Disturbance rejection in the heel contact phase of gait.

Abstract

Compared to other functional deficiencies of Above-Knee (A/K) amputee prostheses, shock-absorption and disturbance rejection characteristics have attracted little attention. The walking process has been linked to creation of shock and impact type loading. This type of loading occurs during the transfer of body weight from the rear to forward foot at the heel contact (HC) phase. This abrupt loading of the forward limb demands proper shock attenuation by the legs. In this work, the effects of rotational degrees of freedom and joint stiffness and damping are analyzed with regard to shock absorption and disturbance rejection characteristics. These characteristics are related to the ratio of the magnitudes of created hip accelerations to that of imposed heel accelerations. This ratio is termed the transmission factor (TF). Mathematical models are used to characterize and compare to the normal and prosthetic leg. Characterization of the prosthetic leg requires the determination of dynamical parameters including that of the artificial feet. Models using these parameters predict that the prosthetic HC dynamics are predominantly in the axial direction as opposed to the flexing mode of the normal legs. The rotational freedom of the ankle joint proved to be the key factor in reducing bandwidth of the disturbance rejection and the peak TF. A parametric study of the knee and ankle joints stiffnesses reveals the importance of the small ankle stiffnesses for obtaining small TF. The knee stiffness has a relatively minor effect. At prosthetic HC a resonance phenomenon produces theoretical TF values of 3 and 7 for SACH and Springlite artificial feet respectively. It is concluded that disturbance rejection properties of conventional prosthesis with the tested feet are insufficient. It is suggested that the peak transmission factors can be reduced by increasing the damping in the prosthetic heel and that future prosthesis designs should utilize increased rotational degrees of freedom.