Crossed reflex reversal during human locomotion

Abstract

During human walking, precise coordination between the two legs is required in order to react promptly to any sudden hazard that could threaten stability. The networks involved in this coordination are not yet completely known, but a direct spinal connection between soleus (SOL) muscles has recently been revealed. For this response to be functional, as previously suggested, we hypothesize that it will be accompanied by a reaction in synergistic muscles, such as gastrocnemius lateralis (GL), and that a reversal of the response would occur when an opposite reaction is required. In the present study, surface EMGs of contralateral SOL and GL were analyzed after tibial nerve (TN), sural nerve (SuN), and medial plantar nerve (MpN) stimulation during two tasks in which opposite reactions are functionally expected: normal walking (NW), just before ipsilateral heel strike, and hybrid walking (HW) (legs walking in opposite directions), at ipsilateral push off and contralateral touchdown. Early crossed facilitations were observed in the contralateral GL after TN stimulation during NW, and a reversal of such responses occurred during HW. These results underline the functional significance of short-latency crossed responses and represent the first evidence for short-latency reflex reversal in the contralateral limb for humans. Muscle afferents seem to mediate the response during NW, while during HW cutaneous afferents are likely involved. It is thus possible that different afferents mediate the crossed response during different tasks.

The effect of crossed reflex responses on dynamic stability during locomotion

“…SLCRs were elicited by applying single electrical stimuli to the iTN (Gervasio et al 2013). Stimuli were delivered at 80% of the ipsilateral gait cycle as at this time the most prominent short-latency facilitation (an increase in the EMG signal) in the gastrocnemius has been observed (Gervasio et al 2013; S. Gervasio, M. Voigt, U. G. Kersting, D. Farina, T. Sinkjaer, and N. Mrachacz-Kersting, unpublished observations).…”

“…Moreover, the interlimb pathway between soleus (SOL) muscles is significantly affected in patients following stroke, and this impairment may contribute to abnormal symmetry between the movement of the legs (Stubbs et al 2012). The functional significance of the SLCR is further supported by the observation that the crossed facilitation elicited in cGL is reverted to an inhibition when an opposite reaction is required (Gervasio et al 2013).…”

“…Recent studies demonstrate that short-latency crossed responses (SLCRs) are elicited in the contralateral triceps surae following ipsilateral tibial nerve (iTN) stimulation; an inhibitory response is elicited in the contralateral soleus (cSOL; Stubbs and Mrachacz-Kersting 2009;Stubbs et al 2011a,b) ϳ40 ms after stimulation, whereas an excitatory response is elicited in the contralateral gastrocnemius lateralis (cGL; Gervasio et al 2013) ϳ69 ms following iTN stimulation. These responses are likely functional since they exhibit a phase-dependent modulation during locomotion with the most prominent responses occurring during the swing-to-stance transition of the stimulated leg (Gervasio et al 2013;Stubbs et al 2011b).…”

“…These responses are likely functional since they exhibit a phase-dependent modulation during locomotion with the most prominent responses occurring during the swing-to-stance transition of the stimulated leg (Gervasio et al 2013;Stubbs et al 2011b). Moreover, the interlimb pathway between soleus (SOL) muscles is significantly affected in patients following stroke, and this impairment may contribute to abnormal symmetry between the movement of the legs (Stubbs et al 2012).…”

“…It was hypothesized that, following an ipsilateral perturbation, the SLCR could be part of a strategy to stop the progression of the contralateral leg and keep the projection of the center of mass to the ground within the margin of stability by inducing a flexion of the knee (Duysens et al 1991;Gervasio et al 2013). Accordingly, it was expected that the SLCR elicited by iTN stimulation would induce a shift of the CoP of the contralateral foot backward compared with the unperturbed trajectory.…”