A modeling effort has been undertaken at the Naval Biodynamics Laboratory in New Orleans to determine the cause of atypical responses of the head/neck anatomy of certain volunteer subjects to -X vector direction acceleration profiles. The results appear to indicate that these subjects, who had been previously run in the +Y and/or -X+Y vector directions, may have learned to exert purposeful control over their head/neck responses in order to lessen the severity of their exposures. Computer simulations appear to verify the theorized explanation that the subjects "locked" their head/neck joints in anticipation of sled firing, and that the joints remained locked during the initial portion of the run until forces and torques increased to a level which forced their joints to unlock.