Impact of elastic ankle exoskeleton stiffness on neuromechanics and energetics of human walking across multiple speeds.
Richard W Nuckols, Gregory S Sawicki
PMID 32539840WHAT IT FOUND
At 1.50 m/s, no ankle exoskeleton stiffness reduced walking energy cost.
At slow and fast walking, 50 Nm/rad reduced cost by 4.2% and 4.7%. The device was tethered and tested in healthy adults.
Key findings
01A stiffness of 50 Nm/rad reduced net metabolic rate by 4.2% at 1.25 m/s and 4.7% at 1.75 m/s compared with no assistance; 100 Nm/rad did not reach significance.
02At 1.50 m/s, exoskeleton assistance increased metabolic cost for all stiffness conditions, and no optimal stiffness was found.
03The calculated optimal stiffness was 70 Nm/rad at 1.25 m/s and 79 Nm/rad at 1.75 m/s.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
The study included 11 healthy adults, so it does not show what happens in patients with gait impairment. The device was a tethered research emulator, not a portable clinical exoskeleton, and the authors state this limits claims about real-world savings. Merely putting on the tethered system increased metabolic cost by about 19%, so the small speed-specific reductions may not translate to a wearable device. The emulator behaved more like an ideal spring than a physical or biological spring, which may differ from a real passive device. Benefit was absent at 1.50 m/s, a speed close to preferred walking speed, so speed matters more than a single stiffness recommendation.
Declared interests
The supplied metadata lists NIH extramural research support. The article text does not include a competing-interests declaration.
The easy way to misread this
Do not conclude that a portable ankle exoskeleton will reduce walking effort for patients. The benefit was only 4.2% at 1.25 m/s and 4.7% at 1.75 m/s, absent at 1.50 m/s, and the device was a tethered emulator.