Principles of obstacle avoidance with a transfemoral prosthetic limb.

In this study, conditions that enable a prosthetic knee flexion strategy in transfemoral amputee subjects during obstacle avoidance were investigated. This study explored the hip torque principle and the static ground principle as object avoidance strategies. A prosthetic limb simulator device was used to study the influence of applied hip torques and static ground friction on the prosthetic foot trajectory. Inverse dynamics were used to calculate the energy produced by the hip joint. A two-dimensional forward dynamics model was used to investigate the relation between obstacle-foot distance and the necessary hip torques utilized during obstacle avoidance. The study showed that a prosthetic knee flexion strategy was facilitated by the use of ground friction and by larger active hip torques. This strategy required more energy produced by the hip compared to a knee extension strategy. We conclude that when an amputee maintains enough distance between the distal tip of the foot and the obstacle during stance, he or she produces sufficiently high, yet feasible, hip torques and uses static ground friction, the amputee satisfies the conditions for enable stepping over an obstacle using a knee flexion strategy.