This study explored how the body position of the tackler and ball carrier was associated with the propensity and incidence of HIAs to both players during tackles in the National Rugby League. Our first important finding, consistent with previous research,7–9 is that the tackler is 1.7 times more likely to require a HIA from a tackle than the ball carrier. Of the 446 HIAs, 63% occurred to the tackler. This is a finding similar to that observed in rugby union.7 This has implications for the injury prevention initiatives designed to reduce risk for HIAs. The application of law typically protects players from the actions of their opponents. However, our data, like that in Rugby Union, shows that the majority of injuries happen to the player initiating, not receiving, the action (i.e., the tackler).
To explore this further, we focused on the body position of the players in the tackle, because it had previously been shown that an upright tackler creates significantly more risk of an HIA than a bent tackler.7 Our second important finding is to confirm this for Rugby League, where the overall propensity for an HIA was 3.2-fold higher when the tackler was upright compared to bent-at-the-waist, and 2.1-fold greater than for a falling/diving tackler (Figure 1). The result is an HIA from upright tacklers every 1.36 matches, compared to one every 4.14 matches for a bent tackler. Because these two body positions account for 83% of all tackles, their relative risks are most important for risk mitigation considerations.
Similarly, we find that HIA propensity and incidence are highest when the ball carrier is upright (Figure 1). Notably, when we explored the HIA risk to each player in the tackle as a function of their body position, we confirm that the overall risk is greatest when tacklers are upright, but also find that this highest risk exists for both the tackler and the ball carrier specifically (Table 1). Upright tacklers thus create greatest risk to themselves, with an HIA propensity that remains 1.9-fold greater than when the tackler is bent-at-the waist (Table 1), as well as to their opponent ball carrier, whose risk is increased by a factor of 12.8 when the tackler is upright (Table 1).
Naturally, the interaction between the tackler’s body position that the body position of ball carriers is an important factor to consider. Here, we have explored many possible interactions between upright, bent-at-the-waist, and other player positions (comprised of falling/diving, jumping kicking, lying and slipping). This analysis confirms that when both players are upright the HIA propensity is greater than (i) when both players are bent in the tackle, and (ii) when the tackler is upright and the ball carrier is bent. Indeed, for any ball carrier body position, the HIA risk was greatest when the tackler was upright, and for any tackler body position, the HIA risk was lowest when the ball carrier was bent. While the specific interactions did not always reach significance (Fig 1), the implication is clear, and consistent with what was found in rugby union7 – the safest body position is when the tackler and ball carrier are bent, and no specific interaction changes this relative risk profile, though certain interactions create similar risk. Overall, however, bent players are considerably less likely to cause head injuries during tackles.
This can be understood when assessing the location of head contact with the opponent that is responsible for causing the HIA. Here, we have examined the HIA risk to the tackler and the ball carrier separately (Table 2). Unsurprisingly, the greatest HIA propensity occurs, generally, for head contact with a hard, bony surface like an elbow, boot, knee or head of the opponent (Table 2). The very highest propensity impacts are however relatively rare, and thus have a low incidence.
Significantly more frequently, tacklers were injured by head-to-head (moderate propensity) and head-to-shoulder (low propensity) impacts, while ball carriers were injured by head-to-head, head-to-arm, and head-to-shoulder impacts (Table 2). Collectively, these findings indicate that tackles where the head is very low (knee or below) or very high (head height) create significantly more risk, with the safest zone at the level of the opponent’s torso.
The game play risks and their association with HIAs caused by the various head contact locations can be understood when appreciating that player body position exposes players to situations where their heads are more likely to encounter higher risk contact locations. That is, given the high propensity of head-to-head, head-to-elbow, head-to-knee, and head-to-arm impacts to cause HIAs (Table 2), the player body position that creates higher likelihoods of these impact locations is going to produce the highest HIA propensity. This occurs when players are upright, as we have shown (Figure 1), or very low (diving to the opponent’s knee).
The strategy that may be explored by regulators to reduce injury risk is therefore to drive tackle technique or execution changes that prevent or reduce the likelihood of head-to-head, head-to-elbow, and head-to-knee impacts. Instead, it would be desirable for heads to be in proximity with, and to make contact with, the torso or the shoulder of an opponent in the tackle, because the HIA propensity for these contacts is very low (Table 2). Importantly, even though the HIA incidence from head-to-shoulder impacts is high (period of 6.09 and 8.20 matches for tacklers and ball carriers, respectively), they are low in risk or propensity (Table 2), indicating the interplay between the frequency of an event and its inherent risk. Therefore, if tacklers and ball carriers were to tackle in such a way as to substitute the highest risk head-to-head, head-to-elbow and head-to-knee impacts for impacts with lower risk at the torso or shoulder, the overall number of head injuries during tackles will decrease. This concept, where one behaviour is substituted for another, requires identification of the behaviour with the higher propensity, so that it can be replaced by the identified safer and thus desirable behaviour with the lower propensity.
In this study, we clearly describe a combination of player body positions and head impact locations that span this spectrum from low propensity (bent players, head impacts with the torso of the opponent) to high propensity (upright players and head impacts with opponent’s heads, or diving players and head impacts with opponent’s knees and feet).
Applying this concept to the body position findings we describe previously, it would thus be desirable for tacklers and ball carriers to more often be bent at the waist, with fewer instances where they are upright. This should reduce the overall number of HIAs because the higher risk behaviour (upright players) is substituted with the lower risk behaviour (bent players). It must be cautioned, however, that if the tackler is too low the risk may increase again as a result of more frequent head to knee and head to boot contacts, which we also found to be high in risk, though very rare. Finally, the elbow-to-head scenario we find to have the highest propensity may be reduced through technique training and law interventions to prevent the use of the elbow on opponent’s heads.
The relatively greater propensity for an HIA when players are upright is in part the result of their head proximity to the higher risk body parts of their opponents (heads and elbows, see Table 2), but may also be the result of dynamic elements and biomechanical factors in the tackle that are beyond the scope of this analysis. For instance, it may be that a bent tackler, whose head is in front of their body while their neck is braced, is less susceptible to the neck forces and head accelerations that can cause a concussion.14 Tierney and colleagues14 have demonstrated, using a passive biomechanical model, that head and neck kinematics and mechanics are significantly affected by the area of contact, with higher linear and angular acceleration of the head for ball carrier during upright, higher contacts.14 Such a phenomenon may contribute to our findings, and may be further moderated by the relative head and neck position of each player when bent at the waist. Also, the context of the tackle may change, with elements of speed, acceleration, direction and tackle technique altered when players are bent compared to upright.
Limitations
There are a number of limitations in the current study. First, the interpretation and coding of the tackle variables are subjective. All events were coded by one analyst to avoid any considerations of between-rater differences, though the subjective nature represents a potential source of error in the analysis. The present method of analysis employs a discreet approach to identify specific tackle characteristics that may be the target of risk mitigation strategies. The characteristics do however interact with one another, and while in this study we have attempted to explore these interactions for body position, head contact and ball carrier evasion method, the tackle is a dynamic and complex event during which many factors may affect risk in subtle ways. Third, a number of the tackle variables occurred rarely, leading to sparse data, and should be interpreted with caution. Third, the results may not be generalisable to other levels of play or women players.