Reducing the energy cost of human walking using an unpowered exoskeleton

Steven H. Collins, M. Bruce Wiggin, Gregory S. Sawicki

★ Must read June 2015 · Nature

TL;DR An ankle exoskeleton with no motor or battery, just a spring that a mechanical clutch engages while the foot is on the ground, lowered the metabolic cost of walking. With the best of five springs, net metabolic rate across nine participants fell 7.2 ± 2.6% compared with walking without the device.

What they do

Earlier devices lowered the energy cost of walking by adding power from pumps or motors. This paper asks whether a device with no energy source can. The authors built an ankle exoskeleton whose spring, held by a mechanical clutch while the foot is on the ground, works in parallel with the calf muscles and Achilles tendon and carries part of their force.

How it works

Rationale. Muscles spend metabolic energy to hold force even when they do no work; a clutch holds force for free. The design copies calf muscle fibers, which appear to act as a clutch holding the spring-like Achilles tendon.
Clutch. A 0.057 kg ratchet and pawl with no motor, battery or computer, switched by ankle motion. It engages at heel strike, holds the spring as it stretches and recoils through stance, and releases it for toe clearance in swing.
Frame. Carbon-fiber shank and foot sections molded for each participant and hinged at the ankle, 0.408-0.503 kg per leg.
Protocol. Nine healthy adults walked on a treadmill at 1.25 m/s without the device, with it but no spring, and with five springs (130-400 N·m/rad), after 21 minutes of training per condition.

Results

Metabolic rate first fell, then rose with stiffness. The 180 N·m/rad spring cut net metabolic rate from 2.88 ± 0.10 to 2.67 ± 0.14 W/kg, a 7.2 ± 2.6% reduction (p = 0.023), similar to earlier powered devices.
Wearing the device without a spring did not raise energy cost (p = 0.9), and joint angles changed little.
With the 180 spring, average biological ankle moment fell 14% and mid-stance soleus activity 22%, mainly in the first 40% of the stride.
Stiffer springs brought metabolic rate back to normal, apparently with more tibialis anterior activity in early and mid-stance and more plantarflexor activity at the end of stance.

Limitations

Nine young, healthy participants walked at one speed on a treadmill, without blinding. Compliance in the frame, rope and human-device interface likely cut effective stiffness by about 33%, varying between subjects, and the clutch sometimes failed during training. Why stiff springs raise the cost is only partly explained; simple walking models predict center-of-mass changes that were not observed.

Figures

From Figure 1: (a) Rigid shank and foot sections hinged at the ankle, with a clutch and spring in parallel with the c... — From Figure 1: (a) Rigid shank and foot sections hinged at the ankle, with a clutch and spring in parallel with the calf muscles and Achilles tendon. (b) A participant walking with the device. (c) The clutch has no electronics: a ratchet and pawl engage the spring when the foot is on the ground and release it when the foot is in the air.

From Figure 2: Over one stride, for each spring: exoskeleton torque (a), the biological part of the ankle moment (b),... — From Figure 2: Over one stride, for each spring: exoskeleton torque (a), the biological part of the ankle moment (b), soleus muscle activity (c) and ankle angle (d), which engages and releases the clutch. Torque rises with stiffness except for the stiffest spring, while biological moment and soleus activity fall.

From Figure 3: Net metabolic rate for normal walking, the exoskeleton without a spring (k = 0) and five spring stiffn... — From Figure 3: Net metabolic rate for normal walking, the exoskeleton without a spring (k = 0) and five spring stiffnesses, with a quadratic fit (dashed). The 180 N·m/rad spring is 7.2% below normal walking (p = 0.023).

Abstract

With efficiencies derived from evolution, growth and learning, humans are very well-tuned for locomotion. Metabolic energy used during walking can be partially replaced by power input from an exoskeleton, 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 fulfill 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 behavior.

Summaries are written automatically from each paper's text. Figures and videos come from the paper and its project page; all credit goes to the authors.

Matt Strong