Lower limb prosthesis

1. A lower limb prosthesis, comprising:

a foot member;

a main body rotatably coupled to the foot member at a joint comprising a joint axis, wherein the main body comprises a housing, an actuator, and a transmission, and wherein the actuator is configured to transmit an actuator torque to the transmission, and the transmission is configured to transmit a final torque to the foot member;

a spring coupled to the foot member and the main body, wherein the spring is configured to apply a spring force to the foot member, and wherein the spring force acts in parallel to the final torque; and

an engagement mechanism configured to engage and disengage the spring, wherein when the spring is engaged, the spring is configured to apply the spring force to the foot member, and when the spring is disengaged, the spring is not configured to apply the spring force to the foot member.

2. The lower limb prosthesis of claim 1 , wherein when the spring is engaged, the spring engages the main body at an engagement position.

3. The lower limb prosthesis of claim 2 , wherein the engagement mechanism is further configured to adjust the engagement position.

4. The lower limb prosthesis of claim 3 , wherein adjusting the engagement position changes a neutral position of the lower limb prosthesis, and wherein the neutral position of the lower limb prosthesis is an angular position of the foot member relative to the main body at which the engaged spring is in equilibrium.

5. The lower limb prosthesis of claim 3 , further comprising a controller in electronic communication with the engagement mechanism, wherein the engagement mechanism is configured to engage the spring, disengage the spring, and adjust the engagement position in response to a signal from the controller.

6. The lower limb prosthesis of claim 5 , further comprising an engagement position sensor configured to provide an engagement position signal to the controller indicating the engagement position of the spring.

7. The lower limb prosthesis of claim 5 , further comprising a torque sensor configured to provide a torque signal to the controller, wherein the controller is configured to control the engagement mechanism in response to the torque signal.

8. The lower limb prosthesis of claim 5 , further comprising an absolute encoder configured to provide a position signal to the controller indicating the position of the foot member relative to the main body, and wherein the controller is configured to control the engagement mechanism in response to the position signal.

9. The lower limb prosthesis of claim 3 , wherein the engagement mechanism comprises a track, and wherein a portion of the spring is configured to slide along the track and reversibly lock into an engagement position on the track.

10. The lower limb prosthesis of claim 3 , wherein the engagement mechanism comprises:

a chamber at least partially filled with a hydraulic fluid;

a piston within the chamber, wherein the piston separates the chamber into a first side and a second side;

a piston rod connecting the piston to the spring; and

a valve fluidly connected to the first side and the second side, wherein the valve is configured to be opened to allow hydraulic fluid to move between the first side and the second side and closed to block hydraulic fluid from moving between the first side and the second side, and wherein the piston is slidable within the chamber when the valve is open and fixed relative to the chamber when the valve is closed.

11. The lower limb prosthesis of claim 10 , wherein the spring is in the engaged state when the valve is closed, and the spring is in the disengaged state when the valve is open.

12. The lower limb prosthesis of claim 11 , wherein the valve is a solenoid valve.

13. The lower limb prosthesis of claim 12 , further comprising a reservoir configured to contain a variable volume of hydraulic fluid in order to compensate for changes in a volume of hydraulic fluid in the chamber.