actually head injury's often manifest themselves hours to days later. in my sport very fit people crash hit there heads feel more or less ok and then sometimes die or go into a coma from internal brain swelling or bleeding. i have gotten concussion's in crashes and im always a bit paranoid for a day or two because of this. the human body is weird sometimes someone will get so fucked up your certain they die and they recover other times something mild snowballs and life is lost you just never know.