Measurements of the impulse frequency response of head impact points on the exterior and the interior of a car were used to calculate the modal mass and stiffness for each point. These points were then arranged in an hierarchy of increasing stiffness and grouped into three classes. Thirty two head impact cases (13 pedestrian, 4 car occupants and 15 fall victims) in which the distribution of injury to the brain had been recorded in detail were grouped according to the stiffness of the object struck and by the location of the impact on the head. The distribution of the brain injury lesions in the anterior, middle and posterior regions of the brain were determined for each class of stiffness (soft, medium or hard) and location of impact (occipital or lateral). Distinctive patterns of brain injury distribution were noted for each class of stiffness and each location of impact. Three probable mechanisms of brain injury were distinguished. They were:​ relative motion between the brain and the skull, local bone deformation and intra-cerebral strains. Each mechanism was related to a range of stiffness and natural frequency of the structure impacted. Hence these theories of brain injury mechanisms are consistent with observed epidemiological data and with conclusions drawn from mathematical modelling.