Adaptation of locomotor control in able and impaired human walking
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Extensive research has documented the stereotypical kinematic and kinetic patterns in healthy human walking, but we have a limited understanding of the neuromechanical control principles that contribute to their execution. Furthermore, the strategies used to adapt human walking to morphological or environmental constraints are poorly understood. After a traumatic injury, like amputation, regaining independent mobility is a primary goal of rehabilitation. Without a clear understanding of the neuromechanical principles governing locomotion, monitoring and quantitatively improving gait rehabilitation outcomes is challenging. The purpose of this doctoral work was to identify controlled variables in able and impaired human walking and to compare the control strategies used to adapt to a novel walking environment both with and without amputation. I apply an uncontrolled manifold (UCM) analysis to test whether likely goal variables of human walking are selectively stabilized through step-to-step variability structure. I found that both able-bodied subjects and subjects with an amputation maintain consistent whole body dynamics and leg power production by exploiting inherent motor abundance. Consistent leg power production is accomplished primarily through step-to-step leg force corrections that are driven by variable timing of ankle torque production. Covariance between ankle and knee torques enable robust motor control in able-bodied individuals, but this stabilizing mechanism is absent in individuals with a transtibial amputation. This coordinated joint torque control also appears to assist able-bodied short-term adaptation, invoked by split-belt treadmill walking. However, loss of ankle motor control and distal sensory feedback due to amputation appears to limit reactive, feedback driven adaptation patterns in subjects with an amputation. Ultimately, this work highlights the role of intact distal sensorimotor function in locomotor control and adaptation. The major findings I present have substantial implications for gait rehabilitation and prosthetic design.