To investigate the human head impact tolerance in terms of changes in vital functions, a series of head impact experiments was performed using live monkeys, which are morphologically analogous to humans. To find a causal relationship between the impact and changes in vital functions, three kinds of experimental conditions were used:​ translational acceleration impact and rotational acceleration impact (both using a head restraint mask with broad contact area), and impact of the unrestrained head against a padded flat surface.

The results indicated that the concussion, cerebral contusion and skull fracture in the monkeys depended on:​ i) the translational and rotational acceleration impact;​ ii) the contact area of the impact;​ iii) the amplitude and duration of the imposed head acceleration*;​ iv) the direction of the impact region (whether frontal or occipital). It was also determined that, of these three patterns of injury, the threshold for the occurrence of concussion is a tolerance threshold (i.e., it indicates transitory and reversible effects). A curve was drawn up for the threshold of concussion occurrence (TCO) in monkeys. Further, the fracture threshold curve for human cadaver skulls was obtained experimentally.

Dimensional analysis and the similarity principle were then used to extrapolate the human threshold for the occurrence of concussion from that obtained experimentally for monkeys, and thus to derive the human head impact tolerance curve. At the same time a comparison was made between the human cadaver skull fracture threshold curve and the monkey skull fractures to confirm the reliability of the estimation of the human head impact tolerance.