Impacts have been analyzed in terms of degree of injury, head injury criterion (HIC), and average acceleration as a function of time for frontal impacts against the following surfaces:
Rigid flat surface-fractured cadaver skull.
Astroturf-head drop of football-helmeted cadaver.
Windshield penetrating impact of a dummy.
Airbag-dynamic test by human volunteers.
It is concluded that the linear acceleration/time concussion tolerance curve may not exist and that only impacts against relatively stiff surfaces producing impulses with short rise times can be critical. The authors hypothesize that if a head impact does not contain a critical HIC interval of less than 0.015 s, it should be considered safe as far as cerebral concussion is concerned.