BIOMECHANICAL PRINCIPLES
Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum,
nisi quatenus a viribus impressis cogitur statum illum mutare.
Biomechanics is about describing movement. You need to use these principles when you teach trampolining
moves. Try not to worry about it! It’s not as complicated as some people think. Most of it is common sense
but it uses some language which people sometimes find scary!!! If you’re one of these people stop
panicking - you just need to learn the language.
Much of what we need to know is based upon Newton’s LAWS OF MOTION
These were stated by Sir Isaac Newton in the late sixteenth century. Sir Isaac Newton proved 3 basic laws
of motion-all of which apply to trampolining.
Unfortunately for us Isaac Newton was a lousy trampolinist, but he was a good scientist. This means that
although what he said is important to us as coaches, it can be a bit frightening for us non-scientists, just
remember, this is just jargon (like we use terms like Barani - ask the average Physicist what this means and
you’ll get some sort of take away meal!!!!!!!!!)
They are the basis of the motion of bodies (actually bodies above sub atomic size and restricted speeds but
unless you are going to do theoretical physics ignore this caveat!). He stated four laws that govern the
application of force to a body. They are therefore the principles with which good trampoline coaches work.
A lot of this is just common sense - don’t get phased!
John Wotherspoon/Dan James 2011
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The movements we are describing can be:
A) LINEAR - which is straight line motion, in which a body moves in a straight line, with all parts
travelling the same distance, in the same direction and at the same time.
B)
ANGULAR - which enables parts of a body to revolve or rotate around an axis, which is fixed. There
are three main axes around which it is possible to revolve, the LONGITUDINAL. LATERAL and DORSO
VENTRAL, of which the first is used for back and front somersault motion, the second for any twisting
motion and the third for specific skills like the side somersault.
It is possible to do both linear and angular movements simultaneously as well as to revolve around more
than one axis at once. However to cause any form of movement some type of force will be required.
A) GRAVITY – (NOT A LAW OF MOTION - LAW OF UNIVERSAL GRAVITATION His law of universal
gravitation doesn't constitute a "4th law of motion", ).
Gravity is the force that attracts everything towards the earth. It acts through your centre of mass.
“Every particle of matter attracts every other particle of matter with a force which varies directly as the
product of the masses of the particles and inversely as the square of the distance between them’.
In simple terms: If a trampolinist goes into the air off the bed he must come down again.
B) CENTRE OF MASS
The centre of mass is the imaginary point on which the body will balance. The position of the centre of
mass will vary according to the shape of the body and may even be outside the body.
The point at which your weight acts. Imagine if you were balanced at any angle on the tip of a sharp spear;
this is the point at which you would balance perfectly. If you change your shape, your centre of gravity can
move as well (and you could fall off).
C) PATH OF THE CENTRE OF GRAVITY
Your flight which is determined at take-off. If you take off so that you are flying off the trampoline, then
there is nothing that you can do about it – you will still fall off. If you change your shape, you can only
change your speed of rotation, NOT where you are going to land.
John Wotherspoon/Dan James 2011
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D) AXES OF ROTATION
There are three axes of rotation that we use to describe trampoline movements. Each axis passes through
the performer’s centre of mass.
E) FORCES AND MOVEMENT
A force is a push or a pull. All movement is caused by forces.
Forces that pass straight through the centre of mass make you move in a straight line.
Forces that do not pass straight through the centre of mass make you move in a straight line and also make
you rotate. (When they cause rotation, these forces are also called torques).
Therefore forces can cause:
1.
2.
3.
4.
Motion (movement) to start
The rate of motion to change
The direction of motion to change
Motion to stop.
F) MOMENTUM
The amount of motion (in any direction).
G) ANGULAR MOMENTUM
The amount of rotation (somersault)
H) CONTROLLING ROTATION - LAW OF CONSERVATION OF MOMENTUM –
What you start with you’re stuck with. So if you take-off with not enough rotation for a somersault you are
always going to land short.
Once in the air, you cannot create any more rotation energy (angular momentum). But you can control the
speed of rotation, even though your overall angular momentum remains the same.
Think of a jump from feet to front landing (a quarter front somersault). Once in the air, you can make
yourself rotate faster by tucking your body and make yourself rotate slower by straightening your body.
It is VERY important that you can see how changes in body shape control a trampolinists’ speed of rotation.
I) ECCENTRIC [OFF-CENTRE] FORCE
Is just simply a force that is NOT JUST UP AND DOWN. It is a force that you apply in the trampoline bed for
rotating forwards or backwards, i.e. when a performer does a front somersault the coach might say “dig
your toes in” or “lift your heels” and this is what they are trying to get you to do in the bed.
John Wotherspoon/Dan James 2011
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Forward somersault
Direction of push
Backward somersault
Direction of push
J) SOMERSAULTING
An off-centre force must be created and applied whilst in contact with the trampoline. It can’t be created
in mid-air.
Once you have taken off, you can only change the angular velocity (speed of rotation) by changing the
shape of the somersault, i.e. if you tuck it will speed up.
K) LAW OF INERTIA – NEWTONS 1ST LAW
“An object remains in a state of rest, or in a state of uniform motion in a straight line, unless acted upon by
some external force”.
The reluctance for anything, (including you) to start moving, or stop moving when it is already moving. The
greater the mass, the greater the reluctance to change, so the greater the force required to make a
change. This means that all objects and persons would remain perfectly still for ever if something did not
make them move.
Examples:
(i) After an oil tanker has turned off its engine it will carry on drifting for miles.
(ii) A 12 stone coach may fairly readily support or catch a 7 stone jumper, but it would be irresponsible for
a small 6 stone spotter to have much effect in catching a 12 stone faller.
(iii) A performer standing on the trampoline must depress the bed before (s)he can be thrown into the air.
Muscular energy has to be directed through the legs to push the bed down (depress the bed) and then an
equivalent recoil from the bed will project the performer into the air.
L) MOMENT OF INERTIA
This is the body’s resistance to somersaulting or twisting. If this is a low value (low resistance), you will
somersault easily, i.e. when you are tucked for a forward s/s.
If this is a high value (high resistance), you will somersault with difficulty, i.e. when you are straight in a s/s.
John Wotherspoon/Dan James 2011
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M) NEWTON’S 3RD LAW OF MOTION - LAW OF REACTION (ACTION-REACTION)
For every ACTION there is an equal and opposite REACTION. If you push down in the trampoline, the
trampoline springs will then push you back up in the air with the same force.
The higher you jump, the more the bed goes down, the springs move through a greater range to return to
their normal state so the jumper is thrown higher.
Pike jump - Legs raise up, an upper body mass of equal amount will be moved downwards. (As legs
and trunk move forwards, hips counter balance- try this - touch your toes; now do the same with
your back flat against a wall and your heels against the skirting board and touch your toes. -Now do
you believe me?????????????)
N) NEWTON’S 2ND LAW OF MOTION - LAW OF ACCELERATION
The force needed to accelerate a body is directly proportional to the mass of the body and to the
acceleration produced.
John Wotherspoon/Dan James 2011
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