“You can almost think of it as a mode of transportation. You could get off a bus, slap on an exoskeleton, and cover the last one-to-two miles to work in five minutes without breaking a sweat.”— STANFORD UNIVERSITY GRADUATE STUDENT RESEARCHER GUAN RONG TAN
Stanford engineers are studying devices to make running easier. In experiments with exoskeleton emulators, researchers investigated two different modes of running assistance: motor-powered assistance and spring-based assistance.
Wearing a switched off exoskeleton made running 13% harder than without the exoskeleton. When powered by a motor, the exoskeleton reduced the energy cost of running, making it 15% easier than running without the exoskeleton and 25% easier than running with the exoskeleton switched off.
In contrast, the study suggested if the exoskeleton was powered to mimic a spring it was 11% harder than running exoskeleton-free and only 2% easier than the non-powered exoskeleton.
“When people run, their legs behave a lot like a spring, so we were very surprised that spring-like assistance was not effective,” says Steve Collins, associate professor of mechanical engineering at Stanford and senior author of a paper covering the results published in Science Robotics.
Powering your step
The frame of the ankle exoskeleton emulator straps around the user’s shin, attaches to the shoe with a rope looped under the heel, and has a carbon fiber bar inserted into the sole, near the toe. Motors produce the two modes of assistance.
The spring-like mode mimics the influence of a spring running parallel to the calf, storing energy during the beginning of the step and unloading that energy as the toes push off. In powered mode, motors tug a cable that runs through the back of the exoskeleton from the heel to the calf. It pulls upward during toe-off to help extend the ankle at the end of a running step.
“Powered assistance took off a lot of the energy burden of the calf muscles. It was very springy and very bouncy compared to normal running,” says Delaney Miller, a graduate student at Stanford working on and run-testing the exoskeletons.
Eleven experienced runners tested the two assistance types on a treadmill, with and without any assistance mechanisms turned on. Researchers measured runners’ energetic output through a mask tracking how much oxygen they were inhaling and how much carbon dioxide they were exhaling. Tests lasted 6 minutes; researchers based findings on the last 3 minutes of each exercise.
The energy savings observed indicate a runner using a powered exoskeleton could boost speed up to 10%, even higher with additional time for training and optimization.