Close your eyes and touch two fingers together. The sense that enables this gesture is proprioception—feedback that tells your brain where body parts are and what they are doing. “Proprioception is essential to all human movement,” says Tyler Clites, a biomedical engineer at the Massachusetts Institute of Technology.

Scientists have made huge strides in controlling robotic limbs with the nervous system, but providing such sensory feedback has proved more challenging. Now, however, a team led by biomechanical engineer Hugh Herr, also at M.I.T., has created a prosthetic leg with proprioception. “That's one of the fundamental pieces of prosthetics that has been missing,” says biomedical engineer Paul Marasco of the Cleveland Clinic, who was not involved in the study.

Muscles that are linked so that one stretches when the other contracts are central to a sense of limb placement. In a traditional amputation, surgeons tie the remaining muscles to bone, limiting movement and breaking this dynamic relationship. The new technique, described in May in Science Translational Medicine, involves grafting new muscle pairs onto the amputation site of a patient with below-knee amputation. Skin electrodes pick up electrical activity in the grafted muscles and use it to control motors in the prosthetic leg's ankle, and sensors in the prosthetic foot transmit proprioceptive feedback to the muscles. “Returning that back into the system that's built for handling it is a pretty big deal,” Marasco says.

The procedure restored near-natural limb control. When climbing stairs, the patient unconsciously flexed his robotic foot like uninjured people do. “This is a first demonstration of emergent reflexive behaviors—these important yet unintentional behaviors that come out as we walk on difficult terrains,” says Clites, the study's lead author. In another first, the researchers showed that including feedback from torque sensors in the ankle allowed the patient to more precisely control how hard he pushed on a pedal. “What's new here is the ability to provide feedback the brain knows how to interpret as sensations of position, speed and force,” Clites explains.

These sensations appear to imbue a sense of ownership. “The patient said things that describe an embodiment, like ‘The robot has become part of me’ and ‘I have my leg back,’” says Herr, who is an amputee himself. When the patient's daughter asked him if he felt like a cyborg, he told her, “No, I felt like I had a foot.”