of monosynaptic reflex excitability in forearm muscles. However, it is unknown whether and how corticospinal excitability of the forearm muscle is modulated during leg stepping. The purpose of the present study was to investigate the excitability of the corticospinal tract in the forearm muscle during passive and voluntary stepping. To compare the neural effects on corticospinal excitability to those on monosynaptic reflex excitability, the present study also assessed the excitability of the H-reflex in the forearm muscle during both types of stepping. A robotic gait orthosis was used to produce leg stepping movements similar to those of normal walking. Motor evoked potentials (MEPs) and H-reflexes were evoked in the flexor carpi radialis (FCR) muscle during passive and voluntary stepping. The results showed that FCR MEP amplitudes were significantly enhanced during the mid-stance and terminal-swing phases of voluntary stepping, while there was no significant difference between the phases during passive stepping. Conversely, the FCR H-reflex was suppressed during both voluntary and passive stepping, compared to the standing condition. The present results demonstrated that voluntary commands to leg muscles, combined with somatosensory inputs, may facilitate corticospinal excitability in the forearm muscle, and that somatosensory inputs during walking play a major role in monosynaptic reflex suppression in forearm muscle.

Effects of movement-related afferent inputs on spinal reflexes evoked by transcutaneous spinal cord stimulation during robot-assisted passive stepping

Studies of robot-assisted passive stepping paradigms have reported that movement-related afferent inputs strongly inhibit the excitability of the Hoffmann (H) reflex in the soleus (Sol) during walking. However, it is unknown if movement-related afferent inputs have the same effect on the excitability of spinal reflexes in the other lower-limb muscles that are involved in normal walking in healthy subjects. The aim of this study was to examine the effects of movement-related afferent inputs on the spinal reflexes in lower-limb muscles during walking. Spinal reflexes that were elicited by transcutaneous spinal cord stimulation (tSCS) were recorded during passive air standing and air stepping at three stepping velocities (stride frequencies: 14, 25, and 36 strides/min). The amplitude of the spinal reflexes was reduced in most of the recorded muscles during passive air stepping compared with air standing. Furthermore, in the Sol and lateral gastrocnemius, the amplitude of the reflexes during air stepping significantly decreased as stride frequency increased. These results demonstrate that movement-related afferent inputs inhibit spinal reflexes in the Sol and other lower-limb muscles during walking.