Biomechanics

Chapter overview

What is biomechanics? page 73Motion page 74Balance and stability page 78Force page 85Projectile motion page 103Fluid mechanics page 109

Now that you’ve finished … answers

What is biomechanics?Page 73

An overview

Biomechanics is the study of the body as a machine.

Biomechanics can be used to: Refine technique Prevent injury Develop equipment Correct errors

Whilst many biomechanical principles are inherent in each sport skill, it is important to discern the most influential principles in your sport in order to look to optimising technique.

MotionPage 74

Motion

To initiate motion, one must overcome inertia. Once in motion, factors such as velocity and acceleration

become influential. Motion can be curvilinear or rectilinear

Curvilinear motion Rectilinear motion

Motion

Factors such as velocity and acceleration are variable amongst athletes and can determine the effectiveness of one’s performance.

Newton’s laws of motion

Newton’s first law of motion states that 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.

His second law states that the sum of the force that moves an object is equal to the object’s mass multiplied by the acceleration. This law can be expressed as the following equation:

Force = mass × acceleration (F = ma)

Newton’s law of action reaction states that for every action there is an equal and opposite reaction.

Balance and stabilityPage 78

Balance and stability

Every athlete must be able to maintain stability and balance.

Failure to do so can result in loss of force and or accuracy.

Personal reflection

Do you find balance easier when you are moving or

stationary? How does this affect your performance in

your current physical activity?

Balance can be dynamic or static.

Factors affecting balance include base of support, centre of gravity and height of gravity.

Personal reflection

Have you ever done the stork stand

balance test? Did you find it

challenging?

Centre of gravity

Personal reflection

Think of a time when you were performing a

dynamic movement. When your centre of

gravity falls outside of your body, what is your natural movement to try

to correct it?

The height of the centre of gravity can have a significant affect on your stability.

Personal reflection

Do you think your height has an effect

on your stability?

ForcePage 85

Force

Force is anything that causes or has the potential to cause the movement, diversion or slowing of the object on which it acts.

All forces have four common properties. They have: magnitude (an amount, or how much is

applied) direction (the angle at which the force is

applied) a point of application (the specific point at

which the force is applied) a line of action (represented by a straight

line through the point of application in the direction that the force is acting).

Integration

Consider the sport you are currently studying.

Describe the forces required in terms of the four properties listed on

the previous slide.

Force production: Most sports require an athlete to be able to both generate and control forces.

Integration

In your sport, can you differentiate between the

skills that require maximum force and those that require control and

accuracy?

Summation of forcesSummation of forces

To obtain maximum force, it is necessary to combine or add up the forces applied by different body parts. This concept is known as the summation of forces.

The summation of force is influenced by the: number of body parts used in the movement order and timing of their involvement force and velocity generated way in which the body and body parts are stabilised and

balanced.

The long jumper combines the forces in his legs, abdominals and arms to propel himself further into the pit.

Summation of forcesSummation of forces

Personal reflection

Choose one physical skill from your current sport. Which body parts do you use to produce maximum

force? In what order?

Momentum

Whether it is a sprinter running along a track or a bowling ball rolling down an alley, any object or person has a certain mass and a certain velocity. The product of these is known as the momentum.

Personal reflection

When you sprint, how many steps does it take you to

come to a complete stop? How does this compare with

your classmates or competitors? Why?

In most sports, mass is constant, so velocity becomes the main factor influencing momentum. So, to increase momentum, simply increase velocity.

In many sports, it is necessary for momentum to be transferred to another object or body part. The greater the momentum an object has, the greater its effect on other objects it collides with.

Personal reflection

Have you ever played pool? Why is that when the white

ball strikes another ball straight on, the white ball

stops moving while the ball that was struck moves

forward?

Stabilisation must occur for effective transfer of gained momentum

For example, in tennis it is common for athletes to brace (tense up) their muscles just before the impact of a forehand. Stopping the rotation of the body by bracing causes a whip-like effect on the arm

Accuracy

It is important in many sports to be accurate in the production of sport in both direction and magnitude.

Application

In the following sporting situations, consider ways in

which the performer can increase their directional

accuracy.

1. Batting into space in softball2. Digging to a setter in volleyball3. Kicking a penalty goal in soccer

Rotary forces

Not all forces are produced in straight lines.

Concentric forces are applied through the centre of an object and cause motion in a straight line

Eccentric forces are applied

off-centre and cause rotation

in the object to which the force

is applied.

Practical example

Place your textbook on the desk in front of you. Push the book away from you

with two fingers, applying the force through the

centre of the book. Now repeat this, applying the

force to one side of centre. What do you notice?

Rotation about an axis (torque)

More torque, and a greater rotation or twisting movement, will be generated when more force is applied further from the centre of an object.

Rotational momentum

A spinning object will have momentum that will allow it to continue to spin until an external force is applied.

Practical example

Sit on a swivel chair and clear a space around the chair. Ask

a friend to gently spin the chair in any direction. You can

control the speed at which you spin by tucking your legs into your body to speed up and extending them out to

slow down.

The velocity of the spin is affected by the distribution of the weight from the axis of the object.

When spinning on the ice, skaters can control the rate of spin by moving their limbs closer to the axis of rotation (to rotate faster) or extending their arms and legs (to rotate more slowly).

Centrifugal and centripetal forces

Centripetal force is the force that causes rotating objects to move towards the centre, or axis, of rotation.

Centrifugal force is the force that causes rotating objects to move away from the centre, or axis, of rotation.

Levers

Three types: First class - fulcrum between the force and

the resistance e.g. oar on row boat. Second class - resistance between the force

and the fulcrum Third class - force between the resistance

and the fulcrum e.g. baseball batter.

Third class levers are the most common in sport. Sports using bats or racquets all use third class levers. The velocity of a lever is fastest at the point furthest from the

fulcrum.

By increasing the length of the bat, the velocity is increased.

It is important to consider strength and accuracy when looking at levers.

It is not ideal to use a 4m long bat/racquet even if a greater velocity could to produced.

Maximising leverage

Due to inertia, a shorter lever is easier to move than a longer one.

To maximise the efficiency of a lever, it is often better to begin a lever’s motion by shortening the lever.

This can be achieved by bending the relevant joint.

E.g. a soccer playing kicking a ball starts with a bent knee, a javelin thrower bends the elbow to bring the javelin through and a tennis server bends their elbow before extending to hit the ball.

The ‘sweet spot’

The sweet spot is the point that, when struck, causes no backwards nor forwards rotation of the bat.

Personal reflection

Have you ever felt the effects of hitting

a ball outside the sweet spot?

Projectile motionPage 103

Projectile motion

Factors that influence flight Height of release Angle of release Speed of release Gravity and air resistance

Angle of release

The optimal trajectory of a projectile is just less than 45 degrees.

Height of release

A projectile thrown from a height will travel further.

Personal reflection

Do you think you have a height advantage

when throwing?

The effects of spinning

The Magnus effect explains how a spinning projectile, such as a ball, curves in flight.

Fluid mechanicsPage 109

Fluid mechanics

Fluid mechanics refers to forces that operate in both the water and in air.

Buoyancy

An object’s buoyancy determines whether or not it will float. Buoyancy is influenced by density. Density is a measure of mass in relation to volume.

Personal Reflection

Have you ever tried floating on your back

in water? What happens when you allow your body to

relax?

The high density muscle and bone in our legs cause them to sink when in the water whilst the low density air and fat in our chests keep them afloat.

Centre of buoyancy

The centre of buoyancy of a floating object is different to its centre of gravity.

In swimmers, the centre of buoyancy can change depending on the position and movement of a swimmer’s body, particularly the legs, and how much of the body is submerged.

A swimmer’s centre of buoyancy is closer to their heads due to the lower density of the chest and core compared to the legs.

Propulsion

Swimmers are required to both push and pull themselves through the water.

In this pushing and pulling action the forces of lift and drag are at work.

Lift

Lift force occurs due to the factors involved in Bernoulli’s Principle.

The difference in velocity of the water travelling over the surface of the hands creates a pressure difference that, when applied correctly, creates lift.

Drag

Drag force opposes the forward movement of a swimmer, providing resistance.

Swimmers can minimise this effect of drag by streamlining their bodies and pitching the entry of their hands to ensure the water surface is cut cleanly.

Drag can also assist a swimmer. Lift and drag can work together to create propulsion.

Water resistance

Skin resistance is the resistance created by water ‘gripping’ the skin as is passes through.

Turbulence resistance is created due to the high pressure created at the front of the swimmer interacting with the low pressure formed at the rear of the swimmer, creating a suction effect.

To minimise turbulence and resistance, good swimmers streamline their bodies in the water.

Wave resistance is caused by the wall of water that builds up at the front of a swimmer, blocking their path.

Now that you have finished …Answers

1. Define biomechanics.

Biomechanics is the study of the body as a machine.

2. How might the science of biomechanics have influenced the sport you are currently studying?

Depending on the sport being studied, look for specific techniques that utilise biomechanical principles such as the sequential summing of forces used in a discus thrower to maximise force produced. There may also be specific design elements in the clothing or equipment used in the sport; e.g., the LZR Racer suit in swimming.

3. Describe the difference between linear (rectilinear) and curvilinear motion.

Linear motion involves a body or object moving in a straight line whilst curvilinear motion involves a body or object moving along a curved path.

4. Explain Newton’s three laws of motion. How might they be used to explain the action of a long jumper from the start of the run-up to landing in the pit?

Newton’s first law (inertia) states that a body will remain at rest or in motion in a straight line unless acted upon by another force. The second law (acceleration) states that the force required to accelerate a body at a given rate is equal to the product of that acceleration and the mass of the body. The third law (action and reaction) states that for every action there is an equal and opposite reaction.

In long jump the athlete uses action and reaction (third law) to begins the run-up by pushing down and backwards on the ground to make the athlete’s foot react by moving forwards and upwards. The rate of acceleration will be determined by the force applied and the mass of the body (second law). Inertia (first law) will keep the body moving forward in a straight line to the pit and will ensure the body does not stop unless the athlete’s muscles to brake the run or until the jumper hits the sand in the pit which stop forward movement through friction. Action and reaction (third law) is also important at the point of take off where the athlete will hit the board and attempt to apply as much downwards and backwards force as possible to gain the maximum of forward and upwards motion.

5. Give three examples in athletics of action and reaction forces.

1. A runner in the starting blocks will apply as much force backwards into the blocks when the gun goes to gain as much forward motion as possible.

2. In high jump, the jumper will apply action and reaction as they take off to clear the bar. The more downwards force they apply with their foot to the ground, the harder the ground will push back on them to propel them in the air.

3. In discus, the greater the forward force applied by the hand to the discus, the more force the discus will apply back onto the hand when released.

6. Using Newton’s second law, answer the following question. If a 15 Newton force is applied to a 1.5 kilogram discus, what is the acceleration of the discus?

Force = Mass x Acceleration

15N = 1.5kg x Acceleration

Ac = 15/1.5

Ac = 10m/s/s

7. Discuss how a gymnast’s centre of gravity may change throughout a routine.

When a gymnast is upright their centre of gravity is situated somewhere towards the centre of the midsection. When they rotate part of their body forwards, their centre of gravity shifts away from the midsection in the direction the body has moved. At some points, the centre of gravity will shift entirely outside of the gymnast’s body.

8. Outline three ways an athlete can maximise force production.

An athlete can maximise force production by ensuring they use summation of force to obtain the greatest amount of force sequentially from all muscles. They can also increase their momentum by increasing the speed or power in their run up and finally by applying the force for a longer period of time where possible to maximise impulse gained.

9. Outline three ways the body can absorb force.

When landing from a jump, the ankles, knees and hips are designed to flex to absorbed the force over a greater length of time and thus reducing impact.

When catching a ball, moving the hands in the direction of the incoming ball as it makes contact, will allow the ball to absorb better and reduce the rebound effect.

Use of larger body parts will also maximise force absorption; e.g., catching with two whole hands rather than just fingers on one hand.

10. Explain the difference between concentric and eccentric force.

Concentric force is force applied through the centre of an object resulting in movement in a straight path. Eccentric force on the other hand, is applied off-centre and causes the object to rotate and move in a curved path.

11. List the two factors that determine rotational momentum.

The two factors that determine rotational momentum are mass and velocity.

12. Identify the principle that causes a rugby union goal-kicker’s left arm to rotate forward, across the body when their right foot kicks the ball through the uprights.

The principle is rotational action and reaction. This response occurs in order for the body to maintain its

equilibrium. When the rugby player’s leg swings through (action force), the arm’s response (reaction force) prevents the body from toppling over. 

13. Explain the term ‘sweet spot’.

The sweet spot is referred to as the point on the equipment where it is ideal to strike the ball to maximum force and accuracy. In this spot, the bat will not rotate forwards or backwards and there will be no vibrating or jarring sensation felt by the striker.

14. Identify five factors that affect the trajectory of a thrown ball.

Factors affecting the trajectory of a thrown ball include the angle of release, height of release, speed of release, spin applied and wind resistance.

15. Explain how you would find a person’s centre of buoyancy.

The centre of buoyancy is found at the geometric centre of the displaced volume.

16. Describe the types of resistance that exist in fluid environments.

The types of resistance that exist in fluid environments include skin resistance, turbulence resistance and wave resistance. Skin resistance is caused by the fluid gripping the skin as it passes through the water. Turbulence resistance is caused by creating areas of high pressure in front of the moving swimmer and low pressure behind. As water flows from the high pressure area to the low, it sucks the swimmer back. Wave resistance is caused by the swimmer making a wave or wall in front of them, blocking their path when they swim.