Neuromechanics of locomotion: Insights from the walk-to-run transition in amputees and pedaling in able-bodied individuals
Norman, Tracy L.
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Afferent feedback is important for modulating locomotion and maintaining stability. Studying locomotor extremes and applying perturbations to normal locomotion allows us to probe the effects of afferent feedback on the control of normal gait. Investigating the walk-to-run gait transition specifically provides a unique locomotor event to investigate the fundamental determinants of legged locomotion (walking or running) and identify the sensory inputs important to the ongoing neuromuscular control of walking and running. The first goal of this dissertation was to investigate the contributions of plantarflexor muscles during stance (Aim 1) and flexor muscles during swing (Aim 2) to the walk-to-run transition. To accomplish this I used unilateral, transtibial amputee subjects as a means to assess the affects of unilaterally eliminating plantarflexor propulsive force production and below-knee flexor activation on the walk-to-run transition speed. The main objective of Aim 1 was to determine the preferred gait transition speeds of unilateral, transtibial amputee subjects, and the influence of kinetics on the walk-to-run gait transition speed. Unilateral, transtibial amputee subjects transition between gaits at a lower speed than able-bodied controls and are still able to generate higher propulsive forces walking at speeds above their preferred gait transition speed. This finding indicates that their walk-to-run transition is not likely dictated by the force-length-velocity characteristics of the intact plantarflexor muscles. Thus, as an experimental model, unilateral, transtibial amputee subjects can provide unique insights for decoupling the previously identified performance limit of plantarflexor muscles from the preferred gait transition speed in order to probe other potential determinants. The main objective of Aim 2 was to quantify the muscle activation during walking and running gaits relative to the walk-to-run gait transition speed for unilateral, transtibial amputee subjects. The swing phase tibialis anterior muscle activation is a major determinant of the walk-to-run transitions in unilateral, transtibial amputee subjects. Swing phase dorsiflexion moments alone do not explain these results and additional work is necessary to probe potential mechanical and neural explanations. Furthermore, in unilateral, transtibial amputee subjects, swing-phase rectus femoris and biceps femoris long head activations and their respective joint moments are a function of changes in absolute speed and thus not indicative of their significantly lower gait transition speed. The second goal of this dissertation was to probe the potential contributions of afferent feedback to the underlying neuromuscular mechanism ultimately responsible for the transition (Aim 3). The main objective of Aim 3 was to evaluate the effects of contralateral sensory loss on the motor output of the ipsilateral leg. Unilateral below-knee, ischemic deafferentation has significant effects on both inter- and intra- limb motor output. The net effect of contralateral sensory loss below the knee is a significant decrease in ipsilateral flexor muscle activations during the transition from flexion to extension in pedaling (Q1). Due to the rapid time course of these responses, I speculate either i) contralateral below-knee afferents (most likely Ia and/or cutaneous) have a net excitatory effect on the ipsilateral flexor muscles or ii) contralateral above knee afferents (most likely Ib) have an inhibitory effect on the ipsilateral flexor muscles.