Disclosed herein a prosthetic or orthotic device having a joint portion and a compliant transmission assembly in operational communication with the joint portion. The device restores the normal capabilities and natural dynamics of a healthy joint for common activities. The compliant transmission assembly includes a compliant element. The compliant transmission assembly absorbs energy when a torque is applied between a prosthetic or orthotic device portion and another adjacent device portion or the adjacent limb segment of the user. The compliant element of the invention absorbs energy during flexion of a joint for the dampening thereof and releases this energy during extension of the joint for assistance thereof. Also disclosed herein is an actuator assembly, a torque sensor and an actuator locking device as well as a method for determining the torque of such prosthetic and orthotic devices.