The relatioship between player technique and forces generated in rugby scrummaging

Introduction
The scrum is integral to the game of rugby union. Although the International Rugby Board (IRB) laws state that players must not “charge” at each other and that players should remain stationary until the ball has been thrown into the scrum (IRB, 2013), modern scrummaging often endeavours to achieve a high-impact engagement. Little is known about how player technique influences the forces experienced at impact and how these forces may then influence the forces produced during the sustained push phase of scrummaging. Therefore, our aim was to understand the relationships between player kinematics, engagement forces, and forces during the sustained push phase.
Methods
Twelve elite professional rugby union forward packs performed up to four scrummaging trials against a scrum machine and one trial was selected for analysis. The scrum machine (Dictator, Rhino Rugby, UK), was instrumented with a bespoke three-dimensional force measurement system (Preatoni et al., 2012). Player kinematics were obtained by manual digitisation of multiple two-dimensional 50 Hz video sequences as described by Preatoni et al. (2012) and parameters recorded at movement onset (@onset), at engagement with the machine (@engage), and during the sustained push phase. Vertical and compression forces were recorded throughout each scrum and peak and average values extracted during both the initial engagement phase and sustained phase. Pearson correlations were used to determine associations between player technique and performance.
Results and Discussion
The further the front row was from the scrum machine @onset, the greater the velocity of the front row @engage (r=0.790, p=0.002). A high velocity of the front row @engage was associated with greater peak compression force, although this was not significant (r=0.434, p=0.158). However, a higher peak compression force was only weakly correlated with a greater compression force during the sustained push (r=0.190, p=0.554), indicating that a large “hit” at engagement did not result in better generation of force during the sustained push phase. Therefore, there was little “carry-over” effect from a large hit at engagement. Furthermore, larger peak compression forces were associated with greater peak downward forces (r=0.530, p=0.076). These results suggest that employment of an aggressive engagement technique which strives to achieve a high-impact engagement may not contribute to better sustained scrum performance, and the accompanying large downward forces may increase the risk of scrum collapse, which disrupts play and is potentially injurious.
References
IRB (2013). Laws of the Game. International Rugby Board, Dublin.
Preatoni E et al. (2012). P I Mech Eng P - J Sports Eng Tech, 226(3/4), 266-273.
Acknowledgement
Funded by the International Rugby Board