Wearable Robotics Only Shifts Stress to Other Parts of Body, It Does Not Eliminate It

A study finds that wearing a commercially available exoskeleton does not turn one into a superhero. Researchers have tested a mechanical arm attached to a harness, which is usually worn by workers to help them lug heavy objects hands-free.

For a study at The Ohio State University, an experimental subject operates a hand tool while wearing an exoskeleton. (Image credit: Eric Weston, courtesy of The Ohio State University)

The researchers report in the journal Applied Ergonomics that the exoskeleton relieved stress on the arms just as it was meant to—but it increased stress on the back by over 50%.

There are tradeoffs with all currently-available exoskeletons, because they innately modify the way one moves, said William Marras, Director of The Ohio State University Spine Research Institute and Honda Chair Professor of Integrated Systems Engineering at Ohio State.

The simplest way to describe it is like dancing with a really bad partner. Someone is tugging and pulling on you in directions you’re not expecting, and your body has to compensate for that. And the way you compensate is by recruiting different muscles to perform the task.

William Marras, Director of The Ohio State University Spine Research Institute

For the research, 12 volunteers used two diverse pneumatic tools, an impact wrench and a torque wrench, as they might in industry. They used the wrenches with and without the assistance of the exoskeleton.

The torque wrench weighed around 10 pounds, while the weight of the impact wrench was 30 pounds. When participants put on the exoskeleton, the wrenches were supported by the mechanical arm, which conveyed the weight to a vest-like harness. The participants then only had to clutch the wrench and move it forward or up as they might to screw bolts in a factory.

Over a period of a few hours, the researchers measured the forces on the volunteers’ spine and back muscles. They discovered that wearing the exoskeleton increased compressive spinal loads up to almost 53% compared to not wearing it. Stress on various muscles in the torso increased anywhere between 56% and 120% while wearing it.

This exoskeleton is meant to offload weight from your arms, so for your arms it’s great. The problem is, the weight of the tool, the weight of the mechanical arm and the weight of the vest you’re wearing - that all goes to your back. At the end of the day, you’re just trading one problem for a potentially even worse problem.

Gregory Knapik, Senior Researcher at the Institute

The volunteers didn’t seem to realize the additional strain on their backs, but they did notice that they were not comfortable, mainly because of the rigid metal rods that lined the harness and prevented them from moving as usual.

You see people wearing this same exoskeleton all the time - workers in industry, camera people at sporting events - so you’d think they’d be more comfortable. But, no. People hated it for the short time that they wore it. Every single person said they would never wear this if they didn’t have to.

Gregory Knapik, Senior Researcher at the Institute

Given that the participants of the study had to wear the harness for just part of a day, the researchers believe that the stresses would be higher for someone who had to wear it for a whole work shift, on a daily basis.

The maker of this specific exoskeleton is aware that it can cause back fatigue. Like manufacturers of similar products, it suggests that users undergo muscle conditioning to prevent injury while using it.

Passive exoskeletons, like the one tested in this research, comprise springs and braces to help support areas of the body. Active exoskeletons, like those worn by Iron Man or Ripley in the movie Aliens, are currently starting to become a reality. They have motors that aid movement—virtually, Marras said, “like power steering for the body.” This fall, he and Knapik will be examining the spinal loads caused by just such a powered exoskeleton.

The study’s co-authors included Eric Weston, Mina Alizadeh, and Xueke Wang, all of Ohio State.

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