In the hope of boosting physical activity, Stanford University researchers are studying devices that people could strap to their legs to make running easier.
According to an article on Stanford University's website, the researchers are experimenting with motor-powered systems that mimic such devices--called exoskeleton emulators—in two different modes of running assistance: motor-powered assistance and spring-based assistance. The results were published March 25 in Science Robotics.
According to the scientists, the mere act of wearing an exoskeleton rig that was switched off increased the energy cost of running, making it 13% harder than running without the exoskeleton. However, wearing a motor-powered 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 that if the exoskeleton was powered to mimic a spring there was still an increase in energy demand, making it 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," said Steve Collins, associate professor of mechanical engineering at Stanford and senior author of the paper, in an article appearing on the university’s website. "We all have an intuition about how we run or walk, but even leading scientists are still discovering how the human body allows us to move efficiently. That's why experiments like these are so important."
The frame of the ankle exoskeleton emulator straps around the user's shin. It attaches to the shoe with a rope looped under the heel and a carbon fiber bar inserted into the sole, near the toe.
The exoskeleton’s 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, the motors tug a cable that runs through the back of the exoskeleton from the heel to the calf. With action similar to a bicycle brake cable, it pulls upward during toe-off to help extend the ankle at the end of a running step.
The researchers had 11 experienced runners test both assistance types while running on a treadmill. They also completed tests where they wore the hardware without any of the assistance mechanisms turned on.
During testing, the researchers measured the runners' energetic output through a mask that tracked how much oxygen they were breathing in and how much carbon dioxide they were breathing out. The energy savings the researchers observed indicate that a runner using the powered exoskeleton could boost their speed by as much as 10%. That figure could be even higher if runners have additional time for training and optimization.
Given the considerable gains involved, the researchers think it should be possible to turn the powered skeleton into an effective untethered device.