The prosthesis provides controllable torque about the ankle axis or axes. The keel(s) are controlled by an external actuator unit throughout the gait cycle, while a passive heel spring provides compliance in the early stance phase.The amount of applied torque is measured with a load cell, and the joint angle is measured with an optical encoder. The external actuator unit is an off-the-shelf Humotech subsystem consisting of a servo-motor, a servo-drive and a drive spindle. Torque is transmitted to the prosthesis via a bowden cable: a cable guided inside a flexible sheath, similar to bicycle brake cables. Off-board actuation enables the emulator to mimic a very wide variety of prosthetic foot & ankle types, while keeping the wearable end-effector very light weight.