OBJECTIVES: Severe brain injury incidence in motorsports has decreased due to driver safety improvements; however, drivers are exposed to non-concussive head acceleration events (HAEs) during active racing maneuvers and crashes. The objective of this study was to evaluate whether Principal Direction of Force (PDOF) influences head impact exposure in National Association for Stock Car Auto Racing (NASCAR) Cup Series drivers. METHODS: Head kinematic data was collected from 20 NASCAR Cup Series drivers during 41 races in the 2024 season using instrumented mouthpiece sensors. Sensors collected linear and rotational head kinematics (1600 Hz, 4 g threshold) and were characterized as crash (i.e., a driver's vehicle interacted with an external object, resulting in a caution) or race (i.e., normal, green flag racing) events and by track type. HAEs with multiple impacts were manually segmented into individual impacts. Impacts were assigned to one of eight PDOF directions characterizing frontal, rear, side, and oblique impacts. Resultant linear acceleration was calculated in the transverse plane with pulse boundaries determined using thresholds (5 g if peak >10 g; 2.5 g if peak ≤10 g). PDOF was calculated from changes in linear velocity in the transverse plane using these boundaries. Peak linear and rotational accelerations and velocities (PLA, PRA, PRV), change in linear velocity (ΔLV), and Diffuse Axonal Multi-Axis General Evaluation (DAMAGE) were calculated per impact. Mixed effects models evaluated associations between PDOF and head kinematics, adjusting for fixed effects of impact classification, track type, and random effects of driver, followed by pairwise comparisons among PDOF directions. RESULTS: 314, 383) having the greatest magnitudes. PRV was greatest in frontal (4.44 rad/s 4.00, 4.94) and frontal oblique impacts (4.29 rad/s 3.86,4.76 right; 4.19 rad/s 3.70,4.75 left). DAMAGE was greatest in right frontal oblique (0.036 0.032, 0.041) impacts. Only DAMAGE saw an oblique direction produce the highest magnitudes. CONCLUSIONS: PDOF was associated with head impact frequency and magnitude in NASCAR Cup Series drivers. Right side impacts were both most frequent and among the highest magnitude, while frontal oblique impacts showed greater effects on estimated brain tissue deformation. This is the first study to characterize directionality using PDOF derived directly from head kinematics. Findings provide a framework for evidence-based, targeted safety interventions in NASCAR, other motorsports, and contact sports.
Smith et al. (2026) studied this question.