The lineout in rugby union is unique. While other ball sports have a method of returning the ball to play after it has crossed the side line to leave the field of play and this is often by a throw (or hit) from the sideline, only rugby has a closely regulated method where several players compete for possession. In many other games the ball may be propelled in any direction into the field of play, aimed directly for a fellow player and opponents usually compete on a one-to-one basis. For example, the soccer or basketball throw in or the hockey hit.
In rugby, if the ball is not thrown in immediately, before a lineout has formed, the ball must be propelled at right angles to the sideline and must travel at least 5m before it is touched by any player. The players contesting the lineout must include a minimum of two players from each team and a maximum number decided by the throwing team. The defending team may have no more players than the throwing team. The two lines of players must stand half a metre from the centre line of the lineout and may not close that gap until the ball is thrown. The ball must be thrown down the centre line. Apart from one player defending the thrower from within 5m of the touch line, and one receiver who, initially at least, stands out of the line, all other players must be at least 10m back from their side of the lineout.
According to the rules of the game, a player may not jump or be supported by a colleague until the ball is thrown, the thrower may not baulk the throw, a player may not be lifted, a player may not be supported before he has jumped and there may be no pre-gripping below the waist. In reality all these rules are ignored and, if the breach is not blatant or no disadvantage occurs to either team, no penalty is imposed.
Where the ball is thrown, how this intention is conveyed to the players and concealed from the opposition and what is subsequently done with the ball are tactical decisions that affect the success of the lineout.
All of the above rules and tactics determine how the ball must be thrown and caught, the physics and biomechanics of throwing and jumping determine whether this occurs successfully.The throw
There are a number of variations where the ball is passed underarm to the player at the front of the lineout or thrown quickly to that player before he has jumped, but in general the thrower uses an overarm throw to a player who has jumped and been lifted/supported as high as possible. The thrown ball thus be considered as a projectile whose relative release height is the difference between the heights of the thrower’s hands and the catcher’s hands. For this type of throw the distances fall into one of three groups: Front (about 6m from the thrower), middle (8 – 10m) and back (10 – 12m). Any throw over the back of the lineout does not normally include a jump and is not included in this analysis. The thrower may change the angle of release and the speed of release, but the height of release is relatively constant. This analysis will attempt to determine how much choice the thrower has over these variables, and how precisely they must be chosen to give an accurate throw.
In initial calculations the drag on the ball is assumed to be zero and the ball behaves as an ideal projectile. In actual practice the drag at the speeds used is as much as 25% of the weight of the ball, decreasing with the square of the speed of the ball. To make the calculations, a spreadsheet was prepared using the kinematic equations for constant acceleration (assuming no drag etc.), and allowing for the initial conditions to be set for each set of circumstances. The results were made conditional on the ball reaching the catcher at the peak of the jump yet clearing an opponent at the same height in front of the catcher.Front of lineout.
For a throw to reach a standing player (no relative release height) the ball must be thrown at at least 7.7m/s, any less and it will drop below the player’s hands. To reach a lifted jumper (relative release height of -2.0m) the throw must be at greater than 9m/s, at which speed the release angle is 54o
, i.e. a lob. For a flat throw (release angle less than 30o
) the speed needs to be at least 12m/s for a lifted jumper. (note that a “flat” throw is not physically possible as the ball will always accelerate down as a result of gravity).
Let us consider a throw to a lifted jumper at the front:
Horizontal distance 6.0m, relative release height -2.0m, speed of release 10m/s and angle of release 40o
For these values the horizontal velocity component (v.cos40) is 7.7m/s and the vertical component (v.sin40) is 6.4m/s.
The time to reach the jumper is time = distance/horizontal speed = 0.78s
In a series of full power jumps by elite junior rugby players during practice lineouts at a range of distances the time for the jump was 1.1s from initiating the jump to the peak height or 0.9s from the time that the feet left the ground. On this basis it would not be possible to complete a legal jump in these conditions. With at least 9m/s and an angle greater than 54o
the time of flight is equal to or greater than the time to jump, but the throw would be a high lob which would give the opposition time to react and disrupt the possession.
At 10m/s and 40o
the height of the ball at 6.0m from the thrower is 4.0m. An error of 3o
in the angle of release, either too large or too small means that the ball is 30cm higher or lower than intended showing the accuracy demanded of the thrower. Similarly, 0.5m/s error in speed of release makes a difference of 30cm at the distance of 6.0m.Middle of the lineout
Using the middle of the line out as 10m from the thrower (we can use any distance and adjust accordingly) we find that the minimum usable throw is 10.9m/s at an angle of 50o
. With these figures the ball actually reaches its highest point as it crosses the front of the lineout and drops towards the jumper from then on, remaining above any jumper closer to the thrower. Only if the target jumper misses the ball does it become available to opponents behind who are not able to out-jump the throwing side. A 5% error in throwing speed in this case will leave the ball within jumping height up to 80cm in front of the target, or will clear the target by 80cm and will not reach catching height until 1.0m behind the target. The same error in release angle will allow the ball to be caught up to 40cm in front of the target or anywhere behind him, suggesting that at this distance at least, the speed with which the ball is thrown is more important than the angle of release for relatively small errors.Back of the lineout
The minimum initial conditions for the ball to be thrown 15m are a release speed of 12.95m/s and a minimum release angle of 49o
. In these circumstances an error of 5o
will make the ball available to opposing jumpers within 30cm in front of the target. A 5% error in speed will make the ball accessible to jumpers up to 1.4m in front of the target or will clear the target by 1.0m and will not be catchable by a jumper until 1.5m behind the target or by a standing player until about 3m behind the target. Again the speed of release has a much smaller tolerance than the angle of release.
For each distance there is a range of combinations of angle and speed which will allow the ball to reach the target at the correct height. A lower angle and higher speed will reduce the time for opponents to react, but will make it easier for them to reach the ball if they do react. A greater angle will make the ball clear the opposition, but will require a greater speed than optimal making the ball travel high and take longer in flight allowing the opponents to challenge the target jumper. The optimal throw will clear the opposition at the smallest possible angle and least possible flight time.
As well as accuracy in release angle it is necessary have a horizontal angle that ensures that the ball remains between the two lines of players otherwise the referee will penalise the offending team. This requires a margin of error of less than 3o
for a throw to the middle of the line.
A lineout thrower, usually the hooker, must, therefore, be capable of throwing the ball with an accuracy of less than 3o
in both the horizontal and vertical axes while varying the vertical angle by up to 20o
between throws to different points in the lineout. He must also be able to judge the speed of the ball to within a few cm/s while varying the speed from 8 to 14m/s.
All these calculations assume that the jumper will be precisely as stated, but in reality positions in the line vary as players move to confuse the opposition so that the actual jump is unopposed. This results in the actual position of the jumper being somewhat indeterminate at the time of the throw so that the thrower must allow for a less than optimal jump by dropping the ball lower than indicated by the calculations above. The competition then becomes not one of precision, but one of timing and communication.The thrower
As we have seen accuracy of the throw depends on the speed and angle of release both in the horizontal and vertical planes. These are determined by the velocity of the hands at release so that these must be moving at right angles to the sideline and along the desired initial trajectory. Using the principle of kinetic linkage (large muscles followed by small muscles) the sequence of events in reverse order is hand motion determined by arm, shoulder, back and if necessary legs to give the required speed. To disguise the destination of the throw, the thrower will normally use the same sequence but with more or less force depending on the length of the throw. In most current teams the thrower is the hooker, but could be any other player and some women’s teams, where strength is an issue, have used a prop to throw especially to the back of the line. Most throwers adopt a two handed throw with one hand behind the ball for power and the other on the side for control. In this format the hands tend to remain on the line of the throw for longer. A few throwers have retained the older ‘bowling’ action using one hand only and a side-on stance.
When a step is taken to increase the force by involving the legs and hips, rotation of the hips and torso results and the thrower must ensure that the upper body is square on to the lineout when the ball is thrown. Small errors of timing in this case can result in the throw being off line. Stepping with the left foot tends to rotate the thrower towards his/her own side while stepping with the right foot has the opposite effect. The main aim in all cases is consistency of action and, therefore, of throw. Mass repetition of each different length of throw is necessary if accuracy is to be achieved.
It has become common for the receiver to move around before the throw is taken to confuse the opposition It is assumed that the thrower can actually aim at a precise point in space into which the jumper will move at the appropriate time. The thrower may find it difficult to avoid focussing on some stationary object in the field of view and thus selecting the throw parameters incorrectly.The Jump
There are two parts to the basic lineout jump, the initiation by the jumper and the lift and support by the lifters.(The term “lifter” is used throughout despite the fact that lifting is against law 19.9 (g), since the jumper rises above the height which he/she could attain without assistance and is thus lifted). In a well timed jump, the acceleration of the jumper comes from his/her leg drive and the rise is then maintained at a near constant speed by the lifters. We can isolate these parts initially and look at each as a separate event.
The jump can be considered as a standing jump which is an area that has been the subject of some research as it is used by biomechanists as a good indication of the power of the athlete. The jump is initiated by flexing the knees hips and ankles either in a static position or squat, or as the result of a small step which is called a countermovement jump. The countermovement jump makes use of the stretch - shorten cycle which stretches the muscles in the relevant groups before they shorten in the jump. The prestretching of the muscle enables the muscle to exert a greater force during the shortening or concentric phase. It would appear that the transfer from stretch to shorten must occur within less than 100ms to be effective. An extreme form of the countermovement jump is the drop jump where a downward jump initiates the shortening phase. The countermovement or drop jump has been shown to give a greater vertical height than the squat jump where the athlete is equally familiar with the techniques. The depth of countermovement is not a major consideration.
Jumpers can initiate the countermovement by sinking slightly on the spot, by stepping forward to the take-off or taking a small forward jump. The last technique, as a form of drop jump using both legs would seem to be the most effective, subject to effective training of the jumper.
To reach a maximum height the jumper then extends all three joints while the feet are in contact with the ground. Since the height risen corresponds to the increase in gravitational potential energy, the jumper must gain the maximum possible kinetic energy in the take-off phase. This in turn corresponds to the maximum possible work done by the muscles and hence the greatest distance moved and force exerted. While a deeper squat might increase the distance moved, it will decrease the initial force and may result in the energy of the countermovement being dissipated rather than being conserved in the elastic stretch of the tendons and muscles.
In a vertical jump from a stationary start, an athlete might expect to rise 40 – 50cm in good conditions. With an additional reach of up to 50cm by raising the arms, a player might be able to catch a ball as much as one metre above standing height. A lock who is normally one of the taller players might expect to catch a ball at 3m in an unassisted jump. With assistance the jumper can reach about 1.8m above the height of the lifter with outstretched arms. This will be approximately 2.2m (1.8m height plus 40cm arm reach) giving a total height reached of about 4.0m.
To maximise the height of the lift, the primary lifter who stands behind the jumper should bind as low as possible within the laws (i.e. not below the shorts) which will tend to be most practicably below the buttocks. In optimum conditions the primary lifter will be tall and will end up erect and on his/her toes at full stretch and will have ended up below and just behind the centre of mass of the jumper. The secondary lifter in front of the jumper provides a lesser lifting force acting more to control and support the lift. The front lifter binds on the thighs of the jumper (commonly now on strapping above the knee designed to give grip) and thus does not need to be as tall as the primary lifter to reach the same height in the lift.
Quantitative calculations based on sample jumpers.
The centre of mass of a standing player is about 1.2m above the ground and this is raised to 1.7m in an unassisted jump and 2.8m in an assisted jump. In preparation for the jump he will sink about 20 – 30cm, either in countermovement to tension the muscles or in the preparatory squat to increase the distance over which the force is applied, so that the total rise in the jump alone is about 75cm.
Thus the work done by the jumper in rising out of the squat (assuming a mass of 100kg) is:
In addition the jumper gains a take-off speed which is typically about
2.5 ms-1 representing a kinetic energy of:313J
or a total amount of work done in the jump of 1050J
Some of this energy will come from conservation of elastic energy in the muscles and tendons during the countermovement. The work of the rise must be done while the feet are in contact with the ground which lasts for about 0.3s.
The power generated by the jumper is thus: 3500W
Taking only the rise above the ground gives work done of 700J, a take-off speed of 2.5m/s and a power generated during the spring phase of 2300W.
If we consider the total energy to be generated over the whole movement, both down and spring, the time to generate the 700J is extended by a factor of about twice and the power generation is about 1200W
In the assisted jump the centre of mass rises 1.6m making the work done: 1570J
or a total of 1900J including kinetic energy
In this case the lift lasts nearer to 0.6s from the lifters making contact with the jumper, or 0.9s from the start of the jump, to the top of the lift if completed at the speed of take-off.
Power in total = 2100W (approximately the same as the power of the jumper during the spring phase)
The lifting portion provides the remaining energy in reaching the high point so that the work done by the lifters is: 1570 – 300 = 1270J which is done in 0.6s giving a net power for the lifters of : 2100W (shared between the lifters).
We can calculate the approximate forces generated during the jump by relating the energy gained to the work done. In jumping, the ankles extend so that the total distance moved while in contact with the ground is extended from 25cm to 30 cm. Usin W=F.d we get a force of 3500N
This is net force of approximately three and one half times the body weight and is in addition to the support of the body so that the total force exerted by the muscles of the leg will be approximately four and one half times the body weight.
The lifters on the other hand do not accelerate the jumper so that the net force exerted between them is approximately the body weight of the jumper or 980N with the rear lifter supporting a larger fraction than the front lifter.
A number of lineouts by elite male and elite female players were studied using 50 field per second video and videopoint analysis software to gain approximate values for speed and height. (in live practice and games, accurate calibration is not possible). The data showed remarkably little difference between male and female jumps with the speed of the lift varying between 2.0 ms-1 and 2.7ms-1 both within a forward pack and between packs, the highest speed being for an international female lock with international props. The two factors that most affected the speed of the lift appeared to be speed of jumper (without assistance), and the timing of the lift. In most jumps the lifters were able to maintain the speed of the jump by applying their force at the time when the jumper left the ground. If the lift was delayed until the jumper was off the ground, the speed began to drop and the lift was slower or the lifters had to apply a greater force to accelerate the jumper and thus expend more energy.
While the male lifters were substantially heavier than their female counterparts, and could thus be expected to exert much greater lifting forces, the male jumpers were heavier than their female counterparts by a greater amount, explaining the similarity of their jumping patterns.
( a coaches perspective can be obtained at www.coachesinfo.com)