'Avatar' experiments to help build better exoskeletons

Researchers are studying human locomotion from the leg up using an "Avatar"-like bio-robotic motor system which may lead to gen-next exoskeletons or prosthetic systems for people with mobility impairments.
North Carolina State University researchers are using an "Avatar"-like bio-robotic motor system that integrates a real muscle and tendon along with a computer controlled nerve stimulator acting as the avatar's spinal cord.
The findings could help create robotic devices that begin to merge human and machine in order to assist human locomotion, researchers said.
NC State biomedical engineer Greg Sawicki and Temple University post-doctoral researcher Ben Robertson found that if you know the mass, the stiffness and the leverage of the ankle's primary muscle-tendon unit, you can predict neural control strategies that will result in spring-like behaviour.

"We tried to build locomotion from the bottom up by starting with a single muscle-tendon unit, the basic power source for locomotion in all things that move," said Greg Sawicki, associate professor in the NC State and University of North Carolina at Chapel Hill Joint Department of Biomedical Engineering.
"We connected that muscle-tendon unit to a motor inside a custom robotic interface designed to simulate what the muscle-tendon unit 'feels' inside the leg, and then electrically stimulated the muscle to get contractions going on the benchtop," Sawicki said.

The researchers showed that resonance tuning is a likely mechanism behind springy leg behaviour during locomotion.

That is, the electrical system - in this case the body's nervous system - drives the mechanical system - the leg's muscle-tendon unit - at a frequency which provides maximum 'bang for the buck' in terms of efficient power output.
Sawicki likened resonance tuning to interacting with a slinky toy.
"When you get it oscillating well, you hardly have to move your hand - it's the timing of the interaction forces that matters," Sawicki said.
"In locomotion, resonance comes from tuning the interaction between the nervous system and the leg so they work together," Sawicki said.
"It turns out that if I know the mass, leverage and stiffness of a muscle-tendon unit, I can tell you exactly how often I should stimulate it to get resonance in the form of spring-like, elastic behaviour," he said.

The findings have design implications relevant to designing exoskeletons for able-bodied individuals, as well as exoskeleton or prosthetic systems for people with mobility impairments, researchers said.
The study is published in Proceedings of the National Academy of Sciences.