A single sports season can physically reshape the brain

When a football player or rugby flier absorbs a typical hit, their brain experiences an acceleration of up to 80 g—nearly ten times the force a fighter pilot feels during a sharp turn. While helmets are excellent at preventing cracked skulls, they cannot stop the brain from sloshing inside its bath of cerebrospinal fluid. This movement creates rotational force, which is far more destructive than a straight-on bump. Instead of just bruising the surface, these twisting forces shear the axons, the long cables that allow different parts of the brain to communicate with one another.