Background: Head acceleration events (HAEs) are an increasing concern in collision sports owing to potential negative health outcomes. Objectives: The objective of this study is to describe the probabilities of HAEs in tackles of differing heights and body positions in elite men`s and women`s rugby union. Methods: Instrumented mouthguards (iMGs) were worn in men`s (n = 24 teams, 508 players, 782 observations) and women`s (n = 26 teams, 350 players, 1080 observations) rugby union matches. Tackle height (i.e. point of contact on ball-carrier) and body positions of tacklers and ball-carriers were labelled for all tackles in which a player wore an iMG. HAEs from the initial impact were identified. Mean player, tackler and ball-carrier exceedance probabilities for various peak linear and angular acceleration thresholds were estimated from ordinal mixed-effects models. Results: Contact with ball-carriers` head/neck resulted in the highest mean HAE probabilities for both sexes. The probability of an HAE to the ball-carrier decreased as tackle height lowered. The highest probability for the tackler was initial contact to the ball-carriers upper leg. Body position influenced the probability of HAEs, with falling/diving ball-carriers resulting in higher mean probabilities. When a player, regardless of role, was bent-at-waist, elevated HAE probabilities were observed in men`s competitions. Women`s data demonstrated similar probabilities of an HAE for all body positions. Conclusions: Initial contact to the ball-carrier`s head/neck had the highest chance of an HAE, whilst role-specific differences are apparent for different tackle heights and body positions. Future player-welfare strategies targeting contact events should therefore consider HAE mechanisms along with current literature.
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