You can't really expect good things to happen when a 305-pound football tackle's knee rams into an opposing player's head, but this clip from one of last Sunday's games is particularly chilling. In the video, Miami Dolphins quarterback Trent Green tries to block defensive tackle Travis Johnson as he pursues one of Green's teammates. The problem here is that Johnson is a very large and very fast man. Green realizes that throwing his 200-pound frame into Johnson's chest won't do much, so he tries to undercut the defender. Unfortunately, his timing is terrible.
As Green throws himself forward, Johnson's right knee comes up at the same time, slamming into the side of the quarterback's helmet. This is where inertia comes into play. Inside the skull, the brain is protected from minor impacts or jolts by a thin layer of fluid. In that instant after Green's head meets Johnson's knee, his helmet and skull stop moving, but his brain keeps going until it bumps up against the inside of his head.
The science of concussions is still being worked out, but there's some evidence that the momentary jarring of the brain affects blood flow. The good news: As Popular Science reported in our August issue, new helmet technology could enable scientists to get a better sense of the biomechanics of concussions, and aid coaches and trainers charged with determining whether or not a player should check back in after a mind-fuzzing hit.
For Green, though, this wasn't even a question. He was done for the day.—Gregory Mone