The mechanics of the scrum in Rugby

In all forms of scrum, loose or set, the forwards contest possession and territory by using strength with technique to move forward or hold ground while moving the ball toward the rear of the pack or to move the opposition back to make it difficult for them to control the ball. The mechanics of pushing and resisting in each form of scrum is similar and good technique in one is transferable to the others. The set scrum, being the most formal, is the most predictable and thus simplest to analyse physically and biomechanically.

The set scrum has two main phases:
Engagement when the two packs are brought together initially from a prepared stationary position determined and controlled by the referee, and
The sustained or secondary shove which is exerted once the ball has been put in and the packs attempt to exert dominance.
The overall position in preparation for engagement is controlled by the referee and should give no advantage to either side as they should react at the same time to commands that are supposed to ensure that the front rows come together in a stable and safe manner. The packs on the other hand are trying to adopt a body position and combination that will give them the greatest speed and hence impulse on the opposition when they come into contact so that they are in the best position for the next phase of the scrum and/or they place the opposition in a weaker position to respond. The rules as interpreted by the referee place strict controls on the means by which the players compete in the interests of safety. A number of the rules correspond to the best scrummaging practice and promote strong body positions which, if applied by both packs will give stability while permitting fair contesting of advantage.
The force exerted by the entire pack is exerted through the front row to the opposition, and by reaction the front row must experience the full force at the points of impact. It is front row players who are most at risk of injury if the engagement, in particular, is incorrect. Since the impulse, and hence the force at impact, depends on the change in momentum rather than the muscular strength, engagement is the time of greatest risk. Thus emphasis is initially placed on the body position of the front row players prior to and during engagement, and on the movement into engagement.
To exert the force onto the opposition the props must adopt a natural back line, slightly hollow at the lumbar back and substantially straight through the upper back and neck. To do this the players look at the opposition, keeping the eyes up but not actually tilting the head back “looking over the top of your glasses”. Tilting the head forward, i.e. looking down, places the neck in the most vulnerable position for possible dislocation if the back of the head is struck on engagement. Looking down also puts a curve into the back which weakens it by producing shear forces rather than compressive forces from in front and behind.
Considering the props alone (though this applies to the hooker in a defensive scrum), they must adopt joint angles that enable the optimum force to be exerted during the move into engagement and that force should be immediate on reaction while allowing the position to remain stable immediately after contact. According to the literature (O’Shea, 2000, p Milburn, 2000, van Heerden 2002 (tug of war)) the optimum angles are: knee 120o (in range 110 - 150), hip 100 (90 – 120).
Video analysis carried out using a Sony DCR TRV240E at 50Hz analysed using “video point” software was carried out video of an international scrum (international competition champions) on a scrum machine, a national representative under age side (world finalists) on a scrum machine and a high school (international competition champions) in live scrummaging practice and on a scrum machine and a leading women’s club side in competition and in practice.
The general results for the international team were as follows:
The front row adopted a position where the shoulder and hips were in a horizontal line (law requires head and shoulders no lower than the hips). The hips were flexed at 80o, the knee was flexed at 120o (outer knee of prop, inner knee not measurable), the foot was plantar flexed and support was on the ball of the foot. The hip angle is slightly below the optimum range, but will extend with the drive.
With the drive into contact both knee and hip extend until at first contact the angles are 95o and 125o respectively and very near the optimum for maximum force production. The feet at all times remained in contact with the ground. The hip extension led the knee extension by about 20ms - 40ms whereas Mills and Robinson() indicate that, in theory, scrummagers with good technique tend to extend simultaneously (the graphs with their paper suggest delay of similar amount in good scrummagers, but a delay of 120ms in poor scrummagers). (The suggested simultaneity was based on the biomechanical push-throw movement for maximum force production). The initial drive carried the joints through to final angles of 105o and 135o respectively, but the shoulders travelled downward into contact.
After contact the settling in preparation for the secondary sustained drive involved a dropping of the hips to bring the angles to 95o and 110o. At all times during the engagement sequence the top grade, experienced, prop maintained joint angles within the optimum range. To date the direction of travel in the third dimension has not been studied and preparations are being made to examine the scrum from vertically above to determine whether lateral movement is substantial.

By contrast the underage international side showed less awareness of the optimum body position. In an identical practice situation the prop had a hip angle of 50o in the crouch position with a knee angle of 105o. In the drive into contact and settling the angles became 90o returning to 70o for the hip and 130o returning to 80o for the knee. Some of the variation resulted from movement of the scrum machine immediately after contact and to stepping after contact. The locks stepped back on contact and the scrum became unsettled until a firm base was re-established.
The High school side when observed in live practice showed a lack of stability which to some degree is explained by the opposition. The initial angles were 70o hip, and 140o knee with little or no ankle flexion so that in the crouch position the hip was above the shoulder and the player was unprepared to engage. On the engage command the prop gained little forward motion from extending the hip and was unable to extend the knee. At the instant of contact a step back was taken to bring the hip to 85o and allowing the knee extension to reach 125o which are nearer the ideals, but this delayed the drive and meant that the scrum was able only to resist the impetus of the opposition. The scrum became unstable and wheeled substantially before settling for the put in. The tendency to adjust position during or just after engagement with a subsequent initial instability of the scrum was noted with almost all the high school front rows observed.
Subsequent study of a women’s scrum has shown similar angles to the boys’ scrum except that the initial hip angle was even smaller (55o), but by maintaining ground contact throughout the drive the hip angle was increased to 105o during the initial contact and settling. The knee was again substantially flexed placing the hips above the shoulders in the crouch position. As the prop moved into engagement the hips extended while the knee initially flexed to bring the hips in line with the shoulders so that on contact and thereafter the joint angles were near optimum. In live scrummaging, both the boys and the women, the timing of the initial drive into engagement was noticeably different for each front row allowing one side to dominate the engagement. Subject to further study of elite players, it would seem that the optimum engagement is obtained when the props are on the balls of the feet, i.e. plantar flexed, with both knee and hip within, but at the lower end of, the optimum range. This then gives maximum acceleration on the “engage” command while leaving the front row stable and ready to shove immediately, leaving the feet in ground contact throughout. In less able scrums the tendency is to sit back by flexing at the hips so that time is taken while the hips extend before the drive proper begins.
Attempts to identify predictors for scrummaging performance within normal testing procedures have shown () that the time for a 30m sprint from standing start gives the closest correlation. Short sprints are a good measure of acceleration and tend to measure the ability to exert force in the forward direction while in a forward leaning position. This more nearly approximates the pushing position than say a vertical jump which also employs countermovement. This would seem to reinforce the advantage of preparing for engagement in a position on the verge of forward over balancing. This position also ensures that the initial movement will involve extension of both hips and knees as indicated by Mills.

The forces of engagement and shove have been measured by the use of force plates, strain gauges etc in the pads of scrum machines and thus indicate the impulsive force on the machine in the former case rather than the accelerating force of the players. We have calculated this impulsive force using video analysis and mass measurements and have found satisfactory agreement with the direct measurements of () giving some confidence in extending this analysis to other aspects of the forces exerted in scrums.
The force in engagement is calculated by measuring the speed of the players immediately before engagement and the time taken to bring them to rest (the scrum must by law come to rest before the ball is put in). From the impulse and change in momentum we calculate the force. To give the greatest impact on the opposition, the player should stop in the shortest time. On impact the shoulder girdle flexes allowing the spine and most of the mass to slow more gradually. To counter this, the prop throws the arm forward into the binding position and making the shoulder more rigid so that the mass stops more abruptly. The stability of the shoulder girdle also promotes stability of the neck.

Studies of the forces on front row players using multi axis force plates by Milburn suggested that the more inexperienced the forwards in the scrum, the more likely it was that there would be instability on contact and during the sustained shove. The lateral or shear forces on the hooker at high school level in particular exceeded the body weight by a substantial amount and could act in opposite directions on engagement and shove even in the controlled situation of a scrum machine. On the other hand the lateral forces experienced in an international standard scrum were shown to be negligible at all stages even though the forward impulse and shove in the international scrum almost doubled that of the school scrum.
While the front row transmits and experiences the full force exerted in the shove the other players contribute substantially to the impulse, especially when they act simultaneously rather than sequentially (McClymont & Cron). As the experience of sides decreases there is a tendency for locks and number 8 to hang back from the engagement so that the contact becomes a series of small impulses rather than one large one. In some scrums it has become a technique for the no. 8 to pull back on the locks until the engage call on the assumption that when released they will accelerate at a greater rate as if from a ‘set’ position in sprinting. It appears to us that this will delay the drive and remove the no. 8 as an effective force.
Simple two dimensional video analysis from the side of the scrum did not allow the lateral forces to be estimated but observation of live scrums show that there are substantial twisting moments in both horizontal and vertical planes on first engagement as a result of the inconsistent techniques of the front row players on each side.
(To be continued)