Position of the Quadriceps Muscle Actuator Influences Knee Loads during Simulated Squat Testing

INTRODUCTION Since the introduction of total knee replacement (TKR) in the 1970s, a variety of devices have been created to test the mechanics of prosthetic knee components. One of these, the ‘Oxford Rig’ simulates knee flexion with a universal joint at the hip, a ball joint at the ankle, and pelvis stage that slides on vertical rails. This simulator provides an indication of knee function during everyday activities that occur under quadriceps load, such as rising from a chair or climbing stairs, while permitting six degrees of freedom each at the tibiofemoral and patellofemoral articulations [1]. Several investigators have published studies of knee mechanics using variants on the original Oxford Rig design. While these design iterations have maintained 12 degrees of freedom for the knee joint, the fixation point of the actuator intended to stand in for quadriceps muscles has often been varied. While the actuator in the original Oxford Rig was attached to the femur [2], quadriceps actuators have also been fixed to the moving pelvis stage [3] and to the ground [4].