West Point Bionic Foot Achieves Powered Running

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By Morgan Stanfield
Morgan Stanfield

Over the past few millennia, lower-limb prostheses have evolved by leaps and bounds. We know that the oldest-known surviving ones, Egyptian big toes made of linen and glue, were followed by items such as the bronze Roman Capua leg, European variations on peg legs, and J.E. Hanger's articulated wooden limb. According to a state-of-the-science article by Terry Supan, CPO, FAAOP, FISPO, written for the American Academy of Orthotists and Prosthetists (the Academy), the past 30 years have seen the greatest strides in foot design, from the SACH foot to carbon-fiber dynamic response feet. (Author's note: For more information, see "Clinical Perspectives on Prosthetic Ankle-Foot Designs." from JPO 2005 Vol. 17, Num. 4S) Ossur's microprocessor-controlled Proprio Foot signaled the beginning of the newest cycle of innovation, feet with microprocessor controls. Now, powered feet such as the PowerFoot One by iWalk, Cambridge, Massachusetts, and the Odyssey ankle by private company Spring­Active, an Arizona State University (Arizona State), Tempe, spinoff, are attracting increasing attention in the profession. As advanced as these latest efforts are, none are designed to facilitate that most primitive of human activities, running.

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Left to right, the first and second generation West Point Bionic Foot. The second motor on the current iteration enables running.

Now, a cooperative effort among Arizona State researcher Tom Sugar, PhD, SpringActive; the U.S. Army's Telemedicine and Advanced Technology Research Center (TATRC); and a student engineering team from the United States Military Academy at West Point (West Point), New York, has proved that powered running is indeed possible. The foot-ankle prosthesis in question, known as the West Point Bionic Foot, is the product of TATRC-funded technologies used in the SPARKy ankle as well as ideas that the West Point team developed under the direction of Lt. Col. Joseph Hitt, PhD. (Editor's note: For more information about the development of SPARKy see "'Robotic Tendon' Puts Spring Back in Steps," in the August 2007 issue of The O&P EDGE.)

On April 23, 2010, a Discovery Channel film crew recorded a military-amputee volunteer using the West Point Bionic Foot to run on a treadmill at a self-selected speed of 8.1 miles per hour—a crisp 7.5-minute mile pace—which may be the fastest recorded gait for any powered foot. The volunteer is shown running on the treadmill belt, then seamlessly hopping off the belt to stand on the treadmill's sides when the test stops. According to Hitt, the device not only enables the wearer to run briskly and stop instantly but it also enables the wearer to smoothly ascend and descend stairs and inclines.

This military-amputee volunteer runs on a treadmill at a self-selected speed of 8.1 miles per hour while demonstrating the West Point Bionic Foot. Photographs courtesy of Paul Tubizidy, U.S. Military Academy (West Point).

Hitt, who was the lead doctoral researcher on Sugar's team before graduating and taking his current position at West Point, says the foot was built with the intention that its technology would lead to the development of "a soldier-ready device with high-performance characteristics." He adds, "We would like a single device to support a soldier in walking and running up and down hills, through various terrain and environmental conditions, and be robust enough to survive the operating environment." The team also believes it can leverage the technology in devices such as powered orthoses and assistive/rehabilitation devices.

Getting Up to Speed

According to TATRC portfolio manager Troy Turner, the leg is powered by two lightweight brush motors that each crank a tensioning spring. Hitt says that the leg's unique capabilities arise from its use of two mechanical principles: regenerative kinetics and shank-based control.

Regenerative kinetics is the same principle that powers pogo sticks, Hitt says. He explains, "When you jump on a pogo stick, it stores your kinetic energy in its spring. However, there's not enough energy in the spring to return you to your previous height, so your leg muscles have to add more energy to the spring to get all the way back up. That's exactly what our device does—during stance phase, the person rotates their leg over the stance foot, pulling on the spring. The motors then provide the...little bit of additional oomph from the other end of the spring to build the peak power required for walking and running."

A test-user takes the first-generation West Point Bionic Foot for a walk.

Because each spring is attached to the motor on one end and the prosthesis on the other, the foot is highly ground-compliant. Compliance also simplifies the problem of control, Hitt notes.

"Let's say the person accidentally walks on a rock," he says. "Since we have compliance in the system to absorb the impulse of that rock, we don't have to have sophisticated sensors and computer algorithms to detect the rock and move the motor—we just allow the spring to absorb those irregularities. That allows us to have a system that is much more robust, simple, and cheap."

Elegant Control

Hitt emphasizes that the credit for the foot's "very, very elegant" control mechanism belongs to Sugar's team at Arizona State and SpringActive.

"In my mind, [the shank-based control mechanism] is completely revolutionary over any other control that I've seen in the new field of powered prosthetics," Hitt states. "The foot manages all this control with one [solid state MEMS] rate gyro. It's a tiny and simple sensor—the size of a dime—that measures the velocity of the limb and the position of the limb with respect to...gravity."

Hitt stresses, "[Other research teams] code their powered prosthetics with rules for various actions, basically making it so that when their foot stops, its sensor says the foot needs to go into a certain position and stop in a certain way. Thanks to an observation that Tom Sugar's team made about human movement, we don't have to develop those elaborate rules. Our only rule is based on a function of the position and the velocity of the limb, which tells the ankle to be in a certain place [according to certain stimuli]."

Modular Muscle

The West Point Bionic Foot has not only unprecedented control and speed, but diverse configurations. The motorized spring unit is modular and intended to be easily removed from the foot.

"The foot won't be heavy with the spring and motor in," Turner says, "but if the battery runs out or the user just wants a much lighter foot or doesn't need much energy return, they don't have to walk around the house or office with it in—they can just remove it."

The final version of the motor-spring complex is intended to be housed in a self-contained waterproof case that is shaped like a human-sized calf muscle. To keep costs down, it will be driven by off-the-shelf lithium polymer batteries, and to keep the prosthesis silent, its motor will be internally lubricated.

Cadet Development

West Point Cadet Elijah Bales has been central to the development of the device, according to both Hitt and Turner. Bales spent a week at SpringActive, followed by a month-long summer internship at TATRC during which time he traveled to various research sites to better understand the historical and technological context of the design. He then drafted many new designs to improve the foot. Now that school has begun for the fall term, Bales and a team of four other cadets are currently working to add the capacity for inversion and eversion to the foot, which currently can only dorsiflex and plantarflex.

The foot's next milestone will be a major one. On April 28, 2011, West Point will host an event in which seniors have the opportunity to present their capstone projects. Bales and his teammates plan to showcase the foot by having a military-amputee wearer use the foot while taking the Army Physical Fitness Test (APFT).

Hitt concludes that just as much as the foot holds the potential to return injured war fighters to their full physical capacity, "In terms of cadet development, this project has been a world-class opportunity to provide a relevant problem with a human dimension that has been incredibility motivating for the cadets."

Morgan Stanfield can be reached at