Neuromechanical Mechanisms of Prosthetic Knee Joint Control: Associations with Prosthetic Alignment

The knee is a major weight-bearing joint that provides both stability and mobility during human locomotion. When disease and trauma, such as transfemoral amputation, eliminate the functional contribution of the knee joint, independence and mobility are inherently challenged. Indeed, many of the gait abnormalities exhibited by persons with transfemoral amputation, including reduced walking speed, bilateral asymmetry, decreased balance, and increased energy expenditure, have been attributed in part to the inability of passive prosthetic knee joints to restore the anthropomorphic and biomechanical features of the amputated limb. Most notably, in the absence of direct muscular control, persons with transfemoral amputation must learn to coordinate the flexion and extension of a passive prosthetic knee joint (i.e., an unactuated degree of freedom) by adopting an alternative control strategy that exploits the inter-segmental coupling of their residual-limb and prosthesis. The objective of this dissertation was to investigate the control strategies used by persons with transfemoral amputation to coordinate the movement of a passive prosthetic knee joint during the stance phase of gait. Specifically, we sought to characterize the interaction between short-term neuromechanical adaptations measured at the user's ipsilateral leg and trunk and mechanical perturbations that influence knee-joint stability. Using prosthetic alignment as a model of variation, we first examined the steady-state response of persons with transfemoral amputation to altered joint mechanics during a level walking task. We then investigated the extent to which this response was exaggerated by the mechanical demands of sloped walking. Overall, we found that subjects responded to a destabilizing alignment perturbation (i.e., an anterior shift in the knee joint) by increasing both their trunk flexion and hip extension moment during early stance phase. This finding suggests that to overcome a reduction in knee-joint stability, amputees rely on a combination of kinematic and kinetic control strategies. In contrast, subjects were relatively insensitive to alignment perturbations that favored excessive knee-joint stability. Collectively, these findings provide new insight into the ability of amputees to control a passive mechanical knee joint during stance phase, which may have important implications for the design and alignment of lower-limb prostheses as well as post-amputation training paradigms.