Exoskeleton use may shift stress around the body, study suggests
Image credit: Ohio State University
Using an exoskeleton to alleviate stress in one part of the body may be achieving localised relief merely by shifting the stress onto another part of the body, according to the findings of a study conducted by a team from Ohio State University.
The research team tested a commercially available exoskeleton - a mechanical arm attached to a harness – that is typically worn by workers to help them carry heavy objects hands-free.
From their analysis, the researchers concluded that the device relieved stress on the arms as it was supposed to, but it consequently increased stress on the back by more than 50 per cent.
For the study, 12 volunteers used two different pneumatic tools, a torque wrench and an impact wrench, just as they might in common and repetitive industry scenarios. They used the wrenches with and without the aid of the exoskeleton.
The torque wrench weighed about 10 pounds, while the impact wrench weighed 30 pounds. When participants wore the exoskeleton, the wrenches were supported by the mechanical arm, which transferred the weight to a vest-like harness. The participants then only had to grip the wrench and move it up or forward as they might to tighten bolts in a factory.
Over the course of a few hours, researchers measured the forces on the volunteers’ back muscles and spine. They found that wearing the exoskeleton increased compressive spinal loads up to nearly 53 per cent compared to not wearing it. Stress on different muscles in the torso increased anywhere from 56 per cent to 120 per cent while wearing it.
There are tradeoffs with all exoskeletons on the market today, because they inherently change the way we move, 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,” Marras said. “Someone is tugging and pulling on you in directions you’re not expecting and your body has to compensate for that. The way you compensate is by recruiting different muscles to perform the task.”
Gregory Knapik, senior researcher at the institute, added, “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.”
The volunteers didn’t seem to notice the extra strain on their backs, but they did notice that they were uncomfortable, chiefly because of the stiff metal rods that lined the harness and prevented them from moving normally.
“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,” Knapik said. “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.”
Given that the study participants had to wear the harness for only part of a day, the researchers expect that the stresses would be higher for someone who had to wear it for an entire work shift, day after day.
The manufacturer of the particular exoskeleton under examination in this study is aware that it can cause back fatigue. The company recommends that users undergo muscle conditioning to prevent injury while wearing it.
Passive exoskeletons, like the one tested in this study, contain braces and springs to help support areas of the body. Active exoskeletons, like those worn by the titular hero in the movie Iron Man, are now starting to become more of a reality. They contain motors that aid movement, almost, Marras said, “like power steering for the body.”
Many companies worldwide are working on exoskeletons to help less able-bodied people achieve greater mobility, such as the robotic leg brace designed to help partially paralysed people walk that was revealed by automotive manufacturer Toyota in 2017. Further technologically advanced exoskeleton designs were on show at CES 2018, from companies such as Honda.
The Ohio State research team is planning to test the spinal loads caused by active powered exoskeletons later in 2018, in order to gather data comparative with the passive variety.
The team’s latest research appeared in the journal Applied Ergonomics.