Photo by Bruno Aguirre | Unsplash Warren Mullisen, 81, of Culver City, Calif., was a runner all his life and competed in numerous marathons. But a few years ago, he slipped on loose gravel while running up a trail and fell backward, hitting his head. He suffered a brain injury that impaired his balance. “The legs don’t seem to move when I’m trying to run,” he said. Despite physical therapy, his neurologist said that most likely won’t change.