Football, neck muscles and head impact.

Any blow to the head may be injurious because it is the manner of response to the blow rather than the blow itself that produces the injury potential. Impacts to inanimate objects or to the heads of anesthetized animals produce data that are simplified by the fact that the involved mass is a constant, and the contact time is consistently negligible. On the football field, all the muscles in the body of the player are tensed at the snap of the ball, and the involved mass varies widely during the resulting prolonged contact time. This causes variable accelerations, variable forces and variable velocity changes as well as a varying mass which can only be measured, therefore, according to the impulse momentum theorem, Ft = deltaMV.