Can Elastic Ankle Exoskeletons Make it Easier for Older Adults to Walk?

Abstract

As we age, we walk slower and expend more energy which can limit our independence and quality of life. The root cause may be reduced Achilles tendon stiffness, which causes calf muscles to operate at shorter lengths and higher activations. In young adults, elastic ankle exoskeletons reduced metabolic cost of walking, or improved biological “gas mileage,” by 7% − potentially by enabling longer muscle lengths and lower activations. We hypothesize adding elastic ankle exoskeletons could mitigate the metabolic penalty associated with more compliant Achilles tendons of older adults. We used a simulation of the human neuromuscular system to investigate the effects of an elastic ankle exoskeleton on walking in aging. Aging was simulated by reducing the tendon stiffness of young adults by ~20% (kTyoung =180 kN/m; kTold = 140 kN/m). First, we measured the ankle moment and ankle angle from a young healthy adult. Then, we determined forces and lengths of the muscle and tendon using skeletal geometry. We calculated the activation of the muscle and estimated the metabolic cost of walking across a range of exoskeleton stiffnesses. We found that adding an elastic exoskeleton shifted the muscle operating lengths toward optimum, reduced activation of the muscles, and reduced the metabolic cost of walking for older adults to levels comparable to young healthy adults. In conclusion, elastic ankle exoskeletons may counteract age-related consequences of reduced tendon stiffness by steering muscle dynamics to elicit more economical muscle contractions.