Influencing Motor Behavior Through Constraint of Lower Limb Movement

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Abstract

Motor control of any movement task involves the integration of neural, muscular and skeletal systems. The integration occurs throughout the sensory and motor system as part of a process to control a body region, i.e., the ankle-foot complex during locomotion. A person utilizing an ankle foot orthosis combined with footwear that encompasses the shank, ankle and foot must integrate the AFO-footwear system with the lower limb as the new lower limb ensemble. Additionally, a person utilizing the AFO-footwear system must adjust to the new motion control environment and the challenges of adopting the external device (i.e. AFO-footwear system), through the mechanical interface between the lower limb and its contact with the AFO-footwear system.

An AFO-footwear system is typically utilized to control motion of lower limb segments and joints in persons with neural, muscular and skeletal systems disorders (i.e., stroke, peripheral nerve injury, muscle-tendon injury, fracture, etc.). In many instances, the decision to utilize AFO-footwear system is based on meeting the biomechanical motion control needs of the individual (i.e., stroke survivor requiring mechanical motion control of the ankle-foot complex during gait due to hemiparesis) but with less regard to the consequence on the neuromuscular system (dependence upon the orthosis). In cases where the AFO-footwear system limits motion of the ankle-foot complex to provide ankle joint stability, a possible consequence is that the neuromuscular system “slacks” by providing decreasing levels of muscle activation when motion is limited.

Factors influencing the neuromuscular response to the AFO-footwear system that limits joint motion are: 1) the central nervous system accounts for reduced ankle motion by adjusting muscle activation-contraction dynamics in proportion to sensorimotor feedback from the limb utilizing the AFO-footwear system, 2) the system alters movements of the remaining body segments to compensate for the new lower limb-orthosis/footwear system ensemble, or some combination of these factors. Understanding how the human sensorimotor system adjusts to the addition of an external mechanical device (i.e., AFOfootwear system) that limits motion can provide useful insight into how the human neural system responds. This approach has the potential to provide additional insights on how humans adjust to and adopt the addition of an AFO-footwear system as the new lower limb ensemble. The research outlined in this dissertation uses a group of healthy, recreationally fit individuals in a newly developed footwear system that maintains rollover that is integrated with a newly developed AFO that limits ankle motion in a STOP condition and maintains ankle motion in a FREE condition and a CONTROL (no AFO) condition.

Subjects walked on a treadmill in CONTROL, FREE and STOP conditions as a model to understand the pattern of biomechanical and neuromuscular “adjustments” necessary to utilize an external device (i.e., AFO and footwear system) for locomotion. Results of these experiments will clarify whether: 1) the motor system accounts for the limitation of joint motion by decreasing muscle activation-contraction dynamics in non-equivalent proportion to the reduction in joint motion or 2) the motor system accounts for the limitation of joint motion by decreasing muscle activation-contraction dynamics in equivalent proportion to the reduction in joint motion.

Results suggest that a decrease in muscle activation contraction dynamics to use of an AFO-footwear system that decreases ankle motion occurs in non-equivalent proportion, and the relationship reflects “motor adjustments” made by the nervous system. These findings may also have relevance to the clinician. This is because in clinical decision making, there is a common, yet unverified notion that an AFO-footwear system immobilizes ankle motion and therefore produces an equivalent elimination of muscle activation by the neuromuscular system. Based on this notion, the clinician is faced with choice whether to a.) accept “learned disuse” by the neuromotor system and to exploit the limitation of joint motion for stability and therefore encourage locomotion that maintains skeletal integrity or b.) to reject the use of an orthosis and to select alternative therapies.

The goal of this research is to clarify the notion of proportional adjustments in muscle activation relative to the proportion of joint motion in order to examine the influence of orthotic motion control on the neural control system. The preliminary findings from this dissertation research have the potential to change the paradigm of thinking – that orthoses and footwear may serve as tools that alter the neural control system to produce a change in the pattern of biomechanical motion and neuromuscular activation of lower limb muscles. The research also has the potential to improve our understanding of the principles of motion control and engineering required to create an orthosis and footwear system that substantially limits motion in a STOP condition, maintains motion in a FREE condition similar to a CONTROL (no AFO) condition.

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