PowerFoot: The Next Step in Ankle-Foot Prostheses

By Laura Calverley

Garth Stewart, a retired Army specialist, lost his left foot and part of his leg four years ago while serving in Iraq. This summer, 24-year-old Stewart tested the PowerFoot, a new powered ankle-foot prosthesis developed by researchers at the Massachusetts Institute of Technology (MIT), Cambridge. The prototype device uses the latest advances in lower- limb prosthetics, including tendon-like springs, microprocessors, and a power source, to replicate natural motion. Stewart says the best thing about the device is that it improves his posture while walking and lets him walk faster than his current high-tech prosthesis allows.

The PowerFoot is designed to provide propulsion for walking much like a biological ankle, allowing the wearer to walk more naturally and with less fatigue.

Hugh Herr, PhD, associate professor at MIT, and a team of researchers at MIT's Biomechatronics Research Lab, of which Herr is the director, created the PowerFoot prosthesis. Herr, who is a bilateral transtibial amputee himself, having lost both legs to frostbite 25 years ago, has tested the device.

Garth Stewart. Photographs by Mary Beth Meehan Courtesy of the Providence (Rhode Island) Veterans Affairs Medical Center.
Garth Stewart. Photographs by Mary Beth Meehan Courtesy of the Providence (Rhode Island) Veterans Affairs Medical Center.

"This new technology will dramatically lower the amount of energy a person requires to walk. In some patients, it will also increase their walking speed. It offers better shock absorption going down steps, for example, and across various terrain surfaces," says Herr.

The PowerFoot is equipped with multiple springs and a small, battery-powered motor. Energy from the user's forward motion is stored in a tendon-like spring and then released, with additional mechanical energy as the foot pushes off. Because conventional prostheses only provide a passive response when walking, the user expends more energy - about 30 percent more - and moves with an unnatural gait.

"In a normal prosthesis, the ankle doesn't really do anything," explains Stewart, who wore the new device during demonstrations at MIT and Brown University, Providence, Rhode Island. "The ankle is a spring, but because it doesn't push off, you have to lean forward into the walk, and after a while that puts a strain on your lower back."

The PowerFoot addresses several issues common among amputees: slow walking; difficulty walking on irregular surfaces; and strain on the leg muscles and back. The device is designed to work harder as a person walks faster. It monitors the speed of the amputee and adapts accordingly. It also varies its force, adjusting for irregular surfaces.

"The prosthesis is actually robotic, meaning it has its own energy supply and is able to move based on its own brain,'?" explains Herr. "There's a microprocessor on the prosthesis that receives sensory information and then makes a decision about how the prosthesis should move."

The energy supply is a small battery attached to the back of the ankle. It is expected to last through 10,000 steps, or one day, before it needs to be recharged. According to MIT, although the PowerFoot is two times heavier than conventional devices, the battery's power helps increase an amputee's "metabolic walking economy."

"Before, with conventional technology, I had to walk the prosthesis," says Herr. "But now, they walk me. It feels remarkable - like the moving walkway at the airport."

Development and Benefits of PowerFoot

Herr created the device through the Center for Restorative and Regenerative Medicine (CRRM), a collaboration between MIT; the Providence, Rhode Island, Veterans Affairs Medical Center; and Brown University. CRRM was created in 2004 with a $7.2 million grant from the U.S. Department of Veterans Affairs (VA), and its goal is to conduct research into restoring natural function to amputees. According to Robert Swift, PhD, MD, associate chief of staff for research at the Providence Veterans Center, the VA has had a large role in the development of this device and others.

"The VA has long been committed to rehabilitation for obvious reasons&. Many soldiers come back with injuries and require rehabilitation, so improving methods for recovery and rehabilitation is very important," Swift says.

The VA has been especially concerned about the number of veterans returning from Afghanistan and Iraq with serious limb injuries, according to Swift. Advances in foot-ankle systems mean faster recovery for many of these veterans.

"One of the differences in this current war over some of the previous wars is that people are surviving injuries that formerly were fatal [because of advances in battlefield medicine and armor]," Swift says. "I think the ratio of the number of people surviving severe injuries compared to the number who die from those injuries is 12-to-1 in the Iraq war, and it was approximately 5-to-1 in Vietnam."

PowerFoot's Next Step

Herr is conducting an expanded patient trial (eight patients have tested it thus far), and a limited launch of the product is scheduled for March 2008. The PowerFoot will be commercialized by iWalk, Cambridge, Massachusetts, and is expected to be available during the summer of 2008.

Herr says his next plan for the ankle-foot involves implants into muscles that may help control the artificial limb. The ultimate goal of the CRRM is the development of "bio-hybrid" limbs, artificial limbs composed of biological and man-made materials.

For more information on the PowerFoot, visit www.iwalkpro.com

Laura Calverley is a freelance writer living in southeastern Massachusetts. She can be reached at lcalverley@aol.com

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