Discussion
The sustained phase of the scrum
We confirmed the results of the study of Preatoni et al
5: the first peak lasted a very brief time but could reach very high level (see table one), in the range of values that studies on cadaveric specimens have indicated as potentially hazardous for (chronic) spine injuries. We also detected a downward oriented peak, occurring after the first peak of force, which could facilitate the collapse of the scrum, a major sources of neck injuries. In addition, we showed that the peaks of force of the transient phase were remarkably synchronised when the players played together. In contrast, their synchronisation, duration and amplitude decreased when the players engaged together. It could be related to the fact that the synchronisation of the players was difficult over a very short and high impact. Finally, in a previous study (Wang et al 2018),10 we showed that the anticipatory postural adjustments during the transient phase were highly variable, which was an additional source of potential injuries.
Altogether, these results showed that the transient phase put players at risk with the rules governing the engagement before 2013. The subsequent studies of Preatoni et al6 7, Cazzola et al8 9 demonstrated that, indeed, the rules change efficiently reduced the dynamics of the engagement and the mechanical stresses acting on forward players.
The DC component
Cazzola8 have shown that while the prebind technique, unlike previous engagement techniques, reduced biomechanical loading, there existed no significant differences between techniques in terms of body kinematics and average force exerted during the sustained push phase. In other words, the result we present here on the sustained phase are still valid, although we tested players who used 2012/2013 engagement processes (referee calls crouch-touch-set) for such was the rule at the onset of this study.
As shown in figure 1, the DC component of the force developed during each trial was very stable along the trials and remained the same whether players were on their own or played together with two other frontliners. In that regard, it contrasts with the engagement. This result tends to support the notion that the decrease observed during the engagement (when players were on their own vs when they played together) has no biomechanical origin but may be due to a double task problem. When playing together, combining a high degree of temporal coordination with an explosive force in a brief lapse of time would be more difficult for players to master than exerting together a sustained effort. In addition, the relationship between the mean and the SD of the DC component for the compression force during the sustained phase was uncorrelated when players played alone, while it exhibited a fair correlation when they played together. Two characteristics of the compression force are thus revealed. First, a player alone displayed large SD for every range of force he exerted. Second, when the same player engaged with other players, the variability of the compression forces he exerted decreased for a given level of force. Moreover, such variability increased with the level of exerted forces in an orderly fashion. From the perspective of a collective motor intelligence, these two characteristics ought to augment the overall pressure exerted on the adverse team during the scrum and facilitate the players coordination during the scrum, rendering the SD of the force they exert somewhat predictable.
Intercorrelation between the oscillatory components
We propose here a new paradigm to assess the degree of cooperation of the player during the scrum and to quantify the effect of mechanical disturbances during the sustained phase. We have shown that the frontliners exhibited a large variability in their ability to synchronise their efforts during the sustained phase. This was true for a given triplet of players and for the same triplet on a trial-to-trial basis. When players succeeded in synchronising their effort, the synchronisation between the hooker and the props was quite efficient, while it was always missing between two props.
The left /right prop pair’s poorer overall synchronisation may be explained by the relative positions of the players. However, at times, the left prop/hooker or hooker/ right prop synchronisation could be as bad as the left prop/right prop synchronisation; this may be related to the difficulty for the hooker to synchronise simultaneously with two other players. It is important to note that our synchronisation measurement tool does not define the best success factors for a pack. Indeed, while a strong synchronisation could increase the collective power in order to destabilise the opposite pack, the desynchronisation could provide interleaved rest-times for players, which could prove beneficial for a long-term effort. This second hypothesis may explain why the mean of the Fx compression force was correlated to its SD when the players play together, whereas no correlation appeared when playing alone (see figure 3): when playing together, each player may have relied on the other player to sporadically reduce their effort, which increased the SD of the force they exerted but also augmented the level of force they could exert during a long stretch of time.
The inter-correlograms were also computed before and after the mechanical disturbances imposed by the machine at random times during the sustained phase. A visual examination of the recordings showed that under such circumstances, the disturbance had some detrimental effect on frontline synchronisation. However, this was obtained only for trials where the disturbance occurred late enough, so that the signal before disturbance was long enough for us to compute relevant values of intercorrelation.
The postural readjustments during the sustained phase of the scrum
During the sustained phase of the scrum, many feet displacement occurred. When the player was alone, feet displacements induced a marked decrease of the compression force he exerted and it is not surprising that these brief episodes of unipodal stances decreased the efficacy of the player.
Feet movements always preceded the loss of force. Obviously, these feet displacement could have indicated that the player was programming a step to progress forward in his contest with the robot. It was not the case since he was instructed to maintain a maximal stable force during the sustained phase. This left us with two hypothesis. Either the player was predicting a loss of efficacy in his effort and, consequently, was optimising his posture pre-emptively or he was interrupting the isotonic muscular contractions underlying the sustained phase, which may have reached their time limits.
When players played together, the timing of feet displacements and the episodes of force decrement were uncorrelated in 45% of the trials. In other words, one of the feet could rise without provoking a subsequent loss of force exerted by the player. This observation may be seen as an outcome of collective intelligence: supporting the body on the neighbouring players compensated the detrimental effect of the transient unipodal posture. In support of that hypothesis, coincident feet displacements in adjacent players were never observed. Consequently, one could speculate that in the remaining 55% of the cases, when foot displacement did induce a force decrement, this was due to an inadequate strategy of the player. In that context, we hypothesised that the hooker could benefit from the support of both the left prop and right prop when he raised his feet. This was only the case in one of the three hooker we tested, which may be due to an inadequate training of the two other players to use two supports moving independently at the same time.
Scrum, a model to study collective intelligence
A recent paper by Sedeaud et al
11has shown that the XV de France’s collective effectiveness relies on a balance between stability and workforce renewal, which allows the building of specific position interactions and builds on skilled forwards packs. It strengthened the notion that selections and shared selections are serious collective performance parameters associated with performance.
In that context, our results demonstrated that collective intelligence exerted its effects at different phases of the scrum. The initial peaks of forces during the engagement could be synchronised when frontliners played together. Two adjacent players were also capable to synchronise their effort during the sustained phase. Finally, supporting the body on the adjacent player could compensate the detrimental effect of transient unipodal postures. But it was also clear that this collective intelligence had its drawbacks and its limits. For instance, the initial peak of force was diminished when the frontliners played together.