The majority of protheses available today that replace the lower leg, ankle, and foot are passive devices that store energy in an elastic element (similar to a coiled spring) at the beginning of a step and release during push-off to give you some added boost. While this type of prosthetic is energy efficient, it doesn't replicate the full power we get from our muscles. In order to provide that kind of energy an actuator is required, and these are often heavy and bulky. Researchers at Belgium's Vrije Universiteit Brussel have streamlined the technology in a device they call the AMP-Foot (Ankle Mimicking Prosthetic Foot).
The latest version, AMP-Foot 2.0, uses an actuator to store energy in springs, which is released when needed. By using a smaller, low-power actuator, the device is lighter and its batteries last longer, even though the actuator is essentially always busy. It is also one of the first prostheses to gather and store energy when the foot naturally bends upwards (towards the shin) during each step. A pair of force sensors (one in the heel and one at the toes) detect the leg's stance, so the locking device knows when to store and when to release its power.
The result is a prosthesis that, despite using actuators, weighs just 5.5 lb (2.5 kg), which is roughly the weight of a healthy foot. Currently it can provide enough power to replicate 100% of push-off for someone weighing up to 165 lb (75 kg) during normal walking on flat ground. It is currently being tested with amputees.
This isn't the only prosthesis with an actuated ankle, but the low power is key. For example, researchers at Arizona State University are working on one called SPARKy, but it relies on a 150W motor, whereas the AMP-Foot 2.0 uses just 30W to 60 W.
Edited from an article by Jason Falconer at Gizmag. Click on the link to watch a video of the AMP-Foot 2.0.