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ItemNeuromechanics of locomotion: Insights from the walk-to-run transition in amputees and pedaling in able-bodied individuals(Georgia Institute of Technology, 2015-10-16) Norman, Tracy L.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.
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ItemInfluencing motor behavior through constraint of lower limb movement(Georgia Institute of Technology, 2015-04-29) Hovorka, Christopher FrancisLimited knowledge of the neuromechanical response to use of an ankle foot orthosis-footwear combination (AFO-FC) has created a lack of consensus in understanding orthotic motion control as a therapeutic treatment. Lack of consensus may hinder the clinician’s ability to target the motion control needs of persons with movement impairment (e.g., peripheral nerve injury, stroke, etc.). Some evidence suggests a proportional relationship between joint motion and neuromuscular activity based on the notion that use of lower limb orthoses that constrain joint motion may invoke motor slacking and decreasing levels of muscle activity. Use of AFO-FCs likely alters the biomechanical and neuromuscular output as the central control system gradually forms new movement patterns. If there is proportional relationship between muscle activation and joint motion, then it could be examined by quantifying joint motion and subsequent neuromuscular output. Considering principles of neuromechanical adjustment, my general hypothesis examines whether orthotic control of lower limb motion alters neuromuscular output in proportion to the biomechanical output as a representation of the limb’s dynamics are updated by the neural control system. The rationale for this approach is that reference knowledge of the neuromechanical response is needed to inform clinicians about how a person responds to walking with motion controlling devices such as ankle foot orthoses combined with footwear. In the first line of research, I hypothesize that a newly developed AFO which maximizes leverage and stiffness will constrain the talocrural joint and alter joint kinematics and ground reaction force patterns. To answer the hypothesis, I sampled kinematics and kinetics of healthy subjects’ treadmill walking using an AFO-FC in a STOP condition and confirmed that the AFO substantially limited the range of talocrural plantarflexion and dorsiflexion motion to 3.7° and in a FREE condition maintained talocrural motion to 24.2° compared to 27.7° in a CONTROL (no AFO) condition. A follow up controlled static loading study sampled kinematics of matched healthy subjects limbs and cadaveric limbs in the AFO STOP and FREE conditions. Findings revealed healthy and cadaveric limbs in the AFO STOP condition substantially limited their limb segment motion similar to matched healthy subjects walking in the STOP condition and in the AFO FREE condition healthy and cadaveric limbs maintained similar limb segment motion to matched healthy subjects walking in the FREE condition. In a second line of research, I hypothesize that flexibility of a newly developed footwear system will allow normal walking kinetics due to the shape and flexibility of the footwear. To answer the hypothesis, I utilized a curved-flexible footwear system integrated with an AFO in a STOP condition and sampled kinematics and kinetics of healthy subjects during treadmill walking. Results revealed subjects elicited similar cadence, stance and swing duration and effective leg-ankle-foot roll over radius compared to walking in the curved-flexible footwear integrated with the AFO in a FREE condition and a CONTROL (no AFO) condition. To validate rollover dynamics of the curved-flexible footwear system, a follow up study of healthy subjects’ treadmill walking in newly developed flat-rigid footwear system integrated with the AFO in a STOP condition revealed interrupted leg-ankle-foot rollover compared to walking in curved-flexible footwear in STOP, FREE and CONTROL conditions. In a third line of research, I hypothesize that use of an AFO that limits talocrural motion in a STOP condition will proportionally reduce activation of Tibialis Anterior, Soleus, Medial and Lateral Gastrocnemii muscles compared to a FREE and CONTROL condition due to alterations in length dependent representation of the limb’s dynamics undergoing updates to the central control system that modify the pattern of motor output. To answer the question, the same subjects and AFO-footwear presented in the first two lines of research were used in a treadmill walking protocol in STOP, FREE, and CONTROL conditions. Findings revealed the same subjects and ipsilateral AFO-footwear system presented in Aim 1 exhibited an immediate yet moderate 30% decline in EMG activity of ipsilateral Soleus (SOL), Medial Gastrocnemius (MG) and Lateral Gastrocnemius (LG) muscles in the STOP condition compared to the CONTROL condition. The reduction in EMG activity in ipsilateral SOL, MG and LG muscles continued to gradually decline during 15 minutes of treadmill walking. On the contralateral leg, there was an immediate yet small increase of 1% to 14% in EMG activity in SOL, MG, LG muscles above baseline. After 10 minutes of walking, the EMG activity in contralateral SOL, MG and LG declined to a baseline level similar to the EMG activity in the contralateral CONTROL condition. These collective findings provide compelling evidence that the moderate 30% reduction in muscle activation exhibited by subjects as they experience substantial (85%) constraint of total talocrural motion in the AFO STOP condition is not proportionally equivalent. Further, the immediate decrease in muscle activation may be due to a reactive feedback mechanism whereas the continued decline may in part be explained by a feedforward mechanism. The clinical relevance of these findings suggests that short term use of orthotic constraint of talocrural motion in healthy subjects does not substantially reduce muscle activation. These preliminary findings could be used to inform the development of orthoses and footwear as therapeutic motion control treatments in the development of motor rehabilitation protocols.
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ItemInjury compensation reveals implicit goals that guide locomotor coordination(Georgia Institute of Technology, 2012-04-08) Bauman, Jay MorrisLocomotion persists despite changes in external and internal circumstances. Motor responses to gait impairment exhibit commonalities across various taxa and types of injury, yet we lack a systematic understanding of compensation strategies. The objective of this dissertation is to uncover principles governing implicit goals within the control of locomotion. I propose that coordination of injured locomotion will demonstrate that these goals follow a hierarchical organization of the neuromuscular system. Accurate quantification of gait deficits in rodents demands sophisticated measurement techniques. I utilize X-ray technology to examine intralimb and interlimb coordination after unilateral injury in rats. My findings indicate that compensation to injury involves the coordination of lower-order motor elements to preserve the pre-injury behaviors of higher-order elements. Specifically I present evidence that preservation of limb angle and limb length are critical task goals that transcend injury states and afferent sensory feedback conditions. Broadening my investigation to include interlimb coordination revealed that task goals may change to satisfy the goals of a higher hierarchical level. This work is a necessary precursor to study locomotor coordination and injury compensation in more complex rodent injury models such as self-reinnervation, sciatic nerve, and spinal cord injury. These results could also translate to clinical gait rehabilitation through future protocols that address motor patterns of the entire limb over the behavior of individual joints.
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ItemRobustness and hierarchical control of performance variables through coordination during human locomotion(Georgia Institute of Technology, 2010-11-03) Auyang, Arick Gin-YuThe kinematic motor redundancy of the human legs provides more local degrees of freedom than are necessary to achieve low degree of freedom performance variables like leg length and orientation. The purpose of this dissertation is to investigate how the neuromuscular skeletal system simplifies control of a kinematically redundant system to achieve stable locomotion under different conditions. I propose that the neuromuscular skeletal system minimizes step to step variance of leg length and orientation while allowing segment angles to vary within the set of acceptable combinations of angles that achieves the desired leg length and orientation. I find that during human hopping, control of the locomotor system is organized hierarchically such that leg length and orientation are achieved by structuring segment angle variance. I also found that leg length and leg orientation was minimized for a variety of conditions and perturbations, including frequency, constrained foot placement, and different speeds. The results of this study will give valuable information on interjoint compensation strategies used when the locomotor system is perturbed. This work also provides evidence for neuromuscular system strategies in adapting to novel, difficult tasks. This information can be extended to give insight into new and different areas to focus on during gait rehabilitation of humans suffering from motor control deficits in movement and gait.