The next time you’re dashing across campus by foot 10 minutes late on your way to your 8 a.m., consider attaching a simple elastic band connecting your legs to improve your running economy.

According to recent research from UCSB mechanical engineering assistant professor Elliot Hawkes, adding a spring linking your feet can help you run more efficiently, reducing the energy needed by about 6.4%.

Running is the most energy-costly mode of locomotion for humans, according to Hawkes’ study published in the Journal of Experimental Biology. Less than one of every 10 calories spent is needed to maintain a constant forward velocity — essentially “wasting” the remaining energy.

“Running is inefficient because every step, you slow your body down as your foot hits the ground, then speed it back up as you push off. This consumes most of the energy. Also, every step, you have to slow your swinging leg down at the extents of the stride, then speed it back up. This consumes a measurable, but much smaller amount of energy,” Hawkes, who conducted the research while at Stanford University, wrote in an email.

While numerous devices have been designed to help increase efficacy for this rather inefficient human activity, Hawkes and his research group took a less-studied approach. Rather than focus on the primary motion of running when the foot hits the ground, they looked at improving efficiency through the leg swing — the smaller, secondary part of the action.

The concept is seen in nature, in which some animals have spring-like tissues or an “exotendon” believed to assist in leg swing.

The scientists realized that adding a light resistance band connecting a runner’s legs would produce this effect and “recycle the energy,” according to Hawkes. They tested this out by measuring energy consumption as runners used the device.

Above is a time-lapse photo showing one complete gait cycle of a runner using UCSB researcher Elliot Hawkes and Stanford researchers’ elastic “exotendon” to reduce energy costs during running. / Courtesy of Journal of Environmental Biology

The band works by applying assistive forces to offload the muscles that normally swing the legs, taking the brunt of the work needed to run.

“Then, with the stored energy in the spring, it helps speed [your legs] back up as they come together, again saving you energy,” Hawkes explained.

By slowing down the runner’s legs at the extents of the swing, the spring boosts the “energy optimal stride frequency,” according to the paper. This causes runners to take shorter and faster strides as they adopt this frequency. Shorter strides decreases biological joint moments and power, reducing the energy costs needed to redirect the runner’s center of mass when their feet hit the ground (during stance) while running.

Hawkes, who personally tried out the “exotendon,” expressed that it “feels great. It makes you feel fast and your legs feel light!”

The investigators initially attached the spring around a runner’s knees. However, they found that it was more comfortable and did not rub against the skin when the band was clipped onto the shoes. Additionally, having the device on a lower location on the body provides less exertion needed to produce the assistive forces to the legs. It also allows for more natural running movements.

The band they used was made from natural latex rubber surgical tubing. Set to 25% of the runner’s leg length, the researchers described this to be “long enough to avoid breaking and short enough to avoid tripping during running” in their study.

So far, Hawkes and his Stanford collaborators have experimented with the band during relatively slow speeds (10 min/mile) and on relatively flat surfaces. The “exotendon” has worked well under these conditions, including on city streets, without any reported tripping incidents. But it probably won’t work for trail running, Hawkes stated.

He expressed interest in testing the band’s effects at higher running speeds.

In addition to improving running economy, the spring shortens your stride, “which some people claim may help with injury prevention,” Hawkes said. However, he noted the research group has not looked into this concept.

In finding that the associated energy savings with using this low-cost, low-tech device can be “much larger than would be expected from savings directly associated with a low-expenditure component of gait,” Hawkes said that “sometimes very simple ideas can work well!”


Jacqueline Wen
Jacqueline serves as the Science and Tech Editor. She enjoys watching movies and learning about science and tech.