Objectives: To investigate head kinematic variables in elite men's and women's rugby union and their ability to predict player removal for an off-field (HIA1) head injury assessment. Methods: Instrumented Mouthguard (iMG) data were collected for 250 men and 132 women from 1,865 and 807 player-matches, respectively, and synchronised to video-coded match footage. Head peak resultant linear acceleration (PLA), peak resultant angular acceleration (PAA) and peak change in angular velocity (dPAV) were extracted from each head acceleration event (HAE). HAEs were linked to documented HIA1 events, with ten logistical regression models for men and women, using a random subset of non-case HAEs, calculated to identify kinematic variables associated with HIA1 events. Receiver operating characteristic curves (ROC) were used to describe thresholds for HIA1 removal. Results: Increases in PLA, and dPAV were significantly associated with an increasing likelihood of HIA1 removal in the men's game, with an OR ranging from 1.05-1.12 and 1.13-1.18, respectively. The optimal values to maximise for both sensitivity and specificity for detecting an HIA1 were 1.96krad.s-2, 24.29g, and 14.75rad.s-1 for PAA, PLA, and dPAV respectively. Only one model had any significant variable associated with increasing the likelihood of a HIA1 removal in the women's game - PAA with an OR of 8.51 (1.23-58.66). The optimal values for sensitivity and specificity for women were 2.01krad.s-2, 25.98g, and 15.38rad.s-1 for PAA, PLA, and dPAV respectively. Conclusion: PLA and dPAV were predictive of men's HIA1 events. Further HIA1 data are needed to understand the role of head kinematic variables in the women's game. The calculated spectrum of sensitivity and specificity of iMG alerts for HIA1 removals in men and women present a starting point for further discussion about using iMGs as an additional trigger in the existing HIA process.

GT and BJ have received research funding from Prevent Biometrics and World Rugby. KS and MB have received research funding from World Rugby. LS, RT, EF, DS, JB are employed by or contracted as consultants to World Rugby. GT previously conducted consultancy work for World Rugby. KS and SK are employed by the Rugby Football Union. SH receives funding for his PhD studies from the Rugby Football Union and Premiership Rugby. BJ is a consultant with Premiership Rugby and the Rugby Football League. MC is employed by Premiership Rugby and was previously employed by the Rugby Football Union. POH has previously been contracted by the Rugby Football Union and is employed by Marker Diagnostics UK Ltd, a company developing salivary biomarker testing for sport related concussion. DA and JT declare they have no conflicts of interest.

Funding was provided by World Rugby, the Rugby Football Union and Premiership Rugby.

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This project was approved by the University's Research Ethics Committee, University of Ulster (#REC-21-0061). The study was performed in accordance with the standards of ethics outlined in the Declaration of Helsinki.

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