Reducing the energy cost of human walking using an unpowered exoskeleton

Abstract

With efficiencies derived from evolution, growth and learning, humans are very well-tuned for locomotion1. Metabolic energy used during walking can be partly replaced by power input from an exoskeleton2, but is it possible to reduce metabolic rate without providing an additional energy source? This would require an improvement in the efficiency of the human–machine system as a whole, and would be remarkable given the apparent optimality of human gait. Here we show that the metabolic rate of human walking can be reduced by an unpowered ankle exoskeleton. We built a lightweight elastic device that acts in parallel with the user's calf muscles, off-loading muscle force and thereby reducing the metabolic energy consumed in contractions. The device uses a mechanical clutch to hold a spring as it is stretched and relaxed by ankle movements when the foot is on the ground, helping to fulfil one function of the calf muscles and Achilles tendon. Unlike muscles, however, the clutch sustains force passively. The exoskeleton consumes no chemical or electrical energy and delivers no net positive mechanical work, yet reduces the metabolic cost of walking by 7.2 ± 2.6% for healthy human users under natural conditions, comparable to savings with powered devices. Improving upon walking economy in this way is analogous to altering the structure of the body such that it is more energy-effective at walking. While strong natural pressures have already shaped human locomotion, improvements in efficiency are still possible. Much remains to be learned about this seemingly simple behaviour. The attachment of a simple, unpowered, mechanical exoskeleton to the foot and ankle results in a net saving of 7% of the metabolic energy expended in human walking. Walking is the most commonplace of activities, yet we know remarkably little about it and no robot has yet reproduced the grace and poise of a human walk. Steven Collins et al. now show that the attachment of a simple mechanical exoskeleton to the foot and ankle results in a 7% reduction of the metabolic energy expended in walking. This work shows that net energy input is not a fundamental requirement for reducing the metabolic cost of human walking, and that reducing calf muscle forces — while also fulfilling normal ankle functions and minimizing penalties associated with added mass or restricted motions — can be beneficial.