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Abstract
Aim Biomechanical studies of the rugby union scrum have typically been conducted using instrumented scrum machines, but a large-scale biomechanical analysis of live contested scrummaging is lacking. We investigated whether the biomechanical loading experienced by professional front row players during the engagement phase of live contested rugby scrums could be reduced using a modified engagement procedure.
Methods Eleven professional teams (22 forward packs) performed repeated scrum trials for each of the three engagement techniques, outdoors, on natural turf. The engagement processes were the 2011/2012 (referee calls crouch-touch-pause-engage), 2012/2013 (referee calls crouch-touch-set) and 2013/2014 (props prebind with the opposition prior to the ‘Set’ command; PreBind) variants. Forces were estimated by pressure sensors on the shoulders of the front row players of one forward pack. Inertial Measurement Units were placed on an upper spine cervical landmark (C7) of the six front row players to record accelerations. Players’ motion was captured by multiple video cameras from three viewing perspectives and analysed in transverse and sagittal planes of motion.
Results The PreBind technique reduced biomechanical loading in comparison with the other engagement techniques, with engagement speed, peak forces and peak accelerations of upper spine landmarks reduced by approximately 20%. There were no significant differences between techniques in terms of body kinematics and average force during the sustained push phase.
Conclusions Using a scrum engagement process which involves binding with the opposition prior to the engagement reduces the stresses acting on players and therefore may represent a possible improvement for players’ safety.
- Rugby
- Biomechanics
- Injury Prevention
- Physical Stress
- Back Injuries
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Introduction
Contemporary rugby union scrummaging involves a dynamic engagement phase and a period of sustained pushing.1 ,2 Previous studies have alluded to the intense physical nature of the scrum,1–3 the moderate acute injury incidence arising from the scrum,4–8 the relatively high injury risk for front row forwards,9 the moderate association with catastrophic rugby injuries10–12 and the potential effect scrummaging has on long-term degeneration of the spine.13–19
The biomechanics of scrummaging has been described in terms of the forces produced1–3 ,20 ,21 and the motions observed,22 but most of these studies focus on scrummaging against a machine. Du Toit et al23 measured forces at the front row interface during live scrummaging using pressure transducers, but this study only included school-age players. Consequently, there is still a gap between the understanding of machine scrummaging and the transfer of this knowledge to the contested scrummaging context, where the forces and the motions might change because of the less controllable counteraction offered by the opposition pack.2 In order to provide more insight into the level of loading experienced by rugby union forwards during scrummaging and whether this level of loading can be modified to potentially reduce injury risk, there is a need to measure the biomechanics of scrummaging under match-like conditions.
Therefore, the aim of this research was to determine whether modifying the engagement technique influences mechanical stresses that represent risk factors for injury during live contested scrummaging. The hypothesis was that an engagement process designed to de-emphasise the dynamics of the initial engagement would reduce the peak biomechanical loading metrics but maintain the forces observed during the sustained push phase.
Methods
Study design
In a repeated measures design, rugby forward packs were analysed at one point in time during the 2012/2013 season and each performed repetitions of trials under three different scrum engagement processes. Multiple force and motion measures were the dependent variables and the engagement technique was the within-group factor.
Engagement techniques
Three different engagement techniques, including the current technique at the initiation of the research programme and two modified processes were tested with each team (table 1 and figure 1). The engagement processes were the 2011/2012 variant (crouch-touch-pause-engage; CTPE), the 2012/2013 variant (crouch-touch-set; CTS) and a process which modified the technique of props to incorporate a prebind with the opposition prior to the ‘Set’ command, to be introduced globally as the 2013/2014 variant (PreBind).
The engagement processes tested
Images of ‘key instants’ of the crouch-touch-pause-engage (CTPE) (left) and PreBind (right) techniques. A=‘TOUCH’ call; B=‘ENGAGE’ (CTPE) or ‘SET’ (crouch-touch-set (CTS) and PreBind) call; C=sustained push phase. CTS has not been reported because visually very similar to CTPE. The PreBind technique evidently differs from CTPE (CTS) because of a lower distance between front rows at ‘TOUCH’ (A), and of the bind maintained by the props on their opponent's trunk from ‘TOUCH’ throughout the engagement phase (B).
Participants
Eleven rugby teams (22 forward packs, n=176 players) were recruited from the professional standard playing level, ranging from senior international forward packs to elite club forward packs (minimum level 2 in the domestic club structure of Tier 1 Rugby Unions). The sample size was determined based on significant differences with six elite teams evaluated during a machine scrummaging study,2 and expecting that in this study the engagement techniques evaluated would have smaller effect of size. For this reason a bigger sample (11) was selected to have an adequate statistical power for evaluating differences between techniques. Mean pack mass was 853.9±28 kg. Individual players provided individual written informed consent prior to participation.
Data collection
All testing sessions took place on natural turf, to mimic match conditions as closely as possible, and the measurement system was fully portable. Before testing, all players undertook a coach-directed warm-up, were provided with an additional verbal description of the different scrummaging techniques to be performed and had the chance of undertaking some practice trials to become familiar with the modified engagement processes. Each team (2 forward packs) performed a complete scrum testing session that typically comprised a total of 12 scrums (4 repetitions per 3 techniques), up to a maximum of 16 scrums to account for mistiming of engagements or scrum collapses. Engagement techniques were presented in random order but all teams performed the trials in a blocked sequence. One forward pack was nominated as ‘Team A’ who was the pack with the ball throw in; the opposing forward pack was nominated as ‘Team B’ (figure 2). Recovery intervals were included between repetitions (1–2 min) and between technique changes (∼5 min).
The camera views (side and top view) of a typical experimental set-up and their relative kinematics parameters. (A) Top view: the trunk centre of mass position for each player was calculated using head, C7 and sacrum anatomical landmarks and referring weighting factors to De Leva anthropometric tables; (B) side views: the props’ centre of mass was calculated using hip and shoulder anatomical landmarks. Sagittal plane (Y horizontal axis–Z vertical axis) joint angles were calculated as the angle between the longitudinal axis of the head and the horizontal axis. In the sagittal plane, the whole scrum centre of mass motion was calculated as the combined centre of mass of players A1 and B3 (left side) and combined centre of mass of players A3 and B1 props.
Instrumentation and data processing
A bespoke control and acquisition system (cRIO-9024, National Instruments, Austin, Texas, USA) was programmed (Labview 2010, National Instruments, Austin, Texas, USA) to synchronously simulate the referee's call as during a real scrummage by delivering consistently timed audio commands and triggering the acquisition (inertial, pressure, video) hardware. Two versions of referee call sequences were used, the ‘crouch-touch-pause-engage’ (duration of full sequence was 5.2 s with t=0.0 s the ‘engage’ command) and the ‘crouch-touch-set’ (duration of full sequence was 4.0 s with t=0.0 s the ‘set’ command).
Inertial measurement system
Each front row player was equipped with an Inertial Measurement Unit (MTw, Xsens Technology B.V., NL) placed on the estimated C7 vertebra position. Raw acceleration signals were sampled at 1800 Hz and transmitted at 50 Hz using the proprietary strap-down integration method. To compare inertial loading across scrummaging techniques, acceleration data were expressed as the module of overall acceleration exerted on an anatomical segment during the trials.
Pressure measurement system
During each scrum session, three pairs of pressure sensors (Model #3005E VersaTek-XL size) were used to collect the pressure distribution between front rows at a sampling frequency of 500 Hz (F-Scan, Tekscan Inc, USA). Each pair of sensors was trimmed to fit into bespoke sleeves and attached on the left and right shoulders of ‘Team A’ front row players (A1—loose head prop, A2—hooker, A3—tight head prop). Pressure data were used to estimate contact forces. All the pressure sensors had been previously calibrated in the lab in comparison with force plate measures by using a method specially designed for force patterns typical of scrummaging.24 The overall force (Ffront row) acting on the ‘Team A’ front row was calculated as the sum of all the single player (A1, A2 and A3) estimated forces.
Video analysis
Four digital video cameras (2 side cameras and 2 top cameras) synchronously captured players’ movements from three different views (top, left and right). Side cameras (HDR-HC9, Sony, Japan, 50 Hz) were placed to view the loose head and tight head props sagittal motion, while top cameras operated at 200 Hz (HVR-Z5, Sony, Japan) and 50 Hz (HVR-Z5), respectively, and were positioned to view transverse motion of the scrum (figure 2). A rigid frame three-dimensional (3D) calibration object was used for multiple 2D calibrations using four-point projective scaling. Video sequences were later captured and digitised using Vicon Motus software (V.9, Vicon Motion Systems, USA) to allow the reconstruction of the position of selected body landmarks and for the estimation of kinematic variables (displacements, angles and their derivatives; figure 2).
Custom-written software (Matlab R2011b, MathWorks, Natick, Massachusetts, USA) was used to process acquired signals and to calculate a set of parameters for each scrum repetition. Parameters were selected with the aim of describing the kinematics (figure 2) and kinetics of contested scrums across the phases of scrummaging (figure 3), primarily in connection with potential injury factors. The phases of the scrum were ‘Approach’, which incorporated initial set-up and lasted from the onset of movement until the initial contact between the two teams; ‘Engagement’ was the interval between initial contact and 1 s after the instant of peak force (Ffront row max value); ‘Sustained Push’ extended from the end of ‘Engagement’ for an additional 1 s.
Characteristic summed force traces for each engagement technique for one Elite team, where t=0 represents the ‘engage’ call for crouch-touch-pause-engage technique and ‘set’ call for crouch-touch-set and PreBind techniques. The force peak values of total compression force (sum of front row players) for each engagement technique are visible in the Engagement phase. The minimum values of the total compression force, used to calculate the ‘rebound’ effect are detectable in the engagement phase. The average total compression force (sum of front row players) for each engagement technique is the average value calculated for each curve during the entire Sustained Push phase.
Statistics
One-way repeated measure analysis of variance (with scrummaging technique as the within-group factor) was applied to test for possible changes across engagement techniques, followed by Bonferroni post hoc comparisons (p<0.05). Sphericity of datasets was checked by applying Mauchly's test. Differences were considered significant for p<0.05 and effect sizes (η2) and observed power were reported. Pairwise effect sizes using Cohen's (d) values were also taken into account (see online supplementary appendix 1–3).
Results
Approach
PreBind (0.32±0.07 m) reduced the distance between the two front rows at set-up by about 0.12 m, compared with CTPE (0.44±0.06 m) and CTS (0.45±0.05 m). PreBind (2.12±0.41 m/s) also significantly reduced the peak engagement speed (ie, the maximum of the sum of the velocities of the two front rows coming together) by 18% compared with CTPE (2.59±0.41 m/s) and CTS (2.59±0.44 m/s).
Props generally had a higher shoulder height and a more ‘shoulder above hips’ position (effect sizes, see online supplementary table A1 (appendix)) prior to movement onset in the PreBind compared with the other two techniques.
Engagement
The PreBind technique resulted in a significant decrease in the biomechanical stresses acting on the front row players during the engagement compared with CTPE and CTS. The forces measured by the shoulder pressure sensors were approximately 35% (PreBind vs CTPE) and 25% (PreBind vs CTS) lower (table 2 and figure 3). In addition, the average peak accelerations of the sensor positioned on the cervical (C7) landmark decreased by about 16% (PreBind vs CTPE) and 14% (PreBind vs CTS), respectively (table 2). Finally, the extent of vertical motion in the sagittal plane once the two forward packs had engaged showed a decreasing trend moving from CTPE to CTS and to PreBind with a moderate-to-large effect size between CTPE and PreBind for the amount of vertical excursion measured on both sides of the scrum (online supplementary table A2 (appendix)).
Kinetic and kinematic measures of the front row players during the engagement phase, across the three different engagement techniques
Sustained push
There were no significant differences between the three engagement techniques in the average force exerted during the sustained push phase (CTPE=4.2±1.4 kN; CTS=3.8±1.4 kN; PreBind=3.8±1.2 kN). The effect sizes for the differences between the three engagement techniques for the vertical offset between the props’ shoulder and hip, over the sustained push phase, were all small (online supplementary table A3 (appendix)).
Discussion
The aim of this study was to determine whether a modified engagement procedure could reduce the biomechanical loading experienced by front row players in live contested rugby scrums. Compared with the CTPE and CTS techniques, the PreBind technique (1) reduced the biomechanical load experienced by front row players during the initial engagement phase; (2) maintained the overall ability to produce an effective sustained push and (3) maintained scrum stability. Points (1) and (3) are potentially important for injury prevention/player welfare, and point (2) suggests that the scrum can be maintained as a contest even with a modified engagement.
All the indicators of mechanical stresses (accelerations and peak forces) acting on the front row players were significantly lower in PreBind than in the CTPE and CTS engagement techniques, with the overall magnitude of this reduction being in the region of 20%. This was likely due to a lower front row distance at the initiation of the engagement and subsequent reduced engagement speed which would have decreased the momentum of the overall system at initial contact.
Repetitive loading/impacts on the spine,16 with magnitudes of force,25–29 speed15 ,25 and/or accelerations30 that are not dissimilar from the load absorbed by players during scrummaging, may induce chronic pain13 ,14 ,16–19 to the cervical and lumbar spine. In general, the determinants of cervical injury mechanics include force characteristics (magnitude, vector direction and rate level),31 head constraints and trunk/neck orientation before impact.32 High magnitude and eccentricity (off-centre application) of the compressive axial load causes bending moments in the cervical column segments leading to buckling mechanisms and consequent ligament disruptions and facet dislocations.31 ,33 ,34 The described situation, with regard to constrained head movement and non-axial external loads, is exactly the one experienced by rugby forwards when scrummaging. For these reasons it is imperative to control the scrum engagement sufficiently to avoid impacts directly on the head and to reduce the overall biomechanical load, in order to minimise the risk of catastrophic injuries and chronic degeneration of the spine. Focusing on the effects of modifying the engagement technique from an injury prevention perspective, it could be speculated that a move to the PreBind technique could be a positive step for reducing chronic injury problems due to scrummaging. In fact, bearing in mind the high scrum rate undertaken by forward rugby players (estimated at up to 60 scrums/week including matches and training), the approximately 20% reductions in loading parameters observed during the engagement phase with the PreBind technique should be viewed favourably when considering the repetitive nature of the task, since these reductions will exist for each scrum undertaken.
The PreBind technique provided a sustained push force magnitude as high as in the other techniques, even with a de-emphasised engagement and a reduced dynamics of the engagement phase compared with CTPE and CTS. In fact, during the PreBind technique, no decrease in the ability to generate force against the other pack was observed, and lower drops in force during the transition between the initial engagement and sustained push were observed (table 2 and figure 3). This last result may indicate a better capability for the team to achieve a more consistent force production during PreBind, which is useful to either produce momentum or to counteract the drive of the opponents. This aspect may also be important from a scrum stability viewpoint where the ‘rebound’ effect in the PreBind was attenuated, and therefore in terms of timing and magnitude of force production the scrum passed through a transient phase with less likelihood of imbalance between action and counteraction. In analogous spring-like terms, the CTPE and CTS techniques are underdamped and the two forward packs continue to oscillate following engagement, whereas the PreBind technique is critically damped and the two forward packs converge quickly to a steady state.
The extent of scrum stability was estimated by considering a number of kinematic variables whereby reduced excursions/range of motion was taken to mean more stability since players were making less postural adjustments. Generally, CTPE showed greater excursions and more instability than CTS and PreBind. These changes reflected scrum centre of mass movement during the engagement phase in the sagittal plane, and hip joint range of motion of the props, which we considered as indexes of stability. A moderate-to-large effect size (online supplementary table A2 (appendix)) indicated a tendency towards increasing stability moving from CTPE to CTS and to PreBind, but without showing a high consistency between variables. In any case, these results suggest that players were making more postural adjustments during the initial stages of the scrum in the CTPE technique compared with the PreBind and CTS techniques. Regarding PreBind, this stability advantage may be due to the prebind action itself, where prop forwards take a legal bind on their opposite number before the engagement phase (before ‘set’ call). First, this means the PreBind technique may decrease the number of missed or slipped binds due to props having to establish a grip prior to the dynamic engagement phase. Second, the PreBind technique may help to establish a more controlled starting body position since props have to stretch out their arm and maintain the bind with the opponent, and therefore a horizontal or downward inclination of the trunk may be difficult and cause a loss of balance. A significantly higher props’ shoulder height measured in the PreBind technique provides support for this assertion. The apparent moderate improvement in stability of the CTS technique over CTPE is harder to explain. The only change was the move to the three-stage call sequence, so possibly elimination of the ‘pause’ command did indeed allow a more coordinated engagement between the two packs, which was one of the tenets of the introduction of this call sequence for the 2012/2013 season.
Focusing on the trunk alignment and building on the ‘spine in line’ reference as the underpinning principle, no significant changes between engagement techniques emerged from the analysis of variables summarising the players’ movements over the engagement phase (table 2): the average deviation from the direction of push (ie, average absolute angle) in the transverse (‘left/right’ rotation) and sagittal (‘down/up’ rotation) planes was similar in the three engagement techniques. This suggests that the PreBind technique did not positively or negatively influence players’ technique in terms of extremes of neck and trunk angles during the engagement phase.
Conclusion
Results on 11 elite rugby teams suggested that a scrum engagement technique which incorporated a prebind between the two forward packs produced the intended effect of reducing the loading experienced by front row players during the engagement process, while maintaining scrum stability and the ability to generate sustained pushing forces. The reduced loading with the PreBind technique was observed across all of the key outcome measures in a consistent manner, producing a reduction in the peak loading values of approximately 20%. The scrummaging forces during the sustained push phase were consistent across the engagement techniques and there were no apparent deleterious effects on players’ technique from the PreBind technique, and some positive results in derived stability measures. For these reasons, the findings of this study are stimulating in terms of injury prevention and performance analysis, proposing biomechanical solutions to minimise potential injury risk and a novel method to evaluate different scrum techniques.
What are the new findings?
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A new scrum engagement technique which includes a prebind between the props of the two forward packs reduces the biomechanical stresses experienced by front row players during the engagement.
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The ability to generate a sustained force after the initial engagement is not decreased using the new ‘PreBind’ technique.
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Scrum stability measures show positive prospective results when using the PreBind technique with a potential minimisation of the number of scrum collapses.
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The biomechanical stresses acting on front row professional players during live contested scrummaging have the potential to cause chronic injuries to the cervical and lumbar spine.
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The engagement technique modification is a viable route to minimising potential injury risk during rugby scrummaging.
How might it impact on clinical practice in the near future?
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This study suggests that a new prebind scrum engagement technique may offer benefits in terms of reducing biomechanical loading experienced by front row rugby players.
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This study provides an evidence base on which to inform discussions relating to the scrum laws of rugby union when seeking to improve player welfare.
Acknowledgments
The authors would like to thank Gavin Williams and Graham Smith for their support as expert coaches in supervising the scrummaging testing sessions. They also acknowledge the assistance provided by Dr Sarah Churchill, Sean Williams, Oly Perkin, Matt Cross, Niki Gabb, Robin Pritchard and Dr Neil Bezodis. Andreas Wallbaum contributed to the design and implementation of all the technical devices used in the study; contributed to data collection for the whole experimental campaign.
Reference
Supplementary materials
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Files in this Data Supplement:
- Data supplement 1 - Online appendix
Footnotes
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DC and EP contributed equally
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Contributors DC contributed to the validation of the data collection equipment; designed data collection software; contributed to data collection during the experimental campaign; designed data processing software; processed data; contributed to statistical analysis design; carried out statistical analysis; contributed to data analyses and interpretation; drafted the paper; contributed to revising the paper; approved the final version of the paper. EP contributed to the definition of the experimental protocol and to its implementation; contributed to the design of the data collection equipment and to its validation; designed data collection software; contributed to data collection for the whole experimental campaign; designed data processing software; processed data; contributed to statistical analysis design; carried out statistical analysis; contributed to data analyses and interpretation; contributed to revising the paper; approved the final version of the paper. KAS initiated the project; contributed to the definition of the experimental protocol and to its implementation; contributed to data collection; contributed to statistical analysis design; contributed to data interpretation; contributed to revising the paper; approved the final version of the paper. MEE contributed to the definition of the experimental protocol; contributed to data interpretation; contributed to revising the paper; approved the final version of the paper. GT is the guarantor, initiated the project and supervised all its phases; contributed to the definition of the experimental protocol and to its implementation; contributed to the design of the data collection equipment and to its validation; contributed to data collection for the whole experimental campaign; contributed to statistical analysis design; contributed to data analyses and interpretation; contributed to revising the paper; approved the final version of the paper.
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Funding This project is funded by the International Rugby Board (IRB), project title ‘Biomechanics of the Rugby Scrum.
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Competing interests None.
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Patient consent Obtained.
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Ethics approval The study was granted by an institutional ethics committee at the University of Bath, School for Health Research Ethics Approval Panel.
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Provenance and peer review Not commissioned; externally peer reviewed.