A new robotic prosthetic leg prototype offers a more natural gait while also being quieter and more energy efficient than other designs. The key is the use of new small and powerful motors, originally designed for a robotic arm on the International Space Station. The streamlined design offers a free-swinging knee and regenerative braking, which charges the battery with energy captured when the foot hits the ground. This feature enables the leg to more than double a typical prosthetic user’s walk