1. CQ3 – How do biomechanical principles influence movement?
PRELIMINARY HSC PDHPE
CQ3 – How do biomechanical principles influence movement?

2. How do biomechanical principles influence movement?
Students learn about:
motion
the application of linear motion, velocity, speed, acceleration, momentum in movement and performance contexts
Students learn to:
apply principles of motion to enhance performance through participation in practical workshops
balance and stability
centre of gravity
line of gravity
base of support
apply principles of balance and stability to enhance performance through participation in practical workshops
fluid mechanics
flotation, centre of buoyancy
fluid resistance
apply principles of fluid mechanics to enhance performance through participation in practical workshops
describe how principles of fluid mechanics have influenced changes in movement and performance, eg technique modification, clothing/suits, equipment/apparatus
force
how the body applies force
how the body absorbs force
applying force to an object.
apply principles of force to enhance performance through participation in practical workshops.

3. Force
A force causes or has the potential to cause, divert or slow the movement of an object upon which it acts. In simple terms, forces can be considered as a push or a pull, a blow or an impact, friction when two surfaces rub together or gravity. When a body is at rest or in motion, forces are acting upon it. Whether you are sitting at a desk, running around a track or jumping out of an aeroplane, forces are acting on your body. Force is measured in a unit called a newton (N).

4. Force
A force can be described as internal or external relative to the system that is being examined. A force can act from inside the system (internal) or from outside (external), depending on how the system is defined at the outset. For instance, if we consider the system as the whole human body, the muscles that contract to exert a force on bones, cartilage or ligaments around a joint are considered inside the system and are therefore internal forces. Any forces exerted outside the body (such as gravity, friction, contact with the ground or another body, air resistance and fluid resistance) are considered external forces.

5. Force All forces have four common properties. They have:
• magnitude (an amount; how much is applied)
• direction (the angle at which the force is applied)
• point of application (the specific point at which the force is applied to a body)
• line of action (represents a straight line through the point of application in the direction that the force is acting).

6. The directions in which forces act are drawn as an arrow called a vector. Figure 6.17 shows the properties of a force.

7. how the body applies force
About 300 years ago, the scientist Sir Isaac Newton devised three theories to describe the relationship between force and motion. These theories have become the basis of trying to solve many problems in biomechanics today. An understanding of the applications of these three theories is essential in developing an appreciation of how biomechanical principles influence the way we move.

8. how the body applies force
Newton’s First Law of Motion states that:
Every body continues in its state of rest or motion in a straight line unless compelled to change that state by external forces exerted upon it. Put simply, no force equals no movement. This seems to be a basic, common sense theory, but it becomes more important when we examine how forces change the state of motion of a body, and the resistance of a body to a change in its state of motion (inertia).

9. how the body applies force
Newton’s Second Law of Motion states that:
The rate of change in motion of a body is proportional to the force causing it, and the change takes place in the direction in which the force acts. This law means that a body will experience a change in its motion in proportion to the force applied to it, and in the direction of the force. For example, a golf ball putted on a green moves in the direction in which it is hit and according to how hard it is hit.

10. how the body applies force
Another, not so obvious interpretation is that when force is applied to a moving body, such as an outstretched hand to a basketball, the motion of the ball is adjusted according to the force. That is, it may slow down or be deflected but it may not be in the exact direction of the force.

11. how the body applies force
This law generates the equation of:
‘For every action there is an equal and opposite reaction’. This is another way of saying Newton’s Third Law. This law illustrates that forces act in pairs, and are equal and in opposite directions.
As the mass of the body increases, a greater force is required to produce the same acceleration. To throw a 4- kilogram discus as far as a 3-kilogram discus, the force applied must be greater. This law also relates to momentum, as the momentum of an object is the change in rate of velocity over time (acceleration) multiplied by its mass.

12. how the body applies force
Newton’s Third Law of Motion states that:
For every force that is exerted by one body on another, there is an equal and opposite force exerted by the second body on the first. You may have heard the saying ‘For every action there is an equal and opposite reaction’. This is another way of saying Newton’s Third Law. This law illustrates that forces act in pairs, and are equal and in opposite directions.
However, the result is not always the same. For instance, when you land after performing a long jump, you apply a force to the ground and it applies one back to you. The effect on you is much greater than your effect on the ground, however, because the earth is much bigger and heavier. In this way, Newton’s Third Law relates to the Second Law.

13. Contact forces
Forces are often described in terms of whether they involve contact between bodies. Contact forces are the forces that involve actions (push or pull) exerted by one object in direct contact with another. Examples are when a foot hits the ground, a bat hits a ball or when players run into each other. Non-contact forces act from a distance and involve no contact between objects. The most common noncontact force is gravity. Weight is also a non-contact force. It is measured in newtons (mass × force of gravity).

14. Contact forces
Body weight and mass are not the same thing, since body weight depends on the force of the gravity acting on your body. If you landed on the Moon, which has a much lower gravity than earth’s, your body would have the same mass as it has now, but a much lighter weight. Most forces are the result of contact between bodies, and can be any one of six types:

15. Contact forces
In most sporting situations, athletes are in contact with the ground, and the reaction force that applies to the athlete is called the ground reaction force.
The force that two bones apply to each other across a joint is called the joint reaction force.
Friction is the force that resists the motion of one surface across another.
In many sports, motion is affected by the fluid in which it is performed; for example, air or water. This is called fluid resistance.
The force of inertia can also affect movement. For example, the ankle swings through when running because of the inertial force placed on it by the legal inquiry
When material changes its length when a force is applied to it, the force is said to be an elastic force. Examples of elastic forces are those provided by diving boards, muscles, sprung floors, trampolines and some running shoes.

16. Contact forces
Humans can apply contact forces to other humans, the ground and implements. For movement to result, the force applied needs to be greater than the external forces acting on the human body. For instance, if a footballer does not step hard into the ground, he or she will not change direction to swerve around an opponent. In the same way, a long jumper will jump further by accelerating to the plate and applying a greater force to it than if the run-up is slower and the plate is hit with less force.

17. Summation of forces
In the example of the long jumper, we also see the application of the principle of summation of forces. This principle explains that the force produced during the movement of one body segment (for example, the lower leg) will be added to the force produced by the next body segment (the thigh), and the next (trunk, chest and arms) and so on until action results. So the long jumper propels further through the air by using his or her legs (lower and upper), trunk, shoulders and arms.

18. Summation of forces Summation of force is therefore influenced by the:
number of body parts used in the movement
order and timing of their involvement
force and velocity generated
way in which body parts are stabilised for other body parts to act upon.

19. Centripetal and centrifugal forces
When objects move along a curved path, a centre seeking force acts towards the centre of the rotation. It is called the centripetal force. There is also an equal and opposite centre- fleeing force (Newton’s Third Law) called the centrifugal force. A gymnast swinging around a bar generates forces both in and away from the bar. The gymnast must retain his or her grip to counter the effects of these forces. In the same way, cyclists and runners lean into corners on tracks.

20. How the body absorbs force
In addition to applying force, the human body also absorbs force. Moving bodies (humans or implements) often need to be stopped and controlled. In order to prevent injury, the momentum of the speeding body must be gradually decreased by joint actions.

21. Propulsive and resistive forces
The external force that acts to cause motion in a body is called a propulsive force. A force that acts to resist the movement created by a propulsive force is called a resistive force. When we land from a height, the momentum of the body causes flexion of the knees, ankles and hip joints. This is resisted by the extensor muscles at these joints, and a sort of ‘giving’ of the legs during landing occurs.
The controlled lowering of a barbell also requires the momentum of that object to be reduced to zero or near zero, hence providing a resistive force. The same is true of the contraction of muscles in preparation for contact and ‘giving’ when catching a ball that is heavy or thrown very hard.

22. Activity/task 1 Define inertia.
2 Describe the effect of inertia on moving bodies.

23. Safety when colliding
Athletes can take a number of simple precautions to ensure safer collisions and impacts between themselves and other bodies (objects and humans). Some of these precautions are listed below:
• Use as large a surface area as possible when landing or catching. (For example, land on two feet; put the body behind the ball when catching.)
• Keep as great a distance as possible between the impacting objects. (For example, avoid running into players.)
• Use as much mass as possible when landing or catching. (For example, land on bigger, heavier body parts; put the body behind the ball.)
• Regulate the position of one’s centre of gravity. (For example, try to keep it stable and low.)
• Use materials other than body parts. (For example, use gloves, mitts or headgear.)
• Protect projections of the body during contact. (For example, during impact avoid using body parts, such as fingers, elbows and shoulders.)
• ‘Give’ with the impact. (For example, conserve momentum by slowing the object gradually.)
In most sporting situations, the next movement can be started by the momentum gained during the catch, landing or impact; for example, a catch then quick throw in softball to effect a double play.

24. applying force to an object.
A range of techniques can be employed to make the body increase the force it exerts in physical activity, or make the play harder to return or intercept. One example of how force can be effectively applied to an object is the top spin serve used in tennis. When executing this serve, the player throws the ball directly above the head and brushes the racquet face across the ball as it descends, imparting top spin on the ball and increasing the difficulty in returning the service.

25. applying force to an object.
Similarly, in cricket, bowlers can impart spin on the ball to alter the ball’s trajectory and force the batter into a position that may lead to his or her dismissal. Spin bowlers alter their wrist and finger motion to make use of the Magnus effect as this causes the ball to drift (deviate sidewards) before it bounces on the pitch.

26. applying force to an object.
1 a Identify four examples of sporting situations in which the performer needs to absorb a force to improve performance and for each, explain how this is done.
1. Cricket catch – the player cups their hands and cradles the ball to soften the impact. They then let their hands move downwards to control the impact and increase the chances of holding onto the catch.
2. Gymnastics landing – after performing a flip or a jump, the gymnast needs to land on their feet and absorb the force of the jump, without moving their feet (or they have a point deduction)
3. Punch in boxing – a boxer needs to absorb a punch in boxing, they do so by moving backwards to soften the impact (or trying to avoid it)
4. Header in soccer – when going for a header, a player must put force into the ball whilst keeping a strong neck. If they fail, the ball may force the neck to move backwards.

27. applying force to an object.
2 Describe what is meant by summation of force.
3 Describe the difference between contact and non- contact forces. Use examples to support your answer.
4 a Identify four examples of sporting situations in which the performer needs to apply a force to improve performance and for each, explain how this is done.

28. applying force to an object.
5 Explain how centrifugal and centripetal forces are related.
6 Describe the process that a cricketer uses when catching a high ball in the outfield to reduce the impact of the ball on his or her hands.
7 Discuss the factors that will affect the amount of force that the participant can apply in each of the following situations:
a a surf lifesaver swimming out through the waves
b a skateboard rider doing tricks on the path
c a person playing basketball in school shoes.

29. applying force to an object.
8 Summarise the safety techniques that can be employed by players of contact sports when encountering forces, such as tackles or collisions.

30. Summary
Performance can be enhanced through understanding the basic biomechanical principles that underpin movement.
Measuring how far, how fast or how consistently a body moves can help athletes improve their speed, velocity and acceleration rate.
Maintaining balance and stability is vital in all sporting activities. The broader the base of support the more stable an object or performer becomes.
Efficient biomechanical technique reduces the negative effects of drag and resistance on performance in fluid environments.
An object will float if the force pushing it up (the buoyant force) is equal to or greater than the force pushing it down (gravity). A body that is partially or totally immersed in a fluid will experience buoyancy that is equal to the weight of the volume of fluid displaced by that body.
Force can divert or slow the movement of an object upon which it acts. Understanding how to apply and absorb force effectively can improve performance.