Biomechanical principals involved in scrum half pass

1.1 - Basic mechanics – Newton’s laws of motion
Sir Isaac Newton was the first to correctly state the basic laws of motion which can be applied to any object under a force.

· Law 1: an object remains at rest or continues to move in a straight line with uniform velocity if there is no unbalanced force acting on it.
· Law 2: the acceleration of an object is proportional to the resultant force acting on it and in the direction of this force. (F=ma) where F is the resultant force acting on the object, a is the objects acceleration and m is the objects mass.
· Law 3: for every action there is an equal and opposite reaction.

Law 1 is included for completeness but is not a factor when analysing a scrum-half pass as it is covered by the statement in law 2, F=ma, if there is no acceleration, F = 0 so there is no unbalance in the forces therefore the object remains at rest or travelling at a constant velocity.

The equation F=ma from law 2 is a vector equation. This means it has a magnitude and specific direction, so when applied to a scrum half pass it can be shown that the ball will be will continue to travel in the direction and release velocity as the hands were moving at the point of releasing the ball from the players hands. Therefore the player should look to be accelerating his/her hands towards the target at the ball release moment. After the ball is released there are no external forces acting upon it in the horizontal plane (disregarding air resistance) so the ball continues to travel at a constant velocity until it is caught or hits something. Gravity is the only force acting on it constantly in the vertical plane after release so the ball is always accelerating towards the ground.
1.2 - Stability
Stability is dependant on two factors, the size of the base of support, and the vertical and lateral position of the centre of gravity. Objects with a low, centralised, centre of gravity and a wide base of support are the most stable e.g. a formula 1 car. Objects with a high centre of gravity and a relatively narrow base of support are far less stable, e.g. a double decker bus.

A humans centre of gravity is generally located around navel height when the person is standing normally. The centre of gravity can be moved laterally by moving body parts in that direction. If the centre of gravity is moved outside the edge of the base of support, the object (or person) is in a state of toppling, meaning, that if left uncorrected, the object or person will fall over. To ensure good stability when the player executes this skill, a wide base and low centre of gravity is required.
1.3 - Summation of forces
The acceleration of the ball during the passing movement is effected by a number of body parts, starting at the ankles, through the legs, hips, trunk, shoulders, arms and wrists. It is the sum of each of these forces which results in the final force applied to the ball which determines its flight trajectory and speed. For a good pass to be produced all of these factors must be performed at optimal velocity and direction.
1.4 - Circular motion – torque and rotation
Torque is the ‘moment’ of a force about a point that tends towards a rotation motion, defined by the equation:

Torque = Force (F) x distance from rotation point (D)

a – no torque b – 50% torque c – full torque

The magnitude of the force and the distance from the point of rotation that the force is applied determine the amount of torque. This means a scrum half with longer arms can generate the same amount of torque with less effort as one with shorter arms. To generate the highest amount of torque during the pass, the scrum half must move the ball through as big an arc as possible so the force is being applied to the ball as far away from the point of rotation as possible.

When a scrum half makes the pass, accuracy and power are the players’ two main concerns. Vertical accuracy is determined by the amount of arm rotation. If the player over rotates the arms in the vertical plane during the swing, the ball will be released too late and the hands will be travelling at an angle higher than desired, this results in the ball flight being too high. If the scrum half releases the ball too early the arms will not have rotated enough in the vertical plane so the resultant ball fight will be too low. To get the desired ball height the player should try to keep a low body position throughout the passing motion from start to finish.

Horizontal accuracy is determined by a combination of hip and arm rotation together. The arms and hips are rotating in the horizontal plane so the ball must be released at the correct point in the motion for it to hit its target otherwise the ball will travel too far forwards (if released early), or too far backwards (if released too late). It is desirable to remove any horizontal rotation from the ball path during the passing motion to increase the accuracy. This can be optimised with good footwork and timing.

A long arc of rotation (big circle) is desirable as this reduces the margin of error of releasing the ball at the correct time. A shorter arc would result in a quicker rotational velocity and therefore make timing the correct release point harder for the player.

Diagrams d and e below show the path the ball follows while it is in the players hands making a pass. It can clearly be seen that there is a much greater deviation from the target with a shorter arc.

Above, d – short arc, poor accuracy
Below, e – extended arc, increased accuracy

1.5 - Acceleration distance
Impulse (I) = Force (F) x Distance force is applied for (D)

An accelerating force is applied to the ball constantly throughout the passing motion, consequently, the longer the accelerating distance, the faster the final launch velocity. This combined with a long passing arc and a wide base of support should result in a fast and accurate pass

2.0 - Common errors associated with poor technique
2.1 - Footwork
As with any sport, footwork is the key to accurate skill execution. It is vital for balance, ball control and body positioning. The scrum half needs to position himself at the correct position to pick up the ball from the trailing foot, with the lead foot pointing towards the target for the pass. If a line were drawn between the players’ feet, it would show the preferred direction of the pass. Therefore it is important that the body is not too square on to the ball and direction of play because the ball must be passed backwards so this could result in the players’ movement being restricted by the leading leg. This would also result in increased trunk and arm rotation in the horizontal plane giving a horizontal rotation of the ball path to compensate for the poor foot positioning, which in turn would result in a less efficient pass, (less power and less accuracy). Good footwork is also required to set the feet in a wide enough stance to give a wide base of support and increased range of motion.
2.2 - High centre of gravity
If the player is too upright, his centre of gravity will be higher. There are several problems associated with the scrum half having his centre of gravity too high. The passing motion should be a long sweeping one. This is not possible if the centre of gravity starts too high, as the arms will have increased vertical rotation to bring the ball up to the release height. This can result in the ball trajectory being too high. The arms will be further outstretched to pick up the ball so some of the power generated by the arms will be lost due to a reduced range of motion in the arms. If the player is more upright, he cannot have as wide a base of support as required so the acceleration distance is reduced, further reducing the power of the pass. The higher centre of gravity and shortened base of support also results in less balance and stability, therefore, reduced accuracy.

For good accuracy and power the centre of gravity should start low and remain low throughout the movement, translating horizontally. All the force generated by the body should move the centre of gravity horizontally with the ball, from trailing foot to leading foot, with a small amount of vertical rotation from the arms.

2.3 - Hand placement
The correct hand placement is vital for a good pass. If the hands are not placed correctly on the ball, the torque imparted onto the ball will be reduced so the ball will not spin through the air as much during flight. The torque is caused by the hands and arms rotating the ball about its centre of gravity through its central axis (end to end). The spinning motion of the ball helps the air flow around the ball so there is less air resistance, enabling the ball to be passed further distances.

If the player places his hands on the ball in the wrong place, it may feel slightly uncomfortable and un-natural so control of the ball is reduced. The player may try to adjust his hand placement on the ball, slowing the pass down and increasing the chance of miss handling the ball. The scrum half should try to spread his hands as wide as possible to increase the points of contact on the ball to give himself more control of the ball (i.e. larger base of support on the ball means greater stability).

It is the rear or “guiding” hand on the ball that causes most of the spin because as the arms come across the body the rear hand and arm have a shorter range of motion as it is restricted by the body so it automatically rotates over the top of the ball. The guiding hand will determine the power of the pass and the magnitude of spin on the ball. The lead hand is responsible for the accuracy successes of the pass by supporting the swing and keeping it in line.

3.0 - Coaching the scrum half pass
3.1 - Wide base
· Increases stability and therefore control as it lowers the players’ centre of gravity.
· Increases range of movement resulting in a longer acceleration period, so in turn, a higher ball release velocity.
3.2 - Foot and body position
The feet should be placed so the body is angled in the intended direction of the pass. This will ensure the scrum half does not restrict his own movement and have to increase the horizontal aspect of the arc motion. The lead foot should be pointing in the intended direction of the pass.
3.3 - Bent Knees
Bending your knees serves two purposes:
· It lowers the scrum halfs centre of gravity, therefore increasing stability and control.
· The scrum half is ready to “spring” into the pass so the ball speed can be increased.

If the centre of gravity is too low it can make it more difficult to optimise other mechanical principals such as leverage and the summation of forces so a compromise must be made.
3.4 - The pick up
Ensuring the correct hand placement on the ball, from the pick-up, the arms move in one movement through an arc to the release point. The scrum half should look to move the ball in a long curve to increase the accuracy of the pass. The passer should be looking at his target all of the time to increase accuracy.
3.5 - Ball spin
The hand position on the ball is essential to generating high amounts of ball spin for distance and accuracy. As the arms come across the body the ball should automatically start to turn. More torque can be imparted onto the ball with the wrists at the end of the passing motion just before the ball is released.

3.6 - Video analysis
Taking a video recording of the player performing the skill being coached can help the player understand exactly which areas of the skill need to be improved by replaying the video in slow motion. Individual frames can be selected to show the player specific points in the skill, e.g. ball release point.

f – typical images from video analysis