Tuning prosthetic foot stiffness to improve lower-limb amputee mobility




Shell, Courtney Elyse

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The foot, ankle and surrounding musculature play key roles in walking and maneuvering. They provide not only body support but also forward propulsion, terrain adaptation, and contributions to mediolateral balance control. Prosthetic feet have previously been designed to provide body support and, to some degree, forward propulsion and sagittal-plane terrain adaptation. However, there has been little exploration of how prosthesis design affects performance in tasks that challenge mediolateral balance, such as turning and walking on cross-slopes. In Chapter 2, the effects of prosthetic foot stiffness on unilateral transtibial amputee gait during turning were explored. Changes found in sagittal-plane kinematics and kinetics caused by more compliant prostheses were similar to those seen previously in studies of straight-line walking. This included decreased body support, increased residual limb propulsion and greater limb flexion. Mediolateral balance, measured by peak-to-peak range of whole-body angular momentum, improved with decreasing stiffness, but adaptations in coronal-plane angles, work and ground reaction force impulses were less systematic. In Chapter 3, forward dynamics simulations of a unilateral transtibial amputee stepping on a cross-slope were used to identify optimal coronal-plane stiffness profiles that improved balance control by decreasing changes in coronal-plane whole-body angular momentum. Profiles that decreased these changes were identified for ankle-inverting and ankle-everting cross-slopes as well as level ground. The change in coronal-plane whole-body angular momentum decreased with an increase in stiffness for ankle-everting cross-slopes but with a decrease in stiffness for ankle-inverting cross-slopes and level ground. Stiffness profiles influenced mediolateral balance control through the medial GRF, but were specific to each surface type. These results highlight the need to identify the surface type encountered (level, ankle-inverting or ankle-everting cross-slope) so that the stiffness profile appropriate for the surface can be set. To that end, in Chapter 4, measurements from the residual limb useful for predicting a cross-slope with a pattern recognition algorithm were identified. Residual limb kinematics, especially measurements from the foot, shank and ankle, were found to successfully predict the surface type with high accuracy. These studies have provided rationale and foundation for designing prostheses that help maintain mediolateral balance control when encountering turning or uneven terrain.


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