The ability to walk upright is a defining characteristic of humans, one that emerged through a long evolutionary history. It's not just a matter of the right bones; our muscular, skeletal, and neural systems have evolved to enable our coordinated movements. The nerves allow us to develop a gait that is optimized to minimize the amount of energy necessary by modulating aspects of our movement such as our step length or arm motions.

Even with all that optimization, walking can be tiring; in fact, people expend more energy walking than any other daily activity. As we age, walking often becomes even more difficult. For decades researchers have explored ways to mitigate the energy cost associated with walking—studies that are typically aimed at helping those who are weaker or disabled.

Recently, scientists and engineers started to look at this issue from a new perspective; they questioned whether the human gait is as efficient as it can be. This interdisciplinary research team developed a device that behaves as an unpowered exoskeleton.

This exoskeleton is composed of a passive clutch that is connected in series above a spring running parallel to the calf. The top of the device is attached below the knee and the bottom to the foot; in the middle, it has a hinge located at the ankle. This design allows for the mechanical clutch to hold the spring as it is stretched and relaxed by ankle movements while the foot is on the ground. In this manner, it is able to fulfill one function of the calf muscles and Achilles tendon.

The team tested the exoskeleton device on healthy volunteers and found that it produced a torque similar to the biological ankle without interfering with normal ankle functions. The normal movement associated with walking stretches the spring, allowing it to store energy for use.

When walking occurs, the spring ratchets as the wearer touches their heel to the ground, initiating a step. When an individual pushes their foot downward to extend their leg and force their body forward, the spring is expanded, storing energy. Once the ankle begins to leave the ground, the spring recoils, allowing the wearer to use the stored energy to decrease their metabolic input required for that part of the motion. The step is then completed and the cycle repeats.

The researchers investigated how the exoskeleton device reduced human metabolic energy consumption. They found that the device is light enough that it does not measurably increase energy cost. They also found that spring stiffness influences metabolic energy used for walking. As spring stiffness is increased, metabolic rate decreases to a minimum and then begins to increase again, demonstrating that a spring with intermediate stiffness is optimal.

This exoskeleton requires no chemical or electrical input; instead, it relies on the mechanical energy provided by the movements inherent of walking itself. It also provides no net mechanical energy input; it simply uses the normal energy expended more efficiently. In doing so, this device reduces the metabolic cost of walking by 7.3 ± 2.6 percent for healthy human users under natural conditions. These energy savings are comparable to what you'd get from powered devices.

The evolutionary changes that optimize our physiological and biological characteristics occur gradually over millions of years, so a device that generates a 7.3 percent increase in efficiency demonstrates that we, as a species, haven't reached our optimal conditions yet. It also paves the way for a new class of mechanical devices that could help those who are weaker walk with ease.

Nature, 2015. DOI: 10.1038/nature14288 (About DOIs).