A Novel Video-Based Method to Estimate American Football Helmet Pad Deformation During Impact Testing

American football is among the sports with the highest incidence of mild traumatic brain injuries (mTBI), associated with long-term neurological consequences including cognitive impairment, chronic traumatic encephalopathy, and Alzheimer's disease. Despite recent rule changes and stricter National Football League (NFL) safety guidelines, mTBI remains common, underscoring the need for improved helmet design and evaluation. Current helmet testing protocols, such as the NFL's, measure head kinematics to derive performance scores but do not characterize helmet component behavior during impact. This study proposes a novel approach through the Pad Deformation Index (PDI)-a quantitative metric estimating compressive deformation of helmet pads during impact.

Standard helmet impact tests were conducted at the Side Upper location, at three velocities (5.5, 7.4, and 9.3 m/s). Complementary mechanical tests on isolated pads using a drop tower at equivalent velocities linked helmet-level outcomes with localized pad response. High-speed videos enabled frame-by-frame reconstruction of the headform-impactor interaction and calculation of the minimum distance between the two rigid bodies, from which PDI was derived.

Higher PDI values generally coincided with reduced peak linear acceleration (PLA), Head Injury Criterion (HIC), DAMAGE, HARM, and partial Helmet Performance Score (HPS), supporting the use of PDI as an indicator connecting impact performance metric to pad deformation. Furthermore, pads exhibiting later densification within high-PDI range produced the lowest performance scores, underscoring the critical role of delayed densification.

PDI offers a practical tool for identifying absorption mechanisms and guiding helmet design.