Prosthetic foot mimics real thing - prosthesis designed by Tufts School of Medicine researcher Mark R. Pitkin mimics the movement and characteristics of a real human foot - Brief Article

USA Today (Society for the Advancement of Education), June, 1997

A prosthetic foot that better mimics how a biological foot works has been developed by Mark R. Pitkin, research assistant professor of bioengineering, Tufts School of Medicine, Boston. Besides helping amputees maintain their balance and stand, walk, or run more naturally, the new design decreases the pressure that can cause painful and dangerous wounds to the area where the prosthesis is attached to the leg.

Current prosthetic feet are fairly good at shock absorption and propulsion or "push-off," but a natural foot also provides balance and control of gait, using 33 complex joints, 26 bones, and more than 100 ligaments, muscles, and tendons. That doesn't even count the foot's partnership with the knee and lower leg muscles, also missing in many amputees.

Pitkin analyzed the electric impulses in biological feet and lower leg muscles during a stance -- the part of a stride when all or part of the foot is on the ground. Using that data, he was able to pinpoint and measure the moments of resistance in the ankle joint produced by muscle and tendon contractions.

Pitkin noticed that pattern of resistance was non-linear and concave, with resistance almost nonexistent in the first part of the stance as the body leans forward and weight is shifted from the heel to the ball of the foot, followed by a sharp increase in resistance as the heel leaves the floor. He then mapped the pattern of resistance created by the most popular prosthetic feet. The moment of resistance comes too soon in most prosthetic feet," notes Pitkin. It does not match the physical reality of the gait."

What [Pitkin's] foot does is give resistance only when the amputee needs it," explains Hugh Herr, a Cambridge, Mass., biophysicist and an amputee who tested Pitkin's prototype. "With other prosthetics, if I lean forward. there's so much resistance that my heels come off the ground right away."

The two most common prosthetic feet on the market today either have no ankle mobility or allow the ankle to rock forward and then be abruptly stopped, short of where it would stop in a biological ankle. Pitkin's device. called a rolling-joint foot (RJF), allows for a progressive buildup of resistance in each stance. "The idea is the foot rolls along, producing resistance in a certain pattern," he says.

Previous designs have failed by missing this pattern. Other designs do not allow for energy reduction, either because they don't permit the ankle to flex at all or because they make it flex too much and too fast. "With these feet, a person expends 40% more energy than with a biological foot. There's a lot less compensatory movement needed with the RJF."