FASTER, PLEASE: Amputee: Bionic leg controlled by brainwaves “blew my mind.” “When a person thinks about moving, a signal is sent from the brain down through the spinal cord. These impulses control the muscles. After an amputation, the muscles are no longer there but the nerves are. Zac underwent surgery to move these nerves to his hamstrings. Sensors relay these nerve signals to a computer, which instructs the knee and ankle how to move.”