Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power.
Samuel Galle, Philippe Malcolm, Steven Hartley Collins and 1 others
PMID 28449684WHAT IT FOUND
A tethered ankle exoskeleton reduced the metabolic cost of walking by 12.3% compared to normal walking when actuation started at 42% of the stride and power was 0.42 W/kg.
Lower power levels were more energy-efficient, though the device itself added a 11.6% penalty when inactive.
Key findings
01The optimal condition reduced net metabolic cost by 12.3% compared to normal walking and 21.4% compared to the inactive exoskeleton.
02Metabolic cost was minimized when actuation timing was 41% of the stride and exoskeleton power was 0.41 W/kg, with no substantial interaction between these two factors.
03Lower power levels (around 0.2 W/kg) achieved metabolic reductions of 14 to 16% compared to the inactive state, with a higher ratio of metabolic savings to mechanical power delivered.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
The exoskeleton was tethered to off-board hardware, which is not representative of the autonomous devices patients might use in clinical or community settings. The study included only healthy young female subjects, so results may not generalize to older adults, males, or individuals with gait pathologies. The penalty for wearing the inactive exoskeleton (11.6%) was higher than in some other recent lightweight designs, meaning the net benefit over normal walking was smaller than the benefit over the inactive state. Individual responses varied, and the study searched for a single global optimum rather than subject-specific optimizations.
Declared interests
Funded by Universiteit Gent. No other conflicts of interest reported.
The easy way to misread this
Do not interpret the 21.4% reduction in metabolic cost as a benefit over normal walking. This figure compares the powered state to the inactive exoskeleton state. The actual reduction compared to walking without any device was 12.3%, and only this specific condition was statistically significant compared to normal walking.