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The researchers mimicked this strategy with a series of springs and a small, battery-powered motor. The kinetic energy of the forward motion of the walker is stored in a power-assisted spring that is then released to help propel the foot forward as it pushes off the ground.

The new ankle is about 20% more efficient than previous prostheses, a significant improvement compared with the 3 to 4% boosts the researchers have achieved in the past.

The team is now trying to make the device lighter and more robust. The researchers aim to have a commercial version available by early next year.