Ankle Exoskeleton Helps Boost Physical Activity

In an attempt to increase and perhaps create a new kind of transportation, Stanford University engineers are analyzing devices that individuals can attach to their legs to make their running easier.

In experiments involving motor-powered systems that imitate such kinds of devices—known as exoskeleton emulators—the scientists inspected two different modes of running assistance: that is, spring-based assistance and motor-powered assistance. The study results, published in the Science Robotics journal on March 25^th, 2020, were truly astonishing.

The energy cost of running was increased just by the mere act of putting on an exoskeleton rig that was deactivated, rendering it 13% harder when compared to running without the exoskeleton.

But the experiments suggested that if the exoskeleton is suitably powered by a motor, it can decrease the energy cost of running, making it 25% easier when compared to running with the exoskeleton switched off and 15% easier when compared to running without the exoskeleton.

On the other hand, the study also indicated that energy demand would still increase if the exoskeleton was powered to imitate a spring, rendering it only 2% easier than the non-powered exoskeleton, while 11% harder than exoskeleton-free running.

When people run, their legs behave a lot like a spring, so we were very surprised that spring-like assistance was not effective. 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.

Steve Collins, Study Senior Author and Associate Professor, Department of Mechanical Engineering, Stanford University

If upcoming designs are able to decrease the energy cost of wearing the exoskeleton, then spring-like assistance at the ankle would slightly benefit runners. Such modes of assistance are anticipated to be less expensive than motor-powered alternatives.

Powering the Step

The frame of the ankle exoskeleton emulator fastens around the shin of the user. This frame binds to the shoe with a carbon fiber bar inserted into the sole, close to the toe, and a rope looped under the heel.

The two modes of assistance are produced by motors located on the rear side of the treadmill (but not on the exoskeleton itself), although a spring-based exoskeleton would not really utilize motors in the end product.

The spring-like mode, as the term implies, imitates the effect of a spring that runs parallel to the calf, and stores energy during the start of the step and unloads that energy as the toes push off.

When the motors are in the actuated mode, they pull a cable that runs via the back of the exoskeleton extending from the heel to the calf. Using this action, just like a bicycle brake cable, the motors tug upward during toe-off to help extend the ankle towards 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. Speaking from experience, that feels really good. When the device is providing that assistance, you feel like you could run forever.

Delaney Miller, Graduate Student, Stanford University

Miller is currently working on these exoskeletons and also assisting in testing the devices.

These two types of assistance were tested by 11 experienced runners while they were running on a treadmill. The runners also completed tests in which they wore the hardware but did not switch on any of the assistance mechanisms.

Before testing, every runner had to become familiarized with the exoskeleton emulator— and its operation was also modified to fit in their gait cycle and phases.

During the real tests, the scientists quantified the energetic output of the runners via a mask that monitored the amount of oxygen being inhaled by the runners and the amount of carbon dioxide being exhaled by them. For each type of assistance, the tests lasted six minutes. The scientists based their results on the last three minutes of every exercise.

The energy savings calculated by the scientists suggest that a runner with the powered exoskeleton can potentially increase his or her speed by as much as 10%. If runners get more time for training and optimization, that percentage could be even greater. In view of the significant gains involved, the scientists believe that the powered skeleton could be converted into an effective untethered device.

The Future

By giving confidence, physical support, and perhaps increased speed, the scientists believe that such a technology could benefit individuals in many different ways.

“You can almost think of it as a mode of transportation,” stated Guan Rong Tan, a graduate student in mechanical engineering who, just like Miller, is continuing this study. “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.”

These are the largest improvements in energy economy that we’ve seen with any device used to assist running. So, you’re probably not going to be able to use this for a qualifying time in a race, but it may allow you to keep up with your friends who run a bit faster than you. For example, my younger brother ran the Boston Marathon and I would love to be able to keep pace with him.

Steve Collins, Study Senior Author and Associate Professor, Department of Mechanical Engineering, Stanford University

Researchers find that a motorized device that attaches around the ankle and foot can drastically reduce the energy cost of running. Video Credit: Stanford University.