Setting of the study: Conventional prostheses for transfemoral amputees are mostly simple mechanic joints, or have micro-processor-controlled damping. The prototype tested in this study is equipped with a motor that can actively move the knee joint. Therefore, movements like, for example, alternating stair climbing, are possible that cannot be realized with pure mechanical and actively damped prostheses. During level-ground walking, it is hypothesized that less compensatory movements are needed. We have developed an actuated prototypical prosthesis and new control strategies which should facilitate both level-ground walking and stair climbing. Parts of the control strategies have been tested in able-bodied subjects [1]. This control strategy has been extended such that only biological stiffness modulation can be rendered [2].