Collisions in adolescent female rugby: incidence, magnitude, tackle technique, cognition and brain integrity across a season of participation : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Rugby is a popular, fast-paced collision sport, but its high contact rate increases injury risks compared to those involved in non-contact sports. While rugby is well-known for its high concussion incidence, more recent concerns focus on former players who, despite never being diagnosed with a concussion, develop brain disorders such as chronic traumatic encephalopathy (CTE). It is suggested that repetitive head impact exposure in rugby places stress and strain on the brain, which, over time, may contribute to the development of neurodegenerative diseases. Youth and female players are particularly vulnerable to brain injury risk because developing brains are more susceptible to injury, and females face higher risks due to anatomical and physiological differences. Despite making up a significant proportion of rugby players in New Zealand, both youth and female players are underrepresented in research to date. Forty-two participants, aged 12-17 yr, were recruited from two under-18 rugby teams in the Canterbury Metro Rugby Sub Union during the 2022 and 2023 seasons. Each participant wore an instrumented mouthguard (iMG) during all games and training sessions to quantify the incidence and magnitude of head acceleration events (HAEs), exceeding 8 g throughout the season, specifically measuring peak linear acceleration (PLA) and peak rotational acceleration (PRA). All sessions were video recorded to verify iMG-detected head impacts. iMG data were analysed to identify key characteristics, including the mechanism of impact (e.g. direct to the head or indirect), head location (e.g. forehead) and rugby event (e.g. ball carrier or breakdown). Tackle-related impacts were further examined to assess the techniques associated with HAEs for both the tackler and ball carrier. Finally, pre- and post-season magnetic resonance imaging (MRI) and neurocognitive testing were conducted to evaluate the effects of seasonal head impact exposure on brain health. iMG data revealed female adolescent rugby players were exposed to similar magnitude of HAEs (PLA and PRA) as matched males and adult females, however, at a much lower incidence of HAEs per game, training, weekly and seasonally compared to adolescent males, adult males, adult females and previously reported adolescent female rugby players. The tackle was the most frequent event to record an iMG event, particularly for the tackler. Very few events followed RugbySmart recommended tackle guidelines for a safe tackle. The ball carrier was more likely to experience an indirect “whiplash” style HAE compared to a direct head impact event. Enhancing the ball carrier’s ability to anticipate a tackle and controlling their body posture while falling may reduce exposure to HAEs. Neurocognitive testing and MRI results revealed no significant change in brain function or structure following a rugby season. When analysing the influence of covariates on brain integrity, we found an association between reduced white matter brain integrity in the body of the corpus callosum, both with wearing headgear and an increased head impact burden. However, results should be interpreted with caution due to low participant numbers, only three participants regularly wearing headgear and the lack of a control group. This thesis provides a comprehensive understanding of the head impacts that adolescent female rugby players are exposed to over a season. Improving tackle technique and training may reduce high-magnitude HAEs, lowering seasonal cumulative head burden and the risk of long-term brain diseases such as chronic traumatic encephalopathy (CTE).