## 2 Materials and Methods  
Ten male players were recruited from a university intra-mural football league. Each player provided written consent, which included 2 defenders (center-backs), 5 midfielders (2 defensive, 1 central, and 2 attacking), and 3 forwards (1 striker and 2 wingers). Ethical approval was granted by the Faculty of Biological Sciences Ethical Review Committee at the University of Leeds (#BIOSCI 19-026). All players were male, with a mean age of 20 (\(\pm\)1) years, a mean weight of 76.2 (\(\pm\)9.4) kg, and a mean height of 182.2 (\(\pm\)6.5) cm. Data were collected from twelve matches within the 2021-2022 season. To be eligible for the study, players had to be active participants during the season, have at least 5 years of experience playing football competitively and have not sustained a concussion in the last year. Prior to the season's commencement, participants underwent dental impressions in order to be outfitted with their custom-fit iMGs (Prevent Biometrics, Minneapolis, MN). iMGs were provided to all athletes prior to each match, and players had the option to decline or remove the iMGs at any point throughout the session. Our methodology for collecting video and iMG data is shown in Figure 1 and Table 1. The iMGs recorded time-stamped HAEs, which were sent via Bluetooth to a central device [1; 6; 23; 29]. Video footage was collected from multiple angles at a resolution of 1080p and a frame rate of 30 fps during each on-field activity. HAEs were time-synchronized with match recordings via iMG timestamps and an on-field timer to facilitate visual verification and qualitative analysis. For each true positive HAE observed on film, the header type, ball delivery method and field location were classified. The raw agreement was 100.0% for the identification of headers. A Cohen's kappa coefficient was performed to assess the inter-rater reliability for contextual characteristics. For the header analysis, the raw agreement was 93.5%, and Cohen's kappa coefficient was 0.81 (95% CI 0.78-0.84) [30]. A Cohen's Kappa value greater than 0.8 is indicative of almost perfect agreement [31]. Every iMG possessed an infrared proximity sensor to assess coupling to the upper dentition, as well as an accelerometer and gyroscope, both sampling at 3200 Hz, with measurement ranges of \(\pm\)200 g and \(\pm\)35 rad/s, respectively. The iMGs were configured to capture 10 ms of pre-trigger data and 40 ms of post-trigger data when the accelerometer detected over 5 g linear acceleration on a single axis of the iMG. A recording threshold of 5 g linear acceleration at the head center of gravity was selected, given that the previous threshold of 10g lacked sensitivity in capturing mild headers [1; 32]. Previous studies have established the validity of the Prevent Biometric custom-fit iMG [25; 33]. Liu et al. [33] found the Prevent Biometric custom-fit iMG to perform best in laboratory dummy headform test \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Pre-Head Contact** & \multicolumn{2}{c}{**Head Contact**} \\ \hline \hline  & & _Header Type_ \\ _Ball Delivery Method_ & & \(\bullet\) & Clearance \\ \(\bullet\) & Cross (incl. Corner, Delivery into Box) & \(\bullet\) & Flick-On \\ \(\bullet\) & Goal Kick & \(\bullet\) & Interception \\ \(\bullet\) & Long Ball (incl. Clearance, High Ball) & \(\bullet\) & Pass \\ \(\bullet\) & Pass (incl. Chip, Headed Pass, Throw in) & \(\bullet\) & Shot \\  & & \(\bullet\) & Headed Control \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variables used to categorize header impacts through video incident analysis. Figure 1: (**a**) The placement of digital cameras to record multi-angle, time-synchronized footage of header impacts, in combination with the (**b**) instrumented mouthguard utilized for the study. Section (**c**) summarizes the methodology of video analysis utilized to categorize each HAE into a contact scenario. ing with lower mean relative errors of 4.9%, 4.6%, and 2.5% for peak angular acceleration, angular velocity, and linear acceleration, respectively.