Transtibial amputees are commonly prescribed passive-dynamic prostheses fabricated with carbon fiber materials that store and return energy. The design of these devices aims to replicate the function of the natural ankle-foot system (NAFS) during steady state walking.

However, a major limitation of passive-dynamic devices is that they cannot return more energy than they store, limiting the amount of net positive work that the device can perform. Active prostheses can be worn to address this limitation, but according to some researchers, motors or actuators may not be necessary in order to replicate the NAFS.

Combined ankle-foot power

To test this theory, researchers from the University of Delaware and the University of North Carolina looked at the total mechanical energy profiles of the combined ankle-foot system during walking, including distal foot structures, which are often overlooked.

“The purpose of the study was to look at the combined energy profiles, how much energy gets either stored or dissipated by the combined ankle-foot system, and see how much energy it generates to be able to assist the person in walking,” Steven J. Stanhope, PhD, professor of kinesiology and applied physiology at the University of Delaware, told O&P Business News. “And if it generates less energy than what it stores, then the overall work profile would be less than 1, and as a result of that, we can actually replicate it with a spring joint.”

The study included 11 healthy subjects who participated in a gait analysis using a six-camera motion system. They walked barefoot at four randomly assigned walking speeds, and strain gauge force platforms collected ground reaction force and center of pressure data.

The researchers found the ankle joint typically underwent a phase of predominantly negative power during early to midstance and a phase of positive power during late stance, while distal foot structures appeared to have primarily absorbed or dissipated energy during stance. They also found the ankle joint performed greater positive work than negative work during stance, and the magnitude of positive work systematically increased as a function of walking speed.

“We were surprised at the extent with which the foot takes energy out of the system, the magnitude of that, and the fact the distal foot powers get significantly more negative as you increase your walking velocity,” Stanhope said. “So the foot was continually taking out energy the ankle was actually putting in.”

They also found ankle joint and distal foot structures exhibit counteracting energy profiles such that the work-ratios of the NAFS are not greater than 1.0 across a range of walking speeds.

“The total system worked in a manner in which, theoretically, it could be replicated by a spring storage and release system,” Stanhope added.

Future designs

Other studies conducted by Stanhope and his colleagues have shown unilateral amputees using powered devices actually have to compensate for that added power by removing power from their sound leg. Although this added power is beneficial for situations such as walking up stairs, it may not be necessary for all users.

“In theory, we could design passive-dynamic prostheses for individuals who don’t need extra energy added into the system to be able to walk,” Stanhope said. “And it is likely that one of the most important factors associated with those prosthetic devices is tuning the stiffness of that device. So each person, depending on their height and weight, should have an ideal spring stiffness.”

Further studies are needed to explore the advantages of this, but according to Stanhope, 3-D printing would be an ideal platform for manufacturing these devices.

“We’re working on techniques to do central fabrication of rehabilitation devices, prostheses and orthoses that are precisely that stiffness-tuned,” Stanhope said. “We are looking at 3-D printing technology, because we realized that every device would be unique.” — by Megan Gilbride