Four years ago Zac Vawter was in a motorcycle collision that severely injured his right leg. He underwent an amputation at the knee, thereby becoming one of the more than one million amputees living in the U.S. Over half of the amputations in the U.S. are due to vascular disease – poor circulation caused by diabetes, smoking, and high cholesterol. Just under half are due to trauma. Modern leg prostheses that replace both the knee and ankle joint use motors that power each joint and multip...