Effects of selected test variables on the evaluation of football helmet performance.

The most important variables in helmet test methods (headform, impact surface, velocity) were studied by systematically changing the test variables and measuring the effect of these changes on the output responses (peak acceleration and severity index) of helmeted headforms. The degree of correlation between variables was also measured. The metal headform yielded consistently higher results than the humanoid headform but the differences between the two were much greater during impacts at the back site. Also, correlation between humanoid and metal headforms was significantly higher for top impacts than for rear impacts. These discrepancies between top and rear impacts were attributed to difficulties in the neck mounting system with the humanoid headform which causes inordinate bending during impacts. The soft impact surface yielded lower responses than the hard surface for both headforms, correlations between soft and hard surfaces were high in all cases. A small change in impact velocity (4.5 to 5 m/sec) resulted in a substantially higher output response. Correlation between velocities were much lower for the back impact site than the top.