Another reminder that wearing a bicycle helmet is crucial. Injuring brain easy as falling off a bike Bicycle helmets can reduce risks of head injury by as much as 85 percent and as much as 88 percent for brain injury...Broken arms and legs can heal, but people's brains are a different story. The skull, which protects the brain, is about ¼-inch to 1/8-inch think, or the size of a cup's...