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.

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Gervasio S, Kersting UG, Farina D, Mrachacz-Kersting N. The effect of crossed reflex responses on dynamic stability during locomotion. J Neurophysiol 114: 1034 -1040, 2015. First published June 10, 2015 doi:10.1152/jn.00178.2015In recent studies, we demonstrated that a neural pathway within the human spinal cord allows direct communication between muscles located in the opposing limb. Short-latency crossed responses (SLCRs) are elicited in the contralateral triceps surae at an onset of 40 -69 ms following electrical stimulation of the ipsilateral tibial nerve (iTN). The SLCRs are significantly affected by lesions of the central nervous system where the patients are unable to attain normal walking symmetry. The aim of this study was to elucidate the functionality of SLCRs by investigating their effects on the center of pressure (CoP) and pressure distribution. SLCRs were elicited by iTN stimulation at the end of the ipsilateral swing phase while the participants (n ϭ 8) walked on a treadmill. CoP location and pressure distribution on the sole of the contralateral foot were recorded using instrumented insoles inserted bilaterally in the participant's shoes. The SLCR induced a significant displacement of the CoP toward the medial and anterior direction, associated with a significant increase in pressure at the level of the first metatarsal head. The SLCR contributed to dynamic stability, accelerating the propulsion phase of the contralateral leg and thus preparing for a faster step in the event that the ipsilateral leg is not able to support body weight. The results presented here provide new insight into the functionality of SLCRs, introducing the perspective that training these reflexes, as shown successfully for other reflex pathways, would increase dynamic stability in patients with impaired locomotion. human locomotion; interlimb reflexes; functionality; interlimb communication

A constant coordination between the left and right leg is required to maintain stability during human locomotion, especially in a variable environment. The neural mechanisms underlying this interlimb coordination are not yet known. In animals, interneurons located within the spinal cord allow direct communication between the two sides without the need for the involvement of higher centers. These may also exist in humans since sensory feedback elicited by tibial nerve stimulation on one side (ipsilateral) can affect the muscles activation in the opposite side (contralateral), provoking short-latency crossed responses (SLCRs). The current study investigated whether contralateral afferent feedback contributes to the mechanism controlling the SLCR in human gastrocnemius muscle. Surface electromyogram, kinematic and kinetic data were recorded from subjects during normal walking and hybrid walking (with the legs moving in opposite directions). An inverse dynamics model was applied to estimate the gastrocnemius muscle proprioceptors’ firing rate. During normal walking, a significant correlation was observed between the magnitude of SLCRs and the estimated muscle spindle secondary afferent activity (P = 0.04). Moreover, estimated spindle secondary afferent and Golgi tendon organ activity were significantly different (P ≤ 0.01) when opposite responses have been observed, that is during normal (facilitation) and hybrid walking (inhibition) conditions. Contralateral sensory feedback, specifically spindle secondary afferents, likely plays a significant role in generating the SLCR. This observation has important implications for our understanding of what future research should be focusing on to optimize locomotor recovery in patient populations.

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