A thesis about Rugby Union Scrum Physics

Back in 2022, a PhD student from the Hydrodynamics laboratory of Ecole Polytechnique in Paris, France, in partnership with Racing 92, published a thesis about the scrum physics.

Here's the link to read it for free : https://theses.hal.science/tel-04105570

Abstract :

The scrum is an essential element of a Rugby Union game. It consists of the ball being put back into play in opposition of strength between two packs of 8 forwards each. Each pack is organized in 3 rows with 3 players in the first row, 4 players behind them, and a last player in the third row. The scrum is orchestrated by the referee's calls which have changed over the history of Rugby to protect the physical integrity of the players. We can divide the scrum into two distinct phases in force production: an impact phase during which each pack accelerates and collides with the other, followed by a continuous pushing phase after the throw-in of the ball under the scrum. During the sustained push phase, the players of each pack try to synchronize to produce the greatest collective force. This thesis, financed by the professional rugby club of Racing 92, focuses on these two phases with experimental studies conducted at the individual level in order to understand the mechanisms of force production. First, using a scrum training machine equipped with force sensors called the "Atelier de la mêlée" and belonging to the French Federation of Rugby, we characterize individual players in order to construct a pusher phase space that allows us to quantify the quality of a scrum impact. Then, thanks to an individual scrum machine that we developed in the laboratory -also equipped with force sensors- we conducted experimental studies to find the individual scrummaging posture that maximizes the force produced by a player. We have also led a study that looks at the influence of the size of the cleats under the shoes on the force produced by a player on synthetic turf. Finally, we study the scrum at the collective scale to understand the mechanisms of force transmission in a professional pack thanks to the "Atelier de la mêlée". The objective is to improve the performance in scrums through the identification of physical variables translating the synchronization of a pack and its efficiency of pushing.

It is quite the hard thing to read, because it refers to fluid and solid mechanics theory, which needs, at least, a bachelor degree in Maths or Physics to be understood fully. An educated lad that watched some popularization of science Youtube videos will understand most of it though.

However, here are interesting conclusions that the now Doctor ès Scrum found :

The development of a phase plane exhibit the difference in impact force signals between a novice player and a professional group of player and could help to detect the dangerousness of some impacts. [...] The physics of the impact depends on the system that is impacting, and the ones that are impacted. Then, it is important to keep the same physical characteristics

The more elongated postures being the less stable postures, there is probably an optimal posture to find between the optimum of stability and the optimum of force production. This study must be repeated a significant number of times with professional or academy-level players to validate the model.

We observed that the length of the cleats influences the individual force production and also the optimal posture of force production. The work done about the influence of the cleats could be used to develop a method to help the forward players select the optimal length of cleats to maximize their pushing forces during the scrums, without being a hindrance to the other tasks they need to accomplish.

We have seen that the sum of the individual forces of the players of a pack is higher than the collective force produced by the pack. These losses could be explained by the di↵erent postures of each player that are constrained by their teammates, by the loss in transmission of the active muscular force in the deformable structure of a scrum, and probably mainly by the problem of synchronization of force production between each player

These conclusions are basically what top notch scrum coaches *felt* for decades now, and I find it sooo much interesting and fascinating that physics calculation just proves them right !

A fascinating read !