Study: Exoskeleton adds ‘spring’ to step, increases walking efficiency

A recently published study that humans can get better ‘gas mileage’ by using an unpowered exoskeleton to modify the structure of their ankles. The device puts an extra spring in each human step, reducing metabolic energy consumption by 7% below walking in normal athletic shoes.

The finding may benefit both able-bodied people who are frequently on their feet as well as those who have been victims of stroke or other gait impairments.

Greg Sawicki, PhD, displays exoskeleton devices, including ones that increase human walking efficiency by 7% in a study published in Nature.

Source: Marc Hall, NC State University


North Carolina State University and Carnegie Mellon University researchers tested the efficacy of a lightweight lower-leg device that uses a spring and clutch system working in tandem with calf muscles and the Achilles’ tendon while people walk. The streamlined, carbon-fiber device was not motorized, so it required no energy from batteries or other external fuel sources.

“The unpowered exoskeleton is like a catapult. It has a spring that mimics the action of your Achilles’ tendon, and works in parallel with your calf muscles to reduce the load placed upon them,” Gregory Sawicki, PhD, biomedical engineer and locomotion physiologist in the joint NC State/University of North Carolina-Chapel Hill Department of Biomedical Engineering and co-author of the paper, stated in a press release. “The clutch is essential to engage the spring only while the foot is on the ground, allowing it to store and then release elastic energy. Later it automatically disengages to allow free motion while the foot is in the air.”

The 9 able-bodied adults in the study strapped the exoskeleton devices on both legs and walked at a normal speed on a treadmill after completing some practice training. The same adults also walked without exoskeletons for a baseline comparison.

The researchers tested exoskeletons with springs that varied in stiffness and found a moderately stiff spring provided the most benefit. Walking with exoskeletons with springs that were too stiff or too compliant resulted in normal or higher-than-normal energy costs for participants.

“A 7% reduction in energy cost is like taking off a 10-pound backpack, which is significant,” Sawicki stated. “Though it is surprising that we were able to achieve this advantage over a system strongly shaped by evolution, this study shows that there is still a lot to learn about human biomechanics and a seemingly simple behavior like walking.”

“Someday soon we may have simple, lightweight and relatively inexpensive exoskeletons to help us get around, especially if we have been slowed down by injury or aging,” Steven Collins, PhD, a mechanical engineer and roboticist from Carnegie Mellon University and paper co-author, stated in the release.


Collins S, et al. Nature. 2015; doi:10.1038/nature14288.

Disclosures: Sawicki received funding for the research from grants provided by the NC State Faculty Research and Professional Development Fund; the NC State Chancellors Innovation Fund; Grant No. 2011152 from the U.S.-Israel Binational Science Foundation; and Award Number R01NR014756 from the National Institute of Nursing Research of the National Institutes of Health. Collins received funding for the research from the National Science Foundation under Grant No. IIS-135571.

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