Quantitative comparison of Hybrid III and National Operating Committee on Standards for Athletic Equipment headform shape characteristics and implications on football helmet fit

Laboratory tests in which dummy headforms are used to evaluate helmet performance must be representative of real-world conditions to ensure helmets perform well in the field. The objective of this study was to quantify shape differences that may affect helmet fit between two dummy headforms commonly used for football helmet testing. Point-cloud models of a 50th percentile male Hybrid III headform and a medium NOCSAE headform were generated using a coordinate measuring machine. The headforms were optimally aligned and shape comparisons were made in the mid-sagittal plane, three coronal planes, and 3D. Planar and 3D differences were quantified by comparing maximum (MRD) and root-mean-square (RMSD) radial deviations. Minor differences were observed in the upper skull contours of all planar cross-sections, where MRDs were less than 3.5 mm and RMSDs were less than 1.7 mm. Larger deviations were observed in other regions including the jaw in the anterior coronal plane, where the MRD was 6.6 mm and the RMSD deviation was 4.5 mm. Substantial differences were noted between the Hybrid III and NOCSAE at the base of the skull, cheeks, jaw and chin. The headforms were also compared to a head model based on medical imaging of a human subject, which the NOCSAE matched more closely than the Hybrid III. The data presented in this study show that the Hybrid III and NOCSAE headforms have substantial shape differences in several regions that are important for helmet fit, possibly making the NOCSAE a better option for realistic helmet fit.