Working by night in a Boulder, Colorado, cabin, Rifkin built something that combined the natural step of a bionic foot with the simplicity and low cost of a mechanical prosthetic. His jointed foot has a heel, a forefoot, a big toe—and no joint at the ankle. Instead, a novel midfoot joint, which connects the heel and forefoot, does the job of both the ankle and the arch. Like an ankle joint, it flexes up and down to give the wearer a more natural step. And, like a real midfoot joint, it creates a flexible arch in the middle of the foot. A spring and cable connect it to a second joint at the toe, to create extra push-off at the end of each step. Other tensioned steel cables serve as the tendons and ligaments that govern its range of motion—the user doesn't control it, it simply responds to the pressure of walking. Because the front and back of the foot can move independently, it can react to uneven terrain.