Researchers at the University of Michigan have developed robotic prosthetic legs which uses motors that were originally designed for use on the robotic arm of the International Space Station. The motors allow the prostheses to move more naturally, producing less stress on the hips of users, and they are also quieter and more energy efficient than pre-existing robotic legs.

Conventional prosthetic legs require the wearer to move their hips abnormally to raise the foot and swing the leg forward. This can make walking difficult, and can place strain on the hips over time. Robotic legs could help to provide a more natural gait for wearers, but current models have several limitations, including joint stiffness, and loud motors. This new design aimed to change that.

“We designed our joints to be as compliant, or flexible, as possible,” said Toby Elery, a researcher involved in the study. “Our robotic leg can perform and even react like a human joint would, enabling a naturally free-swinging knee and shock absorption when contacting the ground.”

Typically, robotic prosthetic legs contain small motors that spin quickly, requiring significant gearing to allow them to apply a reasonable amount of torque. However, this is inefficient and noisy and makes the joints stiffer. These new motors, which the researchers have positioned at the ankle and knee of the prosthetic legs, are more powerful, requiring very few gears to apply a sufficient amount of force for locomotion. The design also incorporates regenerative braking to charge the battery, significantly increasing the energy efficiency of the design.

“Our prosthetic leg consumes approximately half the battery power of state-of-art robotic legs, yet can produce more force,” said Robert Gregg, another researcher involved in the study. “If the joints are stiff or rigid, the force is transferred to the residual limb, and that can be painful. Instead, we use that force to charge the battery.”

The prostheses have been trialed by amputees, and so far, have been positively received, providing a more natural gait and assisting with walking. “In some cases, they have observed that they feel like muscles in their hips and back are working less with our leg, compared to their conventional leg,” said Gregg. “We’re able to reduce compensations at the hips.”