Could this ankle exoskeleton help stroke survivors with mobility problems?April 10, 2015
Previous simulations of human locomotion - the act of walking - have suggested that when we walk on level ground and at a steady speed, theoretically, there should be no expenditure of power at all. But scientists also know that people expend more energy during walking than in any other daily activity.
As this expenditure of energy can cause problems for elderly people or people with mobility issues, scientists have long been concerned with trying to engineer exoskeletons that could make walking easier, but the consistent barrier to realizing this has been the difficulty of how to improve walking without adding an external power source.
Another problem with designing such a device was that placing heavy objects on the legs creates an initial penalty that increases energy expenditure.
Consequently, many engineers, who had grappled with the concept for decades, were convinced that such a project was impossible.
Researchers took 8 years to solve a problem that is over 100 years old
Although this kind of engineering has come a long way since the 1890s, when inventors first tried to boost walking efficiency by using rubber bands, current unpowered exoskeleton models have been unable to cut down energy expenditure. Biomechanists are still not even sure whether the famous running "blades" worn by disabled athletes such as Oscar Pistorius are more energetically efficient than human feet.
Now, in the journal Nature, the Carnegie Mellon and NC State researchers describe how they spent 8 years developing their new, unpowered exoskeleton - which was first conceived on a whiteboard by co-authors Steve Collins and Greg Sawicki while they were graduate students together at the University of Michigan in 2007.
Collins, an assistant professor of mechanical engineering at Carnegie Mellon, says:
"Walking is more complicated than you might think. Everyone knows how to walk, but you don't actually know how you walk."
Key to the success of the new project was the careful attention the researchers paid to how a device might be able to relieve the calf muscle when it was not engaged in productive work.
The team was interested in ultrasound imaging studies that revealed the calf muscle exerts energy not just when it is propelling the body forward, but also when it is holding the Achilles tendon taut - likened by the researchers to a "clutch action."
How to solve the problem of the calf muscle constantly producing force
"Studies show that the calf muscles are primarily producing force isometrically, without doing any work, during the stance phase of walking, but still using substantial metabolic energy," Collins explains. "This is the opposite of regenerative braking. It's as if every time you push on the brake pedal in your car, you burn a little bit of gas."
Therefore, Collins, Sawicki and colleague M. Bruce Wiggin designed their exoskeleton to "offload" some of the calf's clutching muscle force, which they found reduced the overall metabolic rate. To reduce the energy penalty of placing heavy objects on the legs, they fashioned the exoskeleton using ultra-light - yet "rugged and functional" - carbon fiber.
Collins believes that the exoskeleton could be particularly beneficial for people with permanent after-effects of stroke. "We're still a little ways away from doing that," he admits, "but we certainly plan to try."
The team intends to trial the exoskeleton among a people with a variety of mobility issues, so that they can best tailor the designs to different patient groups.
"As we understand human biomechanics better, we've begun to see wearable robotic devices that can restore or enhance human motor performance," says Collins. "This bodes well for a future with devices that are lightweight, energy-efficient and relatively inexpensive, yet enhance human mobility."