The presence of a motorcycle helmet alters the frequency and severity of head injury to the accident-involved motorcyclist, and significant differences are evident in the mechanisms of injury. The conventional motorcycle safety helmet provides impact load distribution and energy absorption, consequently the mechanisms of head injury are affected by the presence of a helmet. There is a consistent typology of head injury mechanisms which distinguishes the presence of a helmet for the accidentinvolved motorcyclist:​ a system of both contact and inertial injuries is evident in unhelmeted motorcyclists but only inertial injuries are the particular threat to the helmeted motorcyclist in more severe impacts. Contact injuries are essentially excluded in the areas covered by the helmet but inertial injuries related to rotation can be reduced but never excluded.