Major questions related to the biomechanics of the rugby league place kick

1. What are the main phases of the rugby league place kick?

2. Why is the skill of the place kick important in rugby league?

3. What role do force and velocity play in the skill of the rugby league place kick?

4.  What are the variable external sources that impact the rugby league place kick?

5. How do the measurements of a standardised rugby league ball affect the place kick?

5. How else is the rugby league place kick relevant in the world of sport?

What are the three main phases of the rugby league place kick?

  

With reference to the above video, below are the three main phases of the place or conversion kick in rugby league along with the biomechanics of each phase. 

1. Initial address to the ball
This phase of the place kick refers to the kicker measuring out their kicking routine to maintain consistency and enhance accuracy. During the run-up, it is vital that the players support foot (the foot that is planted beside the ball) arrives at the tee at the optimal distance and angle that will allow for greater range of motion to build velocity and ultimately, maximize force imparted on the ball by the inner foot.  It is important that the kicker maintains an extended gaze on the ball pre and during the approach to the tee, to ensure the target approach is accurate and gives a greater chance of maintaining correct angles (the optimal run-up angle is said to be 45 degrees to the players non-kicking leg). Step size must be as identical as possible each time a player is marking out in order to maintain consistency and optimize accuracy.  

http://kickcoaching.com.au/category/place-kicking/

  2.Planting of the support leg beside the ball:
The second phase of the place kick is where the kicker maintains balance and control by planting the support leg in the optimal position alongside the ball. Postural support has a direct positive correlation with ball velocity and therefore is an important factor to consider in this phase. The more balance a kicker has upon planting their foot alongside the ball, the greater control they have which tends to result in greater velocity leading to a better kicking accuracy.

http://lermagazine.com/article/kicking-biomechanics-importance-of-balance

   3. Striking of the ball with the instep of the kicking foot   
 The final phase of the place kick is where weight is transferred and the kickers lower extremities are used to generate force and impart angular velocity on the stationary, angled ball. The non-kicking leg is used to create a stable base of support to ensure the foot comes into contact with the sweet sport location on the ball. It is important that segments of the body rotate from proximal to distal (kinematic) to maximise foot speed in order to reach maximal velocity at foot impact. The speed of the foot at contact is said to be the key factor in successful kicking.

http://www.rugby.com.au/Portals/18/Files/Coaching/Level3Papers/Kicking_-_Ian_Fowler_2005.pdf

Why is the skill of the place kick important in rugby league?

In the sport of Rugby League, the place kick is an extremely important aspect of the game and is frequently the difference between winning and losing (Owens & Schwetman 1995). Once a try has been successfully scored in a game, the kicker has the opportunity to take a place kick on the angle from which the try was scored to score an additional two points (Crilly & Pryor 2010). The optimal position for a successful conversion of a place kick is midway between the sides and directly in front of the target area. However, the rules and natures of the game keep the attempts of this particular shot to a minimum (Owens & Schwetman 1995). 

What role do force and velocity play in the skill of the rugby league place kick?

Velocity and angular velocity

With reference to ecological psychology and information (Davids, Button & Bennett 2008), interceptive actions such as the rugby league place kick, require coordination between relevant parts of the movement system components and the object being intercepted. When an individual performs a movement that involves their environment, there is precise information required in order to locate objects, obstacles or surfaces in space (where information) at a particular instant in time (when information) in order to maximise velocity (Davids, Button & Bennett 2008). According to Davids, Button and Bennett (2008), for a skill such as the place kick to be successful the following constraints need to be satisfied:

1.       ensure they contact at an object or surface at an appropriate moment in time

2.       ensure contact at the intended velocity and force

3.       ensure contact at an intended spatial orientation

THE ANSWER
In the case of the place kick, the movement system component that acts on the rugby ball is the inner foot of the preferred kicking leg of the individual taking the conversion place kick. The where information of the place kick would include the ball as the object, the cross bar as the obstacle and the grass and weather conditions as the surface in space. Additionally, the when information would be the entire process leading up to the foot contacting the ball.  Velocity is the speed of something in a given direction (Cronin, McNair & Marshall 2002), which is why it is crucial for place kickers to ensure their approach angle and release angle are on point, to ensure that velocity is exerted in the appropriate direction toward goal (Blazevichs 2007).

Force

The force encompassed in the rugby league place kick can be explained using Newton’s First and Second Laws of Motion:

Newtons First Law of Motion  

  

A still object continues to be still, and a moving object continues to move at its current velocity, unless an external force acts on the object.

 

Newtons Second Law of Motion

The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

According to Newton, an object with a certain velocity will maintain that velocity unless acted on by a force causing it to accelerate at a different rate (resulting in a change in the velocity).

THE ANSWER
Although rugby league is played in an open and dynamic environment, the skill of the conversion place kick is considered a closed skill as it requires internally-directed attentional focus and is performed in a stable environment (Blazevich 2007; Davids, Button & Bennett 2008). For this reason, it is vital that the individual follows a kinematic sequence of limbs to enhance foot speed as they are solely responsible for generating the force to imparted on the ball (Gainor, Pitrowski, and Puhl 1978). Using Newtons First Law, it can be assumed that the greater the force generated by the kicker, the greater the force imparted on the ball which leads to greater acceleration of the ball, resulting in the ball travelling a greater distance (Blazevich 2007). Similarly, using Newtons Second Law of Motion we can view the ball as the object with a particular velocity, before it has a particular force acted on it from the kicker resulting in a change in velocity. Before the kicker makes contact with the ball, it is important that they generate as much force as possible to impart on the ball, in order to increase acceleration which will increase the distance the ball travels (Blazevich 2007). Additionally using the side of the foot allows for greater contact of the kicker’s foot on the surface area of the ball, resulting in greater generation of force and distance traveled (Blazevich 2007).  

What are the variable external sources that impact the rugby league place kick?

Projection angle: The angle at which the ball is released and continues to accelerate, is known as the projection angle (Blazevich 2007). For the rugby league place kick, studies show that the optimal projection angle is less than 45 degrees because the projection velocity that a player can produce decreases substantially as projection angle increases.

The Answer: A study (Linthorne & Stokes 2014) that incorporated more than 10 rugby league players partaking in 49 consecutive place kicks revealed the optimal projection angle to be 30.8 degree as it is less than 45 degrees, and had the greatest projection acceleration and distance traveled.

Air resistance:Air resistance can be defined as an aerodynamic force that acts to oppose the motion of the object traveling through space at a given time (Blazevich 2007; Elert 1998). The speed at which a rugby ball travels will always gradually begin to decrease after the player has kicked it due to air resistance (Watkins 2014). It should be noted that air resistance can be increased or decreased depending on the rate of the wind in that particular moment.

The Answer: Taking the effects of air resistance into account, it can be seen how important it is for a kicker to use summation of forces (i.e. building force through proximal to distal limbs) in order to impart as much force on the ball to enhance acceleration and distance travelled (Blazevich 2007). By maximising force imparted on the ball, the acceleration is greater and therefore, the effect of the frictional force that is air resistance will have less impact on the distance covered (Blazevich 2007). 

How do the measurements of a standardised rugby league ball affect the place kick?

The specific measurement and shape of the rugby league ball (determined by the International Rugby Board) give it certain biomechanical characteristics to be exploited by the players.

As seen in the figure on the right, the measurements of a
standard rugby league ball are as follows:

Shape: Prolate spheroid
Length: 28-30 centimetres
 Width: 18.5-19.7 centimetres
End to end circumference: 74-77 centimetres
Width circumference: 58-62 centimetres
Weight: 410-460 grams

The Answer: The shape of a rugby league ball is described as a rounder, egg like shape which can be referred to as the prolate spheroid (Lemoine & Naami 2013). A prolate spheroid is spheroid that allows a rotation around the major axis, which enforces a backward motion after the ball has been kicked allowing for greater distance to be traveled and a predictable flight path. The length, width and circumference of the ball make it less aerodynamic than an Australian Rules football; however, its wide berth allows it to be more stable so that it can be kicked a greater distance with more accuracy, which is important for the place kick. The weight of the rugby ball is typically heavier than Australian Rules footballs to reduce the effect of air resistance and high winds but, is still light enough to be easily carried during play. 

How else is the rugby league place kick relevant in the world of sport?

Soccer
In the sport of rugby league, many of the biomechanical principles used in the conversion place kick can be applied to the soccer penalty kick. For example, the place kick involves a proximal to distal sequencing (also known as the summation of speed principle) in order to achieve maximal ball speed which involves maximising velocity of the foot before it comes into contact with the ball surface (Zhang, Liu & Shenquan 2011). Similarly when a penalty shot is taken during  a soccer match, the kickers main focus is to obtain maximal ball speed by using a summation of speed model to generate as much force to impart on the ball as possible. Given that both kicking types use similar features to generate force, it can be assumed that they both can have Newton’s First and Second Law applied to investigate force generation. Secondly, like rugby league place kicks, soccer penalty kicks involve a sequence of motions that include an initial address to the ball, planting of the support leg for balance and striking the ball with the instep of the preferred foot. While the patterns of the rugby league place kick and the soccer penalty kick show similarities, the difference in the ball shapes, tee support and release angles allow the rugby league place kick to maintain a more unique technique than the soccer penalty kick. Furthermore, given the difference in ball shape and release angle the effects of air resistance and drag could not be accurately compared for similar outcomes.

American Football

Aside from Canadian football, rugby league is the most similar sport to American football given their shared origin (Britain). Both sports make use of the place kick in order to score points after a try or touchdown has been scored. Similarly, the dynamics of the ball shape and size are considerably alike, with only one centimetre difference in length and four centimetres in circumference. Unlike rugby league and soccer, rugby league and American football can be compared in terms of air resistance and projection angle during a place kick given the similarities of the ball and the use of a tee or mound to hold the ball in place.  Additionally, the American football place kick uses the same three phases as the rugby league place kick showing that they biomechanics of the skill are highly applicable to each other.