Major
Question
Introduction
Correct
technique is important when attempting the rugby tackle as there can be very
serious injury if it is performed incorrectly. In this blog we will look
at the biomechanical principles behind the optimal technique and execution of a
Shoulder Tackle. We will explain what Newtons laws of motion, Impulse Momentum and
Centre of Mass are and address how these effect rugby tackle performance and
look at how we can use this information in other sporting context as well as to
prevent injury.
Biomechanical Principles
Newton's Laws of Motion
Impulse-Momentum
Centre of Mass/ Gravity
Centre of mass is the point
on an object where all of its mass is evenly distributed around. In humans the
centre of mass is close to the belly button. Positioning the centre of mass can
affect balance and stability. The centre of gravity is different to the centre
of mass as the centre of gravity is only used to find an objects centre from a
vertical orientation (Blazevich, 2013).
The Answer
Newton's Laws of Motion
Newton’s first and third law are the most relevant when looking at the
shoulder tackle.
Newton’s first law of motion looks at how every object in a state of
uniform motion usually stays the same in that state of motion unless an
exterior force is applied to it. This law is closely related to Inertia which is
an objects resistance to change (Blazevich, 2013).
Newton’s third law of motion is simple but also powerful. Newton states
that for every action there is an equal and opposite reaction. For example
when the foot of a runner makes contact with the ground they apply a vertical
and horizontal force on the earth. The earth then reacts providing an equal and
opposite reaction onto the foot pushing it in the opposite direction allowing
for acceleration. (Blazevich, 2013).
Impulse-Momentum
Momentum is the objects mass times its velocity. Force and
time both play a part in impulse-momentum. The change in momentum is called
impulse and it’s defined as the force times the time which it take to act,
therefore the larger the impulse, the more change will occur in momentum (Blazevich,
2013).
Centre of Mass/ Gravity
The Answer
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| Jaques Burger- Demonstrating the Rugby Tackle |
Preparation
Phase:
Getting into position
The
tackler needs to run towards the ball carrier in order to a) be in a position
where they are close enough to tackle the ball carrier and more importantly b)
start to build up their momentum in preparation for the tackle. While running the
tackler is using newton’s 3rd law when they push their foot down
against the ground. The ground then exerts an equal and opposite force back
onto their foot allowing the tackler to move towards the ball carrier. (Blazevich, 2013). During this
time the tacklers speed will increase which will increase their momentum in preparation
for the tackle which is important for the tackler if they hope to knock over
the ball carrier.
Increasing Stability
It
is important when tackling to have high stability and momentum when making
contact. This is to ensure that when the tackler collides with the ball Carrier
in the tackle the tackler is able to apply more force than them and they will
be more likely to be able to knock the ball carrier over rather than being
knocked over by them. Stability can be increased by increasing the size of the
base of support and by bringing the centre of gravity lower towards the base of
support (Blazevich, 2013).To build the tacklers stability and momentum they should crouch down and
lean forward before the tackle. The Tacklers feet should be shoulder width
apart. Doing this increases the size of
the tacklers base of support making them more stable and less likely to be
knocked over by the ball carrier and by crouching the tackler is lowering their
centre of gravity towards their base of support which will also aids in
stability. By leaning forward the tackler is putting their centre of gravity
near the front of their base of support which will mean they will require less
force to overcome their own inertia when they go to make contact with the ball
carrier (Blazevich, 2013).During this time the tackler shouldn’t stop, they will need to keep
moving so as to minimize the loss of the momentum they built during their
running phase. It is important that the tackler doesn’t start this phase until
they are no more than a couple of meters away from the ball carrier to prevent
loss of momentum.
The Action Phase
Making contact
To
make contact with the ball carrier the tackler needs to close the distance
between them and the player by taking a few quick steps. During this time the
tackler needs to be keeping their centre of gravity near the front of their
base of support so they are less likely to be pushed backwards.
The tackler should be
trying to hit the ball carrier from the side rather than front on. This is because
the ball carrier will have all their momentum going in the direction that they
are running and little to no momentum going sideways. If the tackler attempts
to tackle the ball carrier front on they will have to overcome a greater force
as they will have to stop the ball carriers forward momentum before they can
knock the ball carrier over (Blazevich, 2013).By approaching the ball carrier from an angle the
tackler doesn’t need to overcome the ball carriers forward momentum to knock
them over so less force is required to knock the ball carrier over making it
more likely to complete a successful tackle. |
| In A. Both players are running at each other front on so when they collide most of the momentum will be lost trying to overcome the opponents momentum and it is less likely that the tackler will get the ball carrier to the ground. In B. The ball carrier is still running towards their score line so their momentum is going forward but this time the tackler comes in at an angle. because of this not as much force from the tackle will be lost trying to over come the ball carriers momentum and they are more likely to have a successful tackle. |
Positioning
When the tackler makes contact they
should be aiming to hit the ball carrier with their shoulder first in a pushing
motion and then wrapping their arms around the ball carrier’s legs. They need
to be keeping their head out to the side for safety reasons. The best position
for the head is for the tackler to rest their ear against the ball carriers
thigh. The tackler should dig their feet into the ground in order to increase the
amount of force they are applying to the ground so that through Newtons 3rd
law an equal and opposite force will be applied to the tacklers feet giving
allowing them to apply a greater force to the ball carrier so that they will be
more likely to be knocked over
The Recovery Phase
The Push
Once
contact is made the tackler must extend their legs using a push-like movement
pattern, so all the joints in the kinetic chain (Ankle, Knees, and Hips)
simultaneously extend to produce a higher cumulative force. Pushing against the
ground utilizes Newtons 3rd law of equal and opposite reactions so that the
opposing energy is pushed back at the tackler providing them with an increased
forward momentum that likely to produce a higher impulse-momentum
relationship giving the tackler the best chance to overcome the ball
carriers inertia and successfully complete the tackle (Blazevich, 2013).
The Arms
The
tackler should at the same time be using their arms which should be wrapped
around the ball carriers legs to try to squeeze the ball carriers legs together.
Bringing their legs together minimizes the ball carrier’s base of support
decreasing their stability making it easier to knock them off balance. It also makes it less likely for the ball
carrier to free themselves from the tacklers grip and will stop the ball
carriers ability to run as their feet are trapped by the tackler and won’t be
able to move.
The Follow Through
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| The Tackler continues to push through the ball carrier to get them onto the ground |
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| When making Contact the body is held at a fairly horizontal angle |
Friction
In Rugby players will tend to have studs on their boots
to increase friction between the bottom of their shoes and the ground. This added
friction can affect the amount of effort required to tackle the ball carrier. Different
types of rugby boots have a different number of and different length studs which
can affect friction.
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| Rugby Boots |
Friction is important in rugby not only to ensure
players don’t slip when running or changing direction but also in the tackle
for both the tackler and the ball carrier. In the tackle both players will be
pushing their feet into the ground to prevent themselves from being knocked
back and to increase their momentum in the tackle. By having the studs on the
bottom of their boots their feet are going to have an increased amount of
friction with the ground and therefore are less likely to slip when applying
force to the ground allowing for greater force being put into the tackle (Blazevich, 2013). In order
for the tackler to prevent the ball carriers studs affecting the tackle the
tackler will have to drive upwards to lift the ball carrier slightly in the
tackle (Blazevich, 2013). This will make it more likely that the ball carrier will be pushed back
in the tackle.
How Else Can We Use This Information
Preventing injury
It is important to teach correct technique as there is a high risk of
injury if the tackle is done incorrectly. Coaches of rugby should use the
information provided on this blog to better understand the best tackling
technique to teach their players. Both the attacking and defending players are
moving towards each other at high velocity and if the incorrect tackling technique
is used (such as the head is not in the correct position) then cervical spinal
injuries can occur not only to the tackling player but also to the ball carrier.
Cervical spinal injuries are injuries caused to the nerves between the C1 and
C7 vertebrae (Shown below). Injuries to this part of the spinal cord are some
of the most severe injuries and can cause difficulties with breathing, speech and
cause paralysis.
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Sumo Wrestling
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| Sumo Wrestlers attempting to throw each other off balance |
Sumo wrestling is a Japanese form of heavyweight wrestling, where by the
wrestler wins a fight by pushing their opponent outside the marked circle or by
making him touch the ground with any part of his body, except the bottom of his
feet. In Sumo wrestling the contenders will crouch in front of each other
before attempting to shove their opponent under the shoulder to lift them up
and throw them off balance making it easier to push them over. A similar technique
is used in rugby during the increasing stability and the push phases explained
above..
Because similar technique is used, similar biomechanics are at play when
Sumo Wrestling. Sumo Wrestlers tend to keep their stance wide so to increase
their base of support and they will crouch before making contact to a) increase
their stability and b) to increase the amount of force they hit their opponent with
when they extend to make contact. In sumo wrestling the contenders lean forward
to maximize their forward momentum in order to increase the likelihood of
overcoming their opponents’ inertia. Similar to rugby, the leaning forward put
the Sumo Wrestlers centre of gravity close to the front of their base of
support so that when they go to make contact with their opponent they require
less energy to overcome their own inertia and can therefore apply more force to
their opponent to try and knock them over.
As a Sumo Wrestlers average
weight is 148kg, they have a much greater inertia than your average rugby
players so they are going to be much harder to knock over. Because of this Sumo
Wrestlers tend to rely mainly on throwing their opponents off balance rather
than just relying on their own strength to push over their opponent.Australian Rules Football
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| An Aussie Rules tackle |
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| A Rugby tackle |
Australian Rules football or Aussie Rules is an Australian
form of football played on a field similar to a cricket pitch, where the main
form of defence is tackling. The Aussie Rules tackle serves a similar purpose
to the Rugby tackle which is to prevent the ball carrier to make any ground and
to try and get a turnover- The major difference between an Aussie Rules tackle
and a rugby tackle is that in Aussie Rules you are not allowed to tackle bellow
the hips. Because of this it is best to tackle above the centre of gravity
around where the elbows are to prevent the ball carrier from being able to
release the ball. Even though there is this difference some of the same techniques
can be applied. Similar to rugby, before contact is made the tackling player
should bend their knees slightly to push off the ground with greater force due
to newtons law of equal and opposite reactions. And should still make contact with their
shoulder first and wrap their arms around the ball carrier, in this case to
prevent them from being able to release the ball rather than prevent them from
moving.
References
Australian Rugby League Referees’ Association, The 2007 ARL Laws of the Game and Notes on the Laws (Approved by the Australian Rugby Football League), http://www arlra org au/pdffiles/International_Laws_2007 pdf, 2007.
Australian Rugby Union, Key Points for the Tackler and Ball-Carrier, Australian Rugby Union SmartRugby: Confidence in Contact, A Guide to the SmartRugby Program, 2008, 18-19
Blazevich, A. (2007) Sports biomechanics the basics: Optimising human performance. Soho Square, London: A&C Black Publishers
Hendricks, S., & Lambert, M. (2010). Tackling in Rugby: Coaching Strategies for Effective Technique and Injury Prevention. International Journal Of Sports Science And Coaching, 5(1), 117-136. http://dx.doi.org/10.1260/1747-9541.5.1.117
International Rugby Board, Law 15 Tackle: Ball Carrier Brought to the Ground, International Rugby Board Playing Charter, 2008
What Is Sumo? - Sumo - Virtual Culture - Kids Web Japan - Web Japan. (2016).Web-japan.org. Retrieved 18 June 2016, from http://web-japan.org/kidsweb/virtual/sumo/sumo01.html
(2016). Aflcommunityclub.com.au. Retrieved 20 June 2016, from http://www.aflcommunityclub.com.au/fileadmin/user_upload/Coach_AFL/Drills__Skills_and_Tactics/Coaching_Tips/How_to_Play/AR13_How_to_play_Pt6_Tackling.pdf
