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A robotic leg with motors that work like bike gears could let a wearer walk up stairs 鈥� which many people find impossible with unpowered prostheses.

Most prostheses have no motors to actively assist walking, but instead are either passive or help only by altering the resistance of a joint鈥檚 swing, controlled by a low-power microchip. The few actively powered prostheses on the market are relatively heavy and bulky, with short battery lives, and have achieved limited success.

at the University of Utah and his colleagues have now created a fully powered leg that has a battery life comparable to resistance-only designs, potentially providing days of use between recharging, while also allowing the wearer to walk up stairs and inclines.

鈥淵ou have to provide a lot of torque power,鈥� says Lenzi. 鈥淚f you try and do that with conventional electric motors and gears, then what happens is you have a leg that is way too heavy, way too inefficient to be usable in real life.鈥�

The Utah Bionic Leg also features an ankle joint and a big toe, which Lenzi says is a first for a powered prosthesis. This helps with stability and makes walking more natural. 鈥淚f you don鈥檛 have a toe, when you push with your ankle, you get your body pushed up and not forward,鈥� he says. 鈥淪o you really need a toe.鈥�

The robotic leg doesn鈥檛 include motors powerful enough for the fastest and strongest tasks, but instead leverages the power of smaller motors to make trade-offs between speed and strength. It does this by internally changing the geometry of the joint and the point at which the motor鈥檚 power acts on it.

The prosthesis is designed for people who don鈥檛 have a leg from above the knee and includes two separate modules, each with its own controller and battery, one for the knee joint and one for the ankle and toe.

The knee module, which is where most strength is needed, senses the amount of power required at any given point, then adjusts the output of its motor up or down as needed.

鈥淚t鈥檚 very similar to what you do on a bike. If you want to go uphill slow, you kind of shoot down [in the gears] and then you can keep going, right?鈥� says Lenzi.

But unlike a bike鈥檚 gears, which have discrete jumps between ratios, the knee joint uses a continuously adaptable linkage that smoothly changes geometry and how the motor鈥檚 power is transmitted. This allows it to move faster when less power is needed, like walking on flat ground, but also provides slower and stronger movement to climb stairs.

The batteries can power the robotic leg for about 15,000 steps on a single charge, which exceeds the average daily number of steps taken by people in the US of between 7500 and 10,000, and is well above the 1500 average daily steps taken by those in the US who walk with the help of leg prostheses, say the authors. Once the battery is empty, the leg continues to function as a passive leg, allowing the wearer to continue walking, albeit only on flat surfaces.