Spring-based actuators are important in the design of wearable robotic systems. These actuators can store and release energy, and reduce the peak power requirements. Reducing these requirements allows the system to function with smaller and lighter-weight motors. Three actuators are compared: a lead screw actuator, a robotic tendon actuator, and a JackSpring™ actuator. The robotic tendon actuator adds a spring in series to the traditional actuator. The JackSpring actuator is a lead screw with a finite stiffness. A formal set of equations for the three actuators is added to Table 1 which summarizes the torque, angular speed, and power for each one. The traditional lead screw actuator cannot store and release energy and the power into the actuator must equal the power out of the actuator. The robotic tendon actuator stores and releases energy, and if a tuned spring is chosen, the power requirements can be greatly reduced. For example, if the desired external motion matches the natural frequency of the system, the motor does not need to rotate. The JackSpring actuator is a unique actuator because the stiffness and motion are coupled. It is shown that if the spring is tuned properly, the power requirements can be greatly reduced, as well.