What a foot does
A natural foot does more than hold the body up. During a step, the heel absorbs the impact, the ankle rolls the body forward over the foot, and the toes push off. An artificial foot must imitate as much of that as it can using only materials and springs, since a passive foot has no muscles. Each major change in foot design has been an attempt to reproduce one more part of that cycle.
Pegs and rigid feet
For centuries, the usual substitute for a lower leg was a peg, a wooden pole strapped to the knee or thigh. A peg is stable and cheap but gives little help with balance on uneven ground. The wooden legs with carved feet that appeared in later periods looked more natural, but the foot was a fixed block with no flex at the ankle. The user walked with a stiff gait, swinging the leg around the side to clear the ground.
The nineteenth century brought hinged ankles. Makers connected the foot to the shank with a pin and added bumpers of rubber, leather or cork to control how far the foot could tip. A well-known nineteenth-century design linked the ankle and knee with cords, so that bending the knee caused the foot to lift. The aim was to prevent the toe from catching the ground during the swing.
Rubber feet
In the mid-1800s, makers began using rubber for feet and heels. Rubber gave some give at heel strike and reduced the hard knock of the wooden block. Vulcanized rubber, which does not turn sticky in heat or brittle in cold, made this practical, and American patents for rubber feet appear in the 1850s and 1860s. Their widespread use after the Civil War helped spread the idea that an artificial foot should absorb shock rather than stay rigid.
The cushioned heel
A major change came after the Second World War. Researchers at American universities and veterans’ programs studied how amputees walked and what made devices fail, and they found that complex jointed ankles were heavy, noisy and prone to wear. A simpler design, the solid-ankle cushion-heel foot, usually called SACH, was developed around the middle of the twentieth century. It used a rigid wooden keel inside a foam rubber body, with a soft wedge at the heel that compresses at heel strike to imitate ankle motion. It had no moving parts, was light and quiet, and was cheap to make.
The SACH foot became a standard around the world, and it is still used. It can be considered the baseline against which later designs are compared. Its limit is that it stores little energy and does not allow much variation in terrain.
The Jaipur foot
Different conditions led to different solutions. In India in the late 1960s, a team including the surgeon P. K. Sethi and the craftsman Ram Chandra Sharma developed a rubber foot that allowed users to walk barefoot, squat, sit cross-legged and walk on uneven ground, which many conventional Western feet did not allow. It could be made locally from simple materials at low cost. Its name comes from the city where it was developed, and it has since been fitted to a large number of people in South Asia and elsewhere. It is a good example of how a design shaped by local needs can be more useful than a technically sophisticated imported one.
Energy-storing feet
In the 1980s and 1990s, designers turned to composite springs. The Seattle foot, developed in the early 1980s, used a flexible keel that stored some energy during stance and returned it at push-off. A few years later, Van Phillips, an American who had lost a leg in a water skiing accident, developed a carbon-fiber design with a curved shape. Carbon fiber is strong, light and springy. These feet compress under the user’s weight and spring back, which gives a livelier step with less effort. Designs that followed added split toes and multiple layers.
Carbon-fiber blades for running, shaped like a long curve, appeared in the 1990s. Their growing use by athletes brought public interest, as well as arguments about whether a blade gives an advantage. In 2008, the Court of Arbitration for Sport overturned a ban that athletics’ governing body had placed on the South African sprinter Oscar Pistorius’s blades, and he later took part in the 2012 Olympic Games. The scientific questions about blades and performance have not been settled to everyone’s satisfaction.
Powered and adjustable ankles
More recent designs include hydraulic ankles that adjust to slopes and stairs, and powered ankles with a motor and lithium-ion batteries that add push at toe-off. Work at university laboratories in the 2000s, including a group at the Massachusetts Institute of Technology, led to powered ankle designs now sold commercially. Microprocessors control some versions, and in that sense the foot has joined the story of smart prosthetics.
Fit and materials
Feet are chosen by activity level, weight and body size. Manufacturers produce feet in a range of stiffness, and the 3D printing industry is now exploring custom foot shapes and keels. Whatever the material, the foot remains one part of a system that includes the socket, a pylon (often aluminum, titanium or stainless steel) and a knee, and a good fit at the socket usually matters more to comfort than the specific foot chosen. The history of the foot shows how much of walking can be copied by springs and rubber, and how much still depends on the human wearing the device.