BIOMECHANICS

What is Biomechanics

Biomechanics is the sport science field that applies the laws of mechanics and physics to human performance.

Biomechanics is the study of how and why the human body

moves.

Why is Biomechanics so Important?

1. Optimise performance by developing the most effective and efficient technique.

2. Correct technique to reduce and prevent overuse or chronic injury.

3. Modify and develop equipment to widen participation eg. Smaller ball

4. Transfer skills from practice field to playing field eg. Batting tees to pitchers ball.

Biomechanics is Concerned with the Following Concepts

ForceMotionProjectile MotionBalance and StabilityEquipment Design

Force

Any pushing or pulling activity that tends to alter the state of motion of a body

(a body can be a human body, ball, discus, javelin, racquet or bat)

FORCES

Forces can be EXTERNALGravityFrictionAir ResistanceWater resistance OR INTERNALAction of muscles and tendons on the skeletal

system

1. Isometric Forces

Force without motionThe muscle length does not change. Muscle contractions may or may not create

movement while applying a force. Eg. Pushing against brick wall, gripping a racquet or bat

2. Isotonic Force

This force changes the state of motion Eg. Pushing out of the blocks for a 100 m

sprint, accelerating a hockey ball with a push pass, decelerating a football

by marking it on your chest.

3. Sub-maximal Force

Force application must be optimal for successful performance, however some activities require less than maximal force.

Eg. Putting in golf, drop shot in badmintonPerformer is required to use limited number of motor units.

4. Maximal Force

Perfect timing, maximal muscle contraction and excellent technique achieve maximal force.

Eg. Throwing for distance, Serving in Tennis

HOW DO WE DEVELOP FORCE?

Force Summation can be achieved:1. Simultaneously – an explosive action of all body

parts at the same time

Eg. A high jump take off, gymnastics vault take off.

2. Sequentially – body parts are

moved in a sequence to generate

greater force

Eg. Kicking for distance, long golf drive

How to use Sequential Force Summation

1. Use as many body parts as possible

2. Use the largest body parts and muscles with greatest mass first (such as legs, quadriceps).

3. Sequentially accelerate each body part so its momentum optimally passes onto the next body part.

How do we apply an effective FORCE?

We need to understand aboutInertiaMomentumImpulseAccuracyForce reception

Inertia

Inertia =A measure of how difficult it is to change an objects motion.

Describes a body’s resistance to change it state of motion. The greater the inertia, the more difficult it is to change the motion of an object.

DEMONSTRATE INERTIA

Chest pass a basketball and medicine ball

1. Which ball has the greater inertia2. Which ball is easier to throw over long distances

The greater the mass of the object = the greater its inertia = greater the force required to change its state of motion

Velocity Measure the rate of change from one position to the next. Velocity has two characteristics: speed (how

quickly an object is covering ground) and direction).

A stationary object has a velocity of zero (position is not changing)

An object moving north and travelling 5 metres per second. Velocity = 5 m/s in a northerly direction.

MomentumMomentum = the motion possessed by a

moving bodyTherefore an object can only have

momentum if it is movingThe momentum of an object = mass x

velocityWhen two things collide. The one with the

most momentum will be less affected.

Impulse

Impulse is the application of force over a period of time which changes the velocity of a body or object. Impulse = Force x Time

The greatest changes in momentum (whether speeding up or slowing down) occur when maximum force is applied for as long as possible

Eg. The rotating discus technique (one and a half turns) allows forces to act longer on the discus.

Impulse and Accuracy

Certain techniques in sport increase accuracy by allowing a greater contact time with the object and greater force application (when required). Eg. Tennis serve, hockey push pass

WRITTEN REPORT p. 59

Flattening the swing arc (refer to p.60 Fig 2.12)

Biomechanists have created techniques that give a zone of flat line motion. This flat zone allows a greater distance over which force

can be applied in the desired direction of travel.

Follow Through – Ensures that the maximum force is applied to the object by avoiding deceleration before contact with ball.

Force Reception

When a ball is travelling at speed, the force (momentum) of the ball must be absorbed or

received over a distance to slow it and stop it. Correct catching technique requires the momentum of the ball to be absorbed over a distance or over a short period of time.

Newtons First Law of Motion - Inertia

An object will remain in its current state of motion (whether that be stationary or moving) unless acted upon by a force.

1. An object at rest will remain at rest unless acted on by an external force. Eg. Golf ball off a tee

2. An object in motion will continue to remain in motion unless acted on by an external force. Eg. Ball travels less distance in the wind

This is called the law of inertia because the size of force required to change the object’s state of motion depends on the object’s weight or mass.

The heavier an object is,

the greater its inertia and the

greater the force required to shift it.

Newtons Second Law of Motion

Acceleration/MomentumThe acceleration of an object is directly

proportionate to the amount of force applied and takes place in the direction in which the force is applied.

Force = Mass x Acceleration

Ie. The smaller the mass, the greater the acceleration. Golf ball travels faster than a heavier object struck with the same implement.

Eg. Short chip causes a golf ball to accelerate more slowly than a drive because smaller amount of force is applied.

Newtons Third Law of Motion - Reaction

For every action there is an equal and opposite reaction.

Eg. The harder you bounce a ball, the harder it will bounce back.

Conservation of Momentum

Relates to Newton’s Third LawThe total momentum of two objects before

impact or compact will equal the total momentum after impact.

The momentum of an object is never lost, but transferred on contact with other objects.

Eg. Billiards – Red ball is struck by the white and takes off with the same velocity and momentum as the white, and the white is now still after having transferred its momentum to the red.

LeversA lever is a simple machine that allows a

greater force to be produced.Consist of:

1. Force Arm – distance between axis and force.

2. Axis – point at which lever rotates 3. Resistance arm – distance between axis

and resistance.

Types of Levers

First Class Lever – Used to generate great speed. Axis is between the force and resistance ( f A r). Eg. Rowing oar, crowbar, scissors.

Second Class Lever – Used to increase the strength that humans can apply to objects. Resistance is between the axis and the force ( a R f). Force arm is always longer than the resistance arm. Eg. Wheelbarrow and bottle opener.

Third Class Lever – The most common lever in the body. The force is between the axis and resistance ( a F r). Eg. Bicep curl, kicking a ball.

Levers in Sport

Velocity is greater at the end of a long lever than a short one. The longer the lever the greater the velocity at impact and greater the momentum.

Motion

There are three basic forms of motion:1. Linear Motion2. Rotary Motion

3. Angular Motion

Linear Motion

All parts of an object travel over the same distance, in the same direction, in the same time.

Eg. Speed ice skater gliding down back straight after finishing the race

Angular Motion

Occurs when an object rotates around an axis

The axis may be at a fixed point (eg. Shoulder joint in a throw) or the centre of gravity of a body

The human body has three axes around the centre of gravity

THE THREE AXES• The longitudinal axis (vertical) is the line

taken from head to toe through COG eg. Ice skater spinning

• The transverse axis (horizontal) is the line taken from hip to hip through COG eg. A diver performing a triple somersault

• The medial axis (horizontal) is the line taken from navel to small of back through COG eg. A gymnast performing a cartwheel

General Motion

A combination of linear and angular motion.

Most actions in sport requires this combination eg. Running – whole

body moves in straight line as result of the rotary motion of the legs around the hip joint

Moment of InertiaThe moment of inertia of a rotating body or

object is its resistance to change, particularly resistance to beginning angular motion.

In linear motion the only factor in moment of inertia is the weight of the object.

Moment of inertia cont..

With rotary motion, moment of inertia depends on weight plus the distance of the weight from the axis of rotation.

(Eg. Easier to rotate the arm around the shoulder when the arm is bent. The closer the mass to the axis of rotation the easier it is to rotate.

Examples of Moment of Inertia:• A young child will grip a baseball bat further

up its handle. The longer the moment arm, the greater the moment of inertia and the resistance to rotation; therefore harder to swing bat.

• Refer to p. 73-74• ACTIVITY p. 75-76

SPIN – Spin is created when a ball or any object is subjected to an rotarional force Eg. Ball and a racquet, or the ball and the ground.

In top spin and back spin, the ball is rotating around its horizontal axis. In side spin, the ball is struck from left or right, and rotates about its vertical axis. (p. 84)

FRICTION

Friction is the force that arises whenever one object moves across the surface of another,

SLIDING FRICTION – Occurs when 2 objects slide on or over one another. In ballroom dancing waxes are used to reduce friction. In athletics friction is increased by using spikes.

ROLLING FRICTION – Occurs because the ball and the surface on which it is rolling are slightly deformed in the process.

In sports rolling friction is affected by nature of balls surface and surface it is rolling along, mass of the ball and diameter of ball.