Abstract
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.
Highlights
Following an unexpected perturbation to one limb a reflex response is often observed in the opposite limb; this response, referred to as a crossed reflex, is PLOS ONE | DOI:10.1371/journal.pone.0168557 January 6, 2017The Role of Afferents in Interlimb Reflexes equipment used during data collection
The results obtained with the described model are consistent with those reported by recent studies for human locomotion: the modulation of the estimated muscle fascicle length changes is in accord with those displayed by [28,39] while muscle force amplitude is consistent with that reported by [40], with a maximum force exerted during walking by the gastrocnemius lateralis being just above 200 N
The force produced by the cGL was 225 N stronger at 80% of the gait cycle of normal walking compared to the 50% gait cycle of hybrid walking
Summary
Following an unexpected perturbation to one limb (ipsilateral) a reflex response is often observed in the opposite limb (contralateral); this response, referred to as a crossed reflex, is PLOS ONE | DOI:10.1371/journal.pone.0168557 January 6, 2017The Role of Afferents in Interlimb Reflexes equipment used during data collection. Following an unexpected perturbation to one limb (ipsilateral) a reflex response is often observed in the opposite limb (contralateral); this response, referred to as a crossed reflex, is PLOS ONE | DOI:10.1371/journal.pone.0168557. The Role of Afferents in Interlimb Reflexes equipment used during data collection. The European Union FP7 program under the project H2R (grant agreement n 600698) provided funding for purchase of equipment used during data collection. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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