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.