Recently, several cases of mild traumatic brain injury to American professional football players have been reconstructed using instrumented Hybrid III anthropomorphic test dummies ATDs. The translational and rotational acceleration responses of injured and uninjured players'' heads have been documented. The acceleration data have been processed according to all current head injury assessment functions including the GSI, HIC and GAMBIT among others.A new hypothesis is propounded that the threshold for head injury will be exceeded if the rate of change of kinetic energy of the head exceeds some limiting value. A functional relation is proposed, which includes all six degrees of motion and directional sensitivity characteristics, relating the rate of change of kinetic energy to the probability of head injury. The maximum value that the function achieves during impact is the maximum power input to the head and serves as an index by which the probability of head injury can be assessed.