Population Variations in Helmet Fit Affect Calculated Head Injury Risk in Blunt Impact

A helmet will not fit the same way on different individuals with different head sizes and shapes. However, the degree to which subject-specific fit differences may affect the risk of injury remains understudied. In this study, we used experimentally measured head/helmet contact force to generate different levels of initial padding compression when applied to a finite element head model. The deformed padding configuration was used as the initial condition for subsequent blunt impact analyses. As the level of initial padding compression increased, the resulting maximum force, total impulse, and the resulting head injury metrics increased by up to 20%. The results of this study highlight the importance of accurate modeling of initial pad deformation before blunt impact and provide a workflow for establishing such a configuration in other helmet applications. Intuitively, two different people wearing an identical helmet will experience a different fit, since the size and shape of their heads will differ. However, it is unclear how that variation in fit will affect the risk of injury for most helmet types. In this study, we generated different helmet fit conditions in a simulated model of a head and helmet, based on previous experimental measurements of the force between the helmet padding and the head. Four conditions were generated: zero, low, medium, and high initial fitting forces. Using the different fit configurations, we simulated two types of impacts on the front, back, side, and top of the helmet, and calculated the risk of injury to the brain and skull. Our results suggest that differences in padding compression arising from differences in head shape can significantly change the risk of injury. Higher initial padding compression resulted in greater force applied to the head and higher values of brain and skull injury metrics. These effects were more apparent for lower energy impacts, suggesting that the differences might be important to consider for helmets where the user sees repeated, low-intensity impacts such as in American football. Our analysis was limited to one type of helmet, and we were unable to vary the geometry of the head, meaning that the data should be interpreted cautiously. However, our results provide compelling evidence that the level of initial compression in the padding due to the user's head is an important factor to incorporate when designing new helmets.