Introduction
Rugby union players are consistently exposed to a relatively high risk of injury across all playing levels.1–12 While the high level of participation suggests that players accept this risk of injury, specific elements within the game remain the target for reduction strategies. The scrummage (hereafter termed the ‘scrum’), involves eight players from each team attempting, en masse, to push their opponents backwards and regain ball possession. This represents the contact event with the highest risk of injury within the sport.13 World Rugby, the game's governing body, has recently evolved the laws in an attempt to reduce this risk of injury; hence, as of the 2013/2014 playing season, the scrum followed the referee's command ‘crouch-bind-set’ (CBS), as opposed to ‘crouch-touch-pause-engage’ (CTPE).14
Scrum injuries can be either acute, chronic or degenerative in nature,15–17 with longer term exposure to this demanding biomechanical environment associated with disc narrowing (35–71% incidence), osteophyte formation (83%), apophyseal joint degeneration (74%), and degeneration of the vertebral endplates (77%).18 ,19 Given the time spent scrummaging compared with other contact events during a game, it is associated with a disproportionately high percentage (6–13%) of all spinal injuries within the sport.20–25 These spinal injuries predominantly happen to players of the front row. Front-row players (ie, the three players from each team, who directly oppose each other) are at particular risk of both chronic and acute injuries, accounting for 78% of all scrum-based acute cervical spine injuries.21 ,26 While it is acknowledged that risk of injury is multifactorial, the biomechanical demands of the scrum are unique and are likely to significantly contribute to the risk of injury. The scrum consists primarily of the ‘hit/impact’, followed by a sustained attempt to ‘shove’. The ‘hit’ is where the two packs initially engage, immediately after the referee's instruction. Both packs then produce a sustained shove, aiming to push the opposition away from the ball. As the packs engage, the shoulders of the front-row players of each team collide, their heads become interlocked and are forced underneath the chest of their opposing player. The impact force of this interaction has previously been reported, varying from 4.4–16.5 kN,27–31 measured by instrumenting either the scrum machine or shoulder pads (point of impact) of front-row players, during live scrummaging.31
Recent kinematic studies have additionally focused on investigating neuromuscular activation patterns during simulated, live and machine-based scrummaging.31 ,32 While no significant differences were observed between engagement sequences for body kinematics, it does appear that the ‘crouch-bind-set’ engagement sequence may prepare the cervical spine by stiffening the joints prior to impact. Furthermore, it was reported that machine scrummaging does not accurately replicate live scrummaging, suggesting the need for future studies to focus on investigations of live scrummaging.
As the scrum has been identified as being specifically associated with injury, a greater understanding of spinal kinematics will further aid in the quest for injury reduction. To date, while studies have considered machine-based and simulated live scrummaging,33 ,34 technical challenges have prevented studies from acquiring spinal kinematics during competitive scrummaging. Given the association between scrummaging and the potential for spinal injury, it is critical that these challenges are met to enable data acquisition in the most relevant physical environment.
This study aimed to determine the resultant spinal kinematics of the hooker during competitive scrummaging using two different sequences (CBS and CTPE), and using CBS when performing machine-based scrummaging. This study focused on the most injury-prone player, the hooker,21 ,23–25 generating the first data sets that allow direct comparison of spinal kinematics between the new and old scrum sequences. It was hypothesised that the hooker's spinal kinematics will be more constrained during the new (CBS), as opposed to the older (CTPE), engagement sequence.