bio mechanics

Two young divers from Shamrock Diving Club which is based in the National Aquatic Centre, Blanchardstown, travelled to Southampton on October 23rd to compete in one of the elite UK platform and springboard diving competitions, the Southampton Invitational.

DIVING SIMPLE LAWS OF BIO MECHANICS :

Anna Lindberg (SWE)

by Derek Travis (GBR)

DEFINITIONS :-

· MASS :- The quantity of matter in a body.

· WEIGHT :- A measure of the effect of the force of gravity on the mass of an object.

· GRAVITY :- The force exerted by the pull of the earth on all matter.

· CENTRE OF GRAVITY :- The point through which the force of gravity seems to act (the balance point).

· FIRST LAW - "THE PRINCIPLE OF INERTIA" A body at rest resists being set in motion, and when set in motion by outside forces, equally resists attempts to alter or stop its motion. The resistance or reluctance to change in the existing state of a body moving in a STRAIGHT LINE (linear movement) is termed INERTIA. When the body is ROTATING about an axis this resistance to change is termed the MOMENT OF INERTIA.

· SECOND LAW - "THE PRINCIPLE OF ACCELERATION" A force acting on a body produces either an acceleration (constant increase of velocity) or a deceleration (constant decrease of velocity). The force of gravity produces an acceleration (or deceleration) of 32 feet(9.80m) per second for every second it acts on a body. The change in velocity of the body is proportional to the magnitude of the force acting on it.

· THIRD LAW - "THE PRINCIPLE OF EQUAL AND OPPOSITE REACTIONS" Every action produces an opposite reaction of equal momentum. An outstretched arm can only be moved across the body by virtue of the fact that the muscle responsible for moving the arm is attached to the trunk, which will react by moving towards the arm with equal MOMENTUM. When the feet are fixed to the earth (or diving board) the trunk and the earth rotate in theory towards the arm. The mass of the earth is so great however that its velocity is so small as to be for all practical purposes non-existent.

THE CENTRE OF GRAVITY OF A DIVER

A diver's mass is the matter of which it is composed. A unique point is associated with every diver, around which the diver's mass is equally distributed in all directions. This is known as the centre of mass of the diver. Since the diver is subject to gravitational force, the centre of mass may also be referred to as the centre of gravity (CG), the point about which a diver's weight is equally balanced in all directions. It can also be viewed as the point at which the entire weight of the diver's body may be considered as concentrated.

A knowledge of the location of the diver's CG for various body positions is useful to the teacher/coach for the following reasons:

o The flight path of the diver can be predicted at the instant of take-off.

o The amount of rotation possessed by the diver can be assessed at the instant of take-off.

o It is the main reference point used to describe a diver's linear and angular motion.

· CREATING ROTATION

During the flight of a diver there is only ONE force acting on the body, the force of gravity, so the diver is always balanced in the air. He cannot overbalance or set himself rotating.

In order to rotate, a diver must employ some other force. The only force available to him is the "reaction" of the earth's surface, it is opposing the force of gravity acting downwards on his body. His muscular system kept him upright. When standing too long, his muscles tire and he falls over, i.e. he overbalances.

The earth's surface reacts upwards in effect through his feet, and as he topples over, his centre of gravity passes outside the base (his feet). He rotates through 90 degrees from vertical to horizontal, die, "falls flat on his face".

When this is performed from a diving board, the extra height will enable him to enter head first.

The body movements needed to provide the turning force necessary to produce the angular momentum required during the flight must be started whilst the feet are in contact with the board, that is during the take-off, and must be completed just before the feet lose contact with the board.

There are basically four methods of producing angular momentum:-

1 Overbalancing (Lean).

2 Momentum Transfer (Jerk).

3 Eccentric Leg Thrust (Hips bent).

4 Eccentric Board Thrust (Springboard only).

· OVERBALANCING (LEAN)

The simplest method of creating rotation, particularly for the beginner.

All forward and back dives require the body to overbalance just prior to the feet leaving the board. This is to ensure that the centre of gravity is set in motion away from the board for reasons of safety. This slight lean at take-off will inevitably provide some angular momentum, however if lean were necessary to create rotation, reverse and inward somersaults would not be possible.

The greater the lean at take-off the greater the angular momentum created, but only at the expense of height and therefore time. It should be reduced to a minimum consistent with the requirements of safety and aesthetics.

Any lean during the take-off for a reverse and inward dive will induce angular momentum in the wrong direction, which will need to be overcome by extra angular momentum in the correct direction if the dive is to be performed successfully.

· MOMENTUM TRANSFER (JERK)

When free in the air, an arm swing will result in the trunk moving "towards" the arm; when the arm stops moving, the trunk stops. If the trunk is prevented from reacting, the momentum stored in the arm as a result of the arm swing will be transferred to the rest of the body, pulling it round in the same direction.

Momentum from the PART has been transferred to the WHOLE.

For the given mass, the momentum stored in the PART of the body set in motion during the take-off will depend on the speed with which it is moving in the required direction at the moment the feet leave the board.

The faster the PART is moving at the moment of take-off, the greater the amount of angular momentum stored in it.

Double the speed produces twice the momentum.

The angular momentum stored in the PART during this important phase of the take-off will be transferred to the WHOLE during the flight, rotating it in the same direction.

MOMENTUM = MASS times VELOCITY

Therefore a MASS moved a SHORT distance at HIGH speed will produce MORE MOMENTUM than when moved a greater distance at a slower speed.

As the initiation of rotation must be co-ordinated with the final leg drive at take-off, this becomes of vital importance, especially in multiple somersaulting dives.

In practice, momentum transfer from the arms can be combined with that of the upper trunk to provide a strong turning force when performing multiple somersaulting dives.

Especially effective with back and reverse movements, as the normal upward arm swing is in the same direction as the body rotation.

With forward and Inward dives this same advantage can be achieved, providing the arm swing is made directional, ie, changes direction when above the head.

· ECCENTRIC LEG THRUST (HIPS BENT) When the legs straighten at take-off, the thrust is along a line projected upwards from the toes passing through the hip joint. If the body is straight at take-off, so that this line passes through the body's centre of gravity as well as the hip joint, the body will rise, but there will be no rotation from this action. If, during the take-off for a forward or Inward dive, the body is bent forward at the hips or the back is rounded so that the body's centre of gravity lies in front of the hip joint at the completion of the leg thrust, an off-centre or eccentric turning force will be applied, causing rotation of the body about its centre of gravity, in addition to the upward motion.

The GREATER the distance between the hip joint and the centre of gravity, the GREATER will be the angular momentum created, but the LESS force there will be available to project the body upwards and vice versa

· ECCENTRIC BOARD THRUST (SPRINGBOARD ONLY)

The line of thrust from the end of a rising springboard is at right angles to its surface. If at the moment of take-off, this line passes outside the body's centre of gravity, then a turning force will be imparted.

In forward and backward take-offs, this will assist the general rotation of the body, but in reverse and inward dives, it provides rotation in the wrong direction.

Xiong Ni (CHI)

· METHODS USED FOR SPECIFIC DIVES :- METHOD DIVING GROUP

1. OVERBALANCING FWD/BK/ARMST SS

2. MOMENTUM TRANSFER ALL GROUPS

3. ECCENTRIC LEG THRUST FWD/INWD

4. ECCENTRIC SPRINGBOARD THRUST FWD/BK

DIVING GROUPS METHOD

o FORWARD DIVES 1 / 2 / 3 / 4

o BACK DIVES 1 / 2 / 4

o REVERSE DIVES 2

o INWARD DIVES 2 / 3

o DIVING - HOW AND WHY by George Rackham

I have been asked if I could present the principles of mechanics applicable to diving as described in " Diving Complete", in a condensed form for easy reference.

The important conclusions reached are : -

10. That the angular momentum (rotational energy) responsible for producing the somersaulting motion during the flight of a dive, must originate during the take-off.

11. That no movement of any part of the body made during the flight can start the body rotating (somersaulting).

12. That a somersault, once started, cannot be stopped by any action made by the diver during the flight of the dive. It can be only slowered.

13. That the amount of angular momentum imparted to the body during the take-off depended mainly on the speed at which the upper body (head, arms and shoulders) was moving in the required direction of rotation, at the moment the feet left the board.

14. That the required upper body movement can be obtained in the simple forward and back dives by over-balancing or leaning.

15. That a greater effect and a more efficient method is achieved by jerking' the upper body in the required direction of rotation (momentum transfer). Used to great advantage in back and reverse dives.

16. That a third method, eccentric leg thrust (hips bent) can be used in any forward rotating dive (forward and inward groups), and can be used at all stages in the teaching of the Plain Header.

17. The rate of rotation (angular velocity) of a somersaulting diver can be increased by making the body more compact, or decreased by making it less compact.

18. When the body is fully stretched, its resistance' or rotational inertia (moment of inertia) is at its greatest, and its angular velocity is at its lowest.

19. When the body is in a tight compact tuck position, its resistance' to rotation is at its lowest, and its angular velocity is at its greatest (about four times as great).

20. The total somersaulting energy (angular momentum) remains constant, irrespective of any change in the body position during the flight.

21. The angular momentum (rotational energy) is a product of the moment of inertia (rotational resistance) and the angular velocity (rate of rotation).

22. For the angular momentum to remain constant, any increase in the moment of inertia will result in a corresponding decrease in the angular velocity and vice-versa.

23. Whilst the body position is altering, the rate of rotation is changing. When the body position remains unchanged, the rate of rotation remains constant.

24. The greater the range of body movement during the flight, the greater the amount of control there will be over the rate of rotation.

After having experience at teaching and coaching dives from the Forward Group, you will arrive at the following conclusions :-

25. That the most important part of the dive was the take-off.

26. That excessive lean during the take-off was the greatest retarding factor to progress in diving.

27. That, whilst recognising that it was possible to employ lean (over-balancing) to create rotation from the 5 or 10 metre platforms, it was impractical to use this technique from the modern flexible springboards.

28. That a better and more efficient means of creating rotation was provided by the more efficient eccentric leg thrusts (hips bent) method.

29. That this method reduced the angle of lean to the minimum necessary to provide enough forward travel to clear the board, whilst at the same time giving maximum height at the take off.

30. That it was advisable for divers to be made aware of this method and to be taught it during the very early stages of instruction.

31. That a further method of creating rotation was achieved by the use of transfer of momentum (jerk).

32. That when both of these techniques were employed together, the maximum amount of rotation could be created with the minimum loss of height

M.E.Ro

Right click press play to play and stop