Head acceleration event frequency among drivers across track types in Stock Car Auto Racing

Repetitive, non-concussive head acceleration event (HAE) frequency has not been characterized across track types in professional stock car racing. The objective of this work was to calculate and compare HAE frequency among drivers across track types in the National Association for Stock Car Auto Racing (NASCAR).

Head kinematics for 61 drivers were monitored over 39 race weekends at 30 distinct tracks during the 2025 season. Drivers across NASCAR's National series (Truck, Xfinity, and Cup) were instrumented with a custom mouthpiece sensor that tightly couples to the maxillary dentition containing a tri-axial linear accelerometer and gyroscope. A HAE was recorded when a 4 g trigger threshold was exceeded on any axis for at least 3 ms at 1600 Hz. HAEs were identified through filters (time-windowing, wear status) for each driver in each race where they participated and data was collected successfully. Events were visually verified and classified as crash events when associated with caution-causing incidents during active racing; all remaining real events were classified as race events. The number of race events for each driver was then divided by the number of laps completed by that driver to determine HAE rate (HAEs/lap). HAE rates were also calculated on a per-hour and per-race basis using transponder timing data and the number of races where the device was worn and data successfully collected. For this analysis, only feature race HAEs were included due to consistent session type and event data collection. Rates were aggregated to determine summary statistics of mean, median, 95^th percentile, and maximum HAE frequencies. Rates were also compared using negative binomial regression to determine rates ratios and 95% confidence intervals.

A total of 5,131 HAEs were collected over 239,337 feature racing laps. The lowest number of races (n = 14), laps completed (n = 14,752), and driver-races (n = 195) occurred at road courses with the greatest occurring at intermediate tracks (n = 44, n = 132,419, n = 621, respectively). Road courses resulted in significantly greater rates (p < 0.05) and the highest mean, median, 95^th percentile, and maximum rates compared to all track types, where per-lap rates were 0.1246, 0.0733, 0.4264, and 0.5565 HAEs/lap. Comparatively, superspeedways had the lowest mean, median, and maximum rates at 0.0046, 0.0027, and 0.0222 HAEs/lap. The lowest 95%ile HAEs/lap occurred at intermediate tracks (0.0172).

Instrumented mouthpieces were used to quantify HAE frequency across track types. Calculating HAE frequency on a per lap basis allows for comparisons of drivers across track types. Road courses had the greatest, mean, median, 95%ile, and maximum HAE rates. This study provides empirical evidence that warrants further investigation into road courses, which may elucidate factors contributing to increased rates. These findings additionally provide a framework for assessing race HAE rates that could provide insights to inform evidence-based decisions to improve driver safety.