Background Head acceleration events (HAEs) are an increasing concern in rugby union due to their association with brain injury and long-term neurological health. Objectives This original research study aimed to establish essential baseline data on HAEs in under-21 professional rugby using instrumented mouthguards (iMGs) and to determine whether a higher competition level (professional club versus international) is associated with elevated magnitude and incidence rate of HAEs. Methods A prospective observational cohort design was used with 32 under-21 club academy players and 38 under-20 international players. Players underwent 3D dental scans and were provided with custom-fit iMGs. The iMGs contained an accelerometer and gyroscope that sampled at 3200 Hz with measured ranges of*±*200 g and*±*35 rad/s. The iMGs recorded peak linear acceleration (PLA), peak angular acceleration (PAA) and peak angular velocity (PAV). Data were collected from academy matches (254 player-matches; 241 h of exposure) and international matches (114 player-matches; 118.2 h of exposure). Descriptive statistics (median, interquartile range [IQR], 95th percentile and peak values) were calculated to characterise HAE magnitude across positions and competition levels. Negative binomial regressions assessed incidence rates across teams, positions and thresholds, adjusting for exposure time. Results A total of 6508 HAEs were recorded across 368 player matches from club U21 and national U20 rugby players. Median and peak values for linear acceleration, angular acceleration and change in angular velocity were generally higher in club players, particularly in the front row and outside backs. Club outside backs demonstrated the highest PLA (72.8 g) and PAA (5740 rad/s2), while the club front row reached the highest PLA (81.8 g) and PAA (8034 rad/s2) overall. The national team back row reported the highest *PAV with 38.3 rad/s. Incidence rate analyses revealed significantly higher rates of severe HAEs (>*30 g,*>*2000 rad/s2,*>*15 rad/s) in club outside backs and the club front row compared with their national counterparts. Conversely, the club halves exhibited significantly lower rates of low-magnitude events across all metrics. Conclusion This study compared HAEs in U21 professional club and U20 international rugby players using iMGs. Contrary to expectations, international players did not consistently exhibit higher HAE magnitude or incidence. Notably, club outside backs and front row players experienced the highest severe HAE rates. These findings highlight the need to consider playing style, position-specific demands and technique in strategies to mitigate concussion risk in developing elite rugby players.
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