Engineers increase walking efficiency with unpowered device
21 April 2015
It has taken millions of years for humans to perfect the art of walking, yet recent research, published in the journal Nature, shows that humans can reduce the energy cost of walking 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 seven per cent 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.
To gain an advantage over nature, North Carolina State University and Carnegie Mellon University researchers tested the effectiveness 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-fibre device weighs around 500g and is not motorised, so it requires 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,” said Dr Gregory Sawicki, a biomedical engineer and locomotion physiologist in the joint North Carolina State/University of North Carolina-Chapel Hill Department of Biomedical Engineering. “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,” he added.
The engineers showed that the metabolic rate of human walking can be reduced by an unpowered ankle exoskeleton. They built a lightweight elastic device that acts in parallel with the user’s calf muscles, off-loading muscle force and thereby reducing the metabolic energy consumed in contractions. The device uses a mechanical clutch to hold a spring as it is stretched and relaxed by ankle movements when the foot is on the ground, helping to fulfill one function of the calf muscles and Achilles tendon. Unlike muscles, however, the clutch sustains force passively. The exoskeleton consumes no chemical or electrical energy and delivers no net positive mechanical work, yet reduces the metabolic cost of walking by 7.2 ± 2.6% for healthy human users under natural conditions, comparable to savings with powered devices.
The study participants, nine able-bodied adults, strapped the exoskeleton devices on both legs and walked at a normal speed on a treadmill after completing some practice training. The same subjects also walked without exoskeletons for a baseline comparison. The researchers tested exoskeletons with springs that varied in stiffness. The spring that provided the most benefit was moderately stiff. Walking with exoskeletons with springs that were too stiff or too compliant resulted in normal or higher-than-normal energy costs for participants.
“A seven per cent reduction in energy cost is like taking off a 10-pound backpack, which is significant. Though it’s surprising that we were able to achieve this advantage over a system strongly shaped by evolution, this study shows that there’s still a lot to learn about human biomechanics and a seemingly simple behaviour like walking,” Sawicki concluded.
“Someday soon we may have simple, lightweight and relatively inexpensive exoskeletons to help us get around, especially if we’ve been slowed down by injury or ageing,” said paper co-author Dr Steven Collins, a mechanical engineer and roboticist from Carnegie Mellon University.
Improving upon walking economy in this way is analogous to altering the structure of the body such that it is more energy-effective at walking. While strong natural pressures have already shaped human locomotion, improvements in efficiency are still possible. Much remains to be learned about this seemingly simple behaviour, the researchers concluded. See below for more.
Steven Collins, Gregory S. Sawicki, M.B. Wiggin. Reducing the energy cost of human walking using an unpowered exoskeleton. Nature ( 2015). DOI: 10.1038/nature14288http://www.engineersjournal.ie/2015/04/21/engineers-increase-walking-efficiency-unpowered-device/http://www.engineersjournal.ie/wp-content/uploads/2015/04/Exoskeleton-walking-efficiency.jpghttp://www.engineersjournal.ie/wp-content/uploads/2015/04/Exoskeleton-walking-efficiency-300x300.jpgBioenergy,innovation,research