In the study, researchers used MRI data from the brains of three patients to determine how much the brain was able to move inside the skull, and in which directions. They were looking for information about the frequency at which the brain rattles around inside the head, measured at cycles per second, or hertz. Using a statistical model along with the physical data, the researchers concluded that the brain could be damaged when the brain oscillates at a rate of 15 hertz. The typical rate is 5 hertz for things like turning your head when someone comes in the room or to look down at your phone, but contact sports can push it to 20 hertz. And unlike concussive injuries that can happen in less than a second, these results suggest that little bumps over the duration of a football game, for example, can cause just as much damage, whether a player is wearing a helmet or not.