Most of the robotic limbs you hear about are meant to replace arms and legs that have been lost to injury, but MIT is working on robotic limbs that are just meant to add on additional ones, giving people three or four arms so that they can get more done. Its researchers demonstrated the limbs — which they call supernumerary robotic arms — at a conference yesterday in China, and videos show that they're already working to a basic extent. The current suit reportedly weighs just 10 pounds, but right now it seems to mainly be useful for holding light objects in place.

"From the wearer's perspective, it behaves like an extension of his own body."

Because the robotic limbs are meant to augment actual arms, they have to be controlled by something else. IEEE Spectrum, which hosted the conference, says that the arms instead base their movements off of the movements of their wearer's entire body, measuring acceleration and orientation to predict what they should be doing. "Once we combine the most significant behavioral modes we are able to control the robot such that, from the wearer's perspective, it behaves like an extension of his own body," Baldin Llorens-Bonilla, an MIT researcher working on robotic limbs, tells IEEE Spectrum.

A separate group of MIT researchers are working on another robotic arm attachment, pictured below, meant to assist people while building planes. It attaches at the waist and allows for the addition two arms or legs, and it's meant to be used to hold objects or brace someone while they're working. Boeing reportedly sponsored much of the development, but IEEE Spectrum doesn't say when MIT expects to be far enough along that Boeing or some other manufacturer will actually be able to put them to use.