Background Rugby Union scrums which collapse possess a higher injury risk than completed scrums. A mechanically stable scrum should limit the motions which generate uncontrolled and hazardous (non-axial) loading on front row players and minimise the risk of scrum collapse.

Objective To compare scrum stability during live contested rugby scrums using two different scrum engagement processes.

Design A cross-sectional design, where teams from different playing levels performed live scrums under two engagement processes: CTS (Crouch-Touch-Set, 2012–2013); and PreBind (Crouch-Bind-Set, 2013–2014), with measurements made via video analysis.

Setting Outdoors, natural turf rugby pitches.

Participants 27 Rugby teams (i.e. 54 forward packs, n=432 players) from five playing levels – International, Elite, Community, Women and University.

Risk factor assessment Scrum engagement process. PreBind differs from CTS for a legal bind that is taken at the “Bind” call and is maintained through the “Set” action.

Main outcome measurements Vertical motion of the (i) shoulder and (ii) centre of mass (COM), and (iii) COM variability in the sagittal plane of prop forwards during initial engagement.

Results The extent of props' vertical shoulder motion were reduced (P<.05, Cohen's d=0.3–0.5) during engagement in the PreBind process compared with the CTS process. The COM vertical motion tended to be reduced in PreBind process (Cohen's d=0.45).There were also reductions in the variability (P<.05, Cohen's d=0.5–1.0) of the props' COM vertical motion, meaning vertical movements were more controlled during PreBind engagement. The International level had reduced vertical motion characteristics with respect to the Community level.

Conclusions Scrum stability improved (moderate effects) in the PreBind engagement process, with props making less extreme postural adjustments during initial engagement. Combined with attenuated peak forces during the engagement process, the PreBind improves the stability of the scrum and should minimise the risk of scrum collapse and subsequent injury.