Effect of stability context on cutaneous reflex modulation during treadmill walking
Date
2011-06-28
Authors
Lewis, Allen
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Abstract
Reflex control during walking has been shown to be specific to the behavioural context of a postural threat. Reflex modulation has been shown to be influenced by perturbations to the trunk, as well as by changes to arm activity that further affected the level of postural threat. The magnitudes of EMG responses in limbs and trunk to mechanical trip perturbations were differentially modulated depending on whether or not an earth- referenced railing was held. To further understand the neural control of limb and trunk muscles during walking contexts where changes to postural stability are solely linked to arm activity, we created 3 treadmill walking tasks each with the arms engaged differently to induce three levels of postural stability. Neurologically-intact participants walked on a treadmill using normal arm swing (NORMAL), holding a wheeled walker (WALKER), or holding the handrails (HANDRAIL). Subjects ranked the tasks according to degree of perceived challenge to stability; WALKER was ranked most challenging, followed by NORMAL and then HANDRAIL. Cutaneous reflexes were evoked via constant current stimulation (5 x 1.0 ms pulses at 300 Hz) of Superficial Peroneal (SP) nerve at the foot and Superficial Radial (SR) nerve at the wrist in separate trials during each walking task. EMG was recorded ipsilateral to nerve stimulation from arm and leg muscles and bilaterally from trunk muscles. Off-line analysis was conducted on eight phases of the step cycle after phase-averaging, contingent upon the timing of stimulation in the step cycle. The number of differences in bEMG and reflexes between tasks was graded with the proximity to the source of instability at the arms, progressively increasing from the legs to the arms. The gradient in bEMG differences suggests that the mechanical constraints of each task required different levels of muscle activation that was greatest for the arms, less for the trunk and least for the legs. The similar gradient seen for reflexes suggests functionally relevant changes in neural control of arm and trunk muscles in order to maintain postural stability during in each task. However, overall there were significantly fewer differences in reflexes between tasks compared to bEMG. We suggest that the mechanical constraints of the tasks yielded more significant changes in the muscle activation while requiring far fewer task-specific changes in reflex control suggesting a conservation of some elements of the neural control mechanisms across tasks.
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Keywords
Human locomotion, reflexes, modulation