Discussion
This study explored how previously researched risk factors for injury—field location, time of match and starters versus substitutes—influence both the incidence and propensity of HAEs during contact events and match play in rugby union to provide insight into the timing and circumstances of HAEs to inform potential interventions that reduce the number of head accelerations players will experience in the game.
Accordingly, the first important finding is that HAE incidence and propensity are not affected by the time in the match. No changes were found for the HAE propensity across a spectrum of PLA magnitudes for tackles, carries or rucks as the game progressed and nor was HAE incidence affected by the match quarter, suggesting that both exposure to contact events and the likelihood that those contact events will cause HAEs at any magnitude is not affected by match time. This has implications for how fatigue and changes in match activities over time affect HAE risk and outcomes. Previous studies have shown that fatigue is a risk factor for injuries,24–27 though not for HIA removals.28 We find no such relationship, though, as discussed subsequently, the interaction of accumulated playing time’s effects on fatigue and substitutions makes this interpretation complex. It is, of course, not as simple as suggesting that players in Q4 are more fatigued than in Q1 since the introduction of substitutes means that a proportion of players in Q4 are playing their first 20 min.29
The second important finding is that substitutes have the same propensity for HAEs during contact events as starters but have a significantly higher HAE incidence. This is intriguing since propensity findings (figure 3) suggest that the per-contact-event-risk is similar between starters and substitutes regardless of playing position, implying that a substitute’s tackle, carry or ruck involvement is as likely to cause an HAE as a starter’s. The higher HAE incidence in substitutes must thus be attributable to greater exposure of substitutes to contact events per unit time on the field. We interpret our combination of findings to show that substitutes have higher rates of HAEs because of greater contact frequency, not greater contact risk. This is explained by the positional bias in substitutions, where five or six of the eight available substitutes are usually forwards, whereas starters are split 8/7 between forwards and backs.29 This accounts for our finding that the HAE incidence in forward substitutes, specifically in the men’s game, is considerably higher than for starters, compared with the relatively smaller difference found between starter and substitute backs (figure 3).
Our comparison of starters and substitutes does not enable a direct evaluation of the effects of relative fatigue (in starters) compared with relative freshness (in substitutes). The dataset, for starters, comprises all their contact events, regardless of match time. Second, we compare the HAE propensity in starters and substitutes, but not whether these roles cause HAEs in other players. To tease out the possible influence of freshness on HAE risk, evaluating the HAE propensity in both the tackler and ball carrier is necessary as a function of whether each is a starter or substitute. In future, research should evaluate HAE propensity in the first 10 min of play, irrespective of starter versus substitute roles and compare this to the HAE propensity in players who have played for more than 60 min, for instance. Future research should also explore the possible integration of other game metrics, such as GPS, to understand further how substitutions affect game involvements, with resultant implications for HAE risk.
The final important finding is that HAE propensity is closely linked to players’ activities at the time of contact and the location on the field where those activities occur. HAE propensity is highest during tackles when players are defending their try line and during carries when attacking the opposition try line (online supplemental tables 1 and 2). This is likely a function of the increased intensity of those tackles and carries to prevent or score a try, which may impact the technique used by players in each situation. This would increase the likelihood of significant HAEs, compared with tackles and carries in the midfield, where ‘winning the collision’ is not as influential. We have not, however, assessed technique in the identified higher-risk situations, but our finding suggests that future research might explore this possibility.
Collectively, the three findings invite consideration of potential mitigation strategies that seek to either (a) reduce the exposure to specific incidents or circumstances identified as causing more HAEs or (b) reduce the likelihood of HAEs in specific circumstances, particularly where propensity is noted to be elevated. Either would reduce overall HAE incidence and exposure. Given that no influence of match time on contact event HAE incidence or propensity was found, there is no specific mitigation that would reduce HAE numbers at the level of match time. Our finding that substitutes have the same propensity as starters regardless of playing position does not support the implementation of strategies that may reduce HAE exposure by limiting the number of substitutes unless it can be shown that substitutes change the frequency of exposure to HAE-inducing contact events.
The final finding that the propensity is greatest for tackler defence and ball carrier attacking situations at the try line is also difficult to mitigate without materially affecting how the game is played. As noted, this is likely explained by the urgency and the resultant intensity of the tackles and carries made in these areas since they are point-scoring (or preventing) opportunities. To reduce these propensities without neutralising the contest would be difficult. One approach may be to limit the number of such occurrences, possibly by considering the introduction of time or phase limits for teams in possession in these areas of the field. This would reduce overall HAE numbers through an exposure reduction rather than propensity reduction. It would, however, represent a significant law change for the sport. Alternatively, the possibility that safe technique is compromised in these situations should be explored, as this may invite coaching interventions to reduce HAE risk by reducing propensity while these situations occur at the same frequency.
More generally, mitigation of HAEs will involve reducing exposure to contact events since these are primarily responsible for HAEs.7 17 30 31 Fatigue remains a potential source of increased HAE risk. Still, the present analysis does not allow the nuance of that potential risk to be explored in a way that may inform whether changes to substitution numbers would increase or decrease overall HAE numbers.
Limitations
One of the main limitations of this study was player compliance with wearing iMG. Where 762 players consented to participate in this study, only 286 provided data that could be examined. This may create a potential bias in the study results as the players who wore the iMG and are included in the presented dataset may not represent all players in the cohort. The current study may not fully represent the various playing styles and conditions at all rugby levels worldwide. Match HAE characteristics may differ in other rugby cohorts, particularly amateur and youth level.
This study focused on high-level player and match characteristics. It did not look at the effect of tackle, carry, ruck technique or other more detailed characteristics of the contact events on HAEs. Technique is understood to be a significant risk factor for injuries, including concussions, and thus likely plays a role in HAE incidence.9 32 Further research needs to look at the effect of the technique on HAE incidence. Understanding the effect of the technique on HAE incidence may allow for the enhancement of technical coaching strategies/cues and/or influence possible law changes in the sport to reduce HAEs and, as a result, potential injury risk. Another limitation is the relatively sparse use of iMGs by players in the same match, which prevents exploration of the interactions between players involved in contact situations. In future, when more players are wearing iMG devices, it will be possible to study and compare HAE outcomes for different role players, characteristics and behaviours. Finally, prevent uses in-house algorithms for data processing and filtering. A fully transparent and common signal processing approach, such as the HEADSport filter method,20 would benefit future study comparisons, particularly if other iMG systems are used.