An unpowered exoskeleton decreases the energy required for walking

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.

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