Robotic-assisted stepping modulates monosynaptic reflexes in forearm muscles in the human.
ABSTRACT Although the amplitude of the Hoffmann (H)-reflex in the forelimb muscles is known to be suppressed during rhythmic leg movement, it is unknown which factor plays a more important role in generating this suppression-movement-related afferent feedback or feedback related to body loading. To specifically explore the movement- and load-related afferent feedback, we investigated the modulation of the H-reflex in the flexor carpi radialis (FCR) muscle during robotic-assisted passive leg stepping. Passive stepping and standing were performed using a robotic gait-trainer system (Lokomat). The H-reflex in the FCR, elicited by electrical stimulation to the median nerve, was recorded at 10 different phases of the stepping cycle, as well as during quiet standing. We confirmed that the magnitude of the FCR H-reflex was suppressed significantly during passive stepping compared with during standing. The suppressive effect on the FCR H-reflex amplitude was seen at all phases of stepping, irrespective of whether the stepping was conducted with body weight loaded or unloaded. These results suggest that movement-related afferent feedback, rather than load-related afferent feedback, plays an important role in suppressing the FCR H-reflex amplitude.
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ABSTRACT: Presynaptic inhibition of transmission between Ia afferent terminals and alpha motoneurons (Ia PSI) is a major control mechanism associated with soleus H-reflex modulation during human locomotion. Rhythmic arm cycling suppresses soleus H-reflex amplitude by increasing segmental Ia PSI. There is a reciprocal organization in the human nervous system such that arm cycling modulates H-reflexes in leg muscles and leg cycling modulates H-reflexes in forearm muscles. However, comparatively little is known about mechanisms subserving the effects from leg to arm. Using a conditioning-test (C-T) stimulation paradigm, the purpose of this study was to test the hypothesis that changes in Ia PSI underlie the modulation of H-reflexes in forearm flexor muscles during leg cycling. Subjects performed leg cycling and static activation while H-reflexes were evoked in forearm flexor muscles. H-reflexes were conditioned with either electrical stimuli to the radial nerve (to increase Ia PSI; C-T interval = 20 ms) or to the superficial radial (SR) nerve (to reduce Ia PSI; C-T interval = 37-47 ms). While stationary, H-reflex amplitudes were significantly suppressed by radial nerve conditioning and facilitated by SR nerve conditioning. Leg cycling suppressed H-reflex amplitudes and the amount of this suppression was increased with radial nerve conditioning. SR conditioning stimulation removed the suppression of H-reflex amplitude resulting from leg cycling. Interestingly, these effects and interactions on H-reflex amplitudes were observed with subthreshold conditioning stimulus intensities (radial n., ∼0.6×MT; SR n., ∼ perceptual threshold) that did not have clear post synaptic effects. That is, did not evoke reflexes in the surface EMG of forearm flexor muscles. We conclude that the interaction between leg cycling and somatosensory conditioning of forearm H-reﬂex amplitudes is mediated by modulation of Ia PSI pathways. Overall our results support a conservation of neural control mechanisms between the arms and legs during locomotor behaviors in humans.PLoS ONE 01/2013; 8(10):e76313. · 3.53 Impact Factor
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ABSTRACT: ABSTRACT It is well established that remote whole-limb rhythmic movement (e.g., cycling or stepping) induces suppression of the Hoffman (H-) reflex evoked in stationary limbs. However, the dependence of reflex amplitude on the phase of the movement cycle (i.e., phase-dependence) has not been consistent across this previous research. The authors investigated the phase-dependence of flexor carpi radialis (FCR) H-reflex amplitudes during active walking and in kinematically matched static postures across the gait cycle. FCR H-reflexes were elicited in the stationary forearm with electrical stimulation to the median nerve. Significant phase-dependent modulation occurred during walking when the gait cycle was examined with adequate phase resolution. The suppression was greatest during midstance and midswing, suggesting increased ascending communication during these phases. There was no phase-dependent modulation in static standing postures and no correlation between lower limb background electromyography levels and H-reflex amplitude during active walking. This evidence, along with previous research demonstrating no phase modulation during passive walking, suggests that afferent feedback associated with joint position and leg muscle activation levels are not the sole source of the phase modulation seen during active walking. Possible sources of phase modulation include combinations of afferent feedback related to active movement or central motor commands or both.Journal of Motor Behavior 12/2013; · 1.04 Impact Factor
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ABSTRACT: Although sensory inputs from the contralateral limb strongly modify the amplitude of the Hoffmann (H-) reflex in a static posture, it remains unknown how these inputs affect the excitability of the monosynaptic H-reflex during walking. Here, we investigated the effect of the electrical stimulation of a cutaneous (CUT) nerve innervating the skin on the dorsum of the contralateral foot on the excitability of the soleus H-reflex during standing and walking. The soleus H-reflex was conditioned by non-noxious electrical stimulation of the superficial peroneal nerve in the contralateral foot. Significant crossed facilitation of the soleus H-reflex was observed at conditioning-to-test intervals in a range of 100–130 ms while standing, without any change in the background soleus electromyographic (EMG) activity. In contrast, the amplitude of the soleus H-reflex was significantly suppressed by the contralateral CUT stimulation in the early-stance phase of walking. The background EMG activity of the soleus muscle was equivalent between standing and walking tasks and was unaffected by CUT stimulation alone. These findings suggest that the crossed CUT volleys can affect the presynaptic inhibition of the soleus Ia afferents and differentially modulate the excitability of the soleus H-reflex in a task-dependent manner during standing and walking.Experimental Brain Research 06/2014; · 2.22 Impact Factor