An exoskeleton can reduce the metabolic cost of walking not by adding energy or by recycling energy from one gait phase to another, as other exoskeletons have done, but by removing the kinetic energy of a striding person’s swinging leg so they don’t have to tense their muscles so much.

Tested in ten healthy males, it also converted the extracted kinetic energy to useable electricity.

Although humans are exceptional walkers, walking is metabolically expensive and requires more energy than any other activity of daily living.

Exoskeletons and exosuits – wearable devices designed to work along with the body’s musculoskeletal system – have been shown to reduce this cost by adding or recycling energy to assist the body’s movement.

These and other devices have also been designed to “harvest” the body’s mechanical energy and convert it into useable electrical energy. However, these biomechanical energy harvesters have not provided their users with a net metabolic benefit.

Here, Michael Shepertycky and colleagues present an exoskeleton device that can harvest mechanical energy from walking and convert it to electrical energy while also reducing the overall metabolic energy consumption of the user. Instead of adding energy to supplement the movement, Shepertycky et al.‘s device strategically removes kinetic energy during the knee swing phase of the walking gait cycle, effectively saving on the body’s costs to operate upper leg muscles as “biological brakes” as they control and slow the leg’s outward swing.

The removed kinetic energy is converted into electrical energy using an integrated generator. According to the results, in 10 healthy male users, this approach reduced the metabolic cost of walking by as much as 3.3% while also converting the removed energy into roughly .25 watts per gait cycle. In a Perspective, Raziel Reimer and colleagues discuss the study’s implications in greater detail.