Unpowered ankle exoskeleton takes a load off calf muscles to improve walking efficiency

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The passive-elastic ankle exoskeleton makes walking easier without the use of an external energy source (Photo: Stephen Thrift, North Carolina State University)

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We might have started off in the water, but humans have evolved to be extremely efficient walkers, with a walk in the park being, well, a walk in the park. Human locomotion is so efficient that many wondered whether it was possible to reduce the energy cost of walking without the use of an external energy source. Now researchers at Carnegie Mellon and North Carolina State have provided an answer in the affirmative with the development of an unpowered ankle exoskeleton.

The result of eight years of work begun by Steve Collins and Greg Sawicki when they were graduate students together at the University of Michigan in 2007, the device has been shown to reduce the metabolic cost of walking by around seven percent. The researchers claim this is roughly equivalent to taking off a 10-lb (4.5 kg) backpack and equates to the same savings provided by electrically-powered exoskeletons.

The researchers analyzed the biomechanics of walking and examined ultrasound imaging studies that showed the calf muscle not only exerts energy when pushing a person forward, but also when performing a clutch-like action to hold the Achilles tendon taut.

"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 explained. "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."

By offloading some of the clutching muscle forces from the calf to the passive-elastic device, the researchers were able to reduce the overall metabolic rate of walking. This is accomplished through the use of a mechanical clutch that produces force without consuming any energy. The clutch engages a spring in parallel with the Achilles tendon when the foot is on the ground to offload force from the calf, and disengages when the foot is in the air so as to avoid interfering with toe clearance.

To offset the initial penalty that sees an increases in energy costs when heavy objects are placed on the legs, it was imperative the device be kept very light, while still being rugged enough to withstand the rigors of walking. This was done through the use of carbon fiber, which resulted in the device weighing around 1 lb (450 g) per leg.

"You can imagine these lightweight efficient devices being worn on the affected limb to help people with the permanent aftereffects of stroke," Collins says. "We're hopeful that designs that use similar techniques can help people who have had a stroke walk more easily. We're still a little ways away from doing that, but we certainly plan to try."

Despite their success with the ankle exoskeleton, the team believes even bigger benefits could be seen with exoskeleton components for the knees and hip. They are hoping to examine such avenues in the future.

"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."

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