Design optimization of an above-knee prosthesis based on the kinematics of gait

A dynamic model of an above-knee prosthesis during the complete gait cycle was developed. The model was based on a two-dimensional multi-body mechanical system and included a hydraulic and an elastic controller for the knee and a kinematical driver controller for the prosthetic ankle. The equations of motion were driven using Lagrange method. Simulation of the foot contact was conducted using a two-point penetration contact model. The knee elastic and hydraulic controller units, the knee extension stop, and the kinematical driver controller of the ankle were represented by a spring and a dashpot, a nonlinear spring, and a torsional spring-damper within a standard prosthetic configuration. The hip trajectory and net joint moment were considered as the initial conditions of the coupled differential equations. Design optimization of the prosthesis, to achieve the closest knee flexion pattern to that of the normal gait, resulted in a good correlation; the average differences with normal data were 3.3 and 3.4 deg for prosthetic knee and ankle joints, respectively. A parametric study showed that both increase and decrease of the stiffness by 50% caused an earlier knee flexion in stance phase and a lower knee flexion in swing phase. The effect of hydraulic controller damping coefficient on the flexion pattern of the prosthetic knee and ankle was only significant in the swing phase of the gait cycle.