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EDU 2462 Biophysical Foundations of Human Movement 1. Lecture 2 BIOMECHANICS. 2 Broad Categories. Kinematics - describes movement - “How far?” - “How fast?” - “Where did it go?” - “At what rate?”. Kinetics investigates causes of movement and measures the underlying forces at work.

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EDU 2462

Biophysical Foundations of Human Movement



2 broad categories
2 Broad Categories
  • Kinematics
      • - describes movement
      • - “How far?”
      • - “How fast?”
      • - “Where did it go?”
      • - “At what rate?”
  • Kinetics
    • investigates causes of movement and measures the underlying forces at work
function of biomechanics
Function of Biomechanics
  • Internal and external forces acting on the human body determine the performance of a motor skill
  • The nature of these forces, their impact and our adaptations to them is crucial to the effectiveness of the skill’s performance
  • Objects move when acted upon by a force greater than the resistance to movement provided by the object.
  • The force may:
    • produce motion (a golf club striking a ball)
    • stop motion (brakes on a bike)
    • positively accelerate (from jog to sprint)
    • negatively accelerate (from sprint to jog)
    • change the direction of the object (tennis serve).
Forces can be measured by:
  • magnitude eg. size/number of muscle fibres
  • direction eg. running = horizontal movement
  • point of force application

eg. feet placement prior to jump

  • line of action of force eg. topspin in tennis
summation of forces
Summation of Forces
  • If several forces act at any one time, the net force will cause the object to move or


Summation of forces may be...

1 sequential
1. Sequential
  • When the correct sequence of bodyparts is used to generate great force
  • To achieve the maximum power, each preceding segment must contribute maximally before the next segment follows on

eg. Volleyball spike; T-ball hit;

cricket bowl


Larger body parts are the slower


A Throw

  • F1








The speed of the projectile depends on

the speed of the last part of the

body at the time of release/contact

Smaller body parts are the fastest



2. Simultaneous

  • summation of the forces involves an explosive action of all body parts at the same time to generate the greatest force eg. the high jump; vault take-off
  • If a body is at rest, it will tend to stay at rest
  • Once an object is moving in a straight line, it will continue to move with constant velocity


a force acts upon it to change its speed/direction

Inertia =

the object’s resistance to this change in its state of motion.

in linear motion
In linear motion...
  • In linear motion (motion in a straight line),

inertia = the mass of the object

  • The larger the mass, the greater the inertia

(and vice versa)  the harder it is to move.

eg. to increase inertia, a greater mass is added

angular motion
Angular Motion
  • Refers to circular motion around an imaginary line called anaxis of rotation.
  • There are 3 primary axes of rotation:

1.Longitudinal (top-bottom)

2. Transverse (left-right)

3. Sagittal (front-back)

A body’s resistance to angular motion is called itsmoment of inertia.
  • It is dependent on two factors:
    • the mass of the object
    • how this mass is distributed relative to the axis about which the rotation is occurring

Longitudinal axis

eg. A figure skater when doing a spin has his arms and free leg extended away from the longitudinal axis of his body. His spin is slow. When he brings these limbs closer to his body, he is bringing his mass closer to the axis of rotation, reducing the moment of inertia and spinning faster.

Photo : - Technical Figure Skating Spin page


Film: - Technical Figure Skating Spin page

centre of gravity

In the picture the red triangle is the fulcrum or the

  • Balancing point.
  • Generally the Centre of Gravity of the body is
  • about half an inch above the navel and straight back
  • near the backbone.
  • At this point there would be equal weight on the
  • left and right side of the fulcrum and thus the
  • body can be balanced as shown in the diagram.
Centre of Gravity
  • The center of the mass of an object.
  • The location around which the mass of an object is balanced.
  • Not necessarily within the mass of the object.
The human body’s centre of gravity can be altered by merely lifting your arms over your head (heightens the C of G).
  • Males and females have different

C of G locations:

    • Adult males - more mass in chest and shoulders  higher C of G (57% of standing height)
    • Adult females - 55% of standing height
centre of gravity and stability
Centre of Gravity and Stability
  • The force of

gravity always acts


  • Stability (or equilibrium)

is attained when the

Centre of Gravity is over the base of


  • Stability = the body’s resistance to movement

Photos: - Grand Sumo Photos Jan 21, 1999


Stability depends on:

a) The height of the C of G - the closer it is to the base of support, the more stable.

eg. more stable -standing or lying?

b) The size of the base of support - the larger the base of support, the more stable

Photos: - Grand Sumo Photos Jan 21, 1999


c) Line of gravity - a body becomes less stable as the line of the C of G moves closer to the edge of the base of support

Photos: - Grand Sumo Photos Jan 21, 1999


d) Mass - generally the larger the mass, the greater the stability IF the force is applied

e) Friction - applies when you move any two surfaces against each other. Friction works in a direction opposite to the direction of motion.Generally, the greater the degree of friction, the greater stability

eg. studs in football boots  friction

 more stability and more effective motion

* too much friction however can lead

to injuries (eg. long studs in dry ground)

  • All bodies will rotate easiest about their Centre of Gravity.
  • If a force is applied on either side of the Centre of Gravity, the object will rotate.
  • This rotational force is called torque.
  • Torque = the product of the amount of force applied

the distance from the centre of gravity at which the force applied.

Practical Application:

The Tackling Process

Coaches often tell their players to tackle a runner low. In this way, the runner's feet will be rotated in the air in the direction of the tackle.


  • Centre of Gravity - the point in a body's

distribution of mass at which all of the

mass can be considered to be concentrated.

  • Torque -a force that tends to produce rotation or



eg. Compare the force required to

push a person off-balance when

they have their arms folded in

front of themselves and when their

arms are outstretched.

  • Because torque is a product, the same torque can be applied to an object at different distances from the center of mass by changing the amount of force applied
  • Less force is required farther out from the Centre of Gravity than closer in.

So, by tackling a

runner low, far from

the centre of gravity,

it takes less force to

tackle him than if he

were tackled high



Furthermore, if a runner is hit

exactly at his centre of gravity,

he will not rotate, but instead

will be driven in the direction

of the tackle eg. the “big hit”

or impact tackle

Photos: Planet-Rugby Photo gallery


Velocity =

the rate at which a

body moves from

one location

to another

  • a bar or rigid structure
  • hinged at one point around which the structure can turn when a force is applied to it
  • the bones act as levers which are pulled and moved by the forces generated by muscles.
  • can be used to enhance either

the force, or the velocity

of movement


Every lever has three components:



Resistance arm

Force arm


  • There are three different types of levers, that vary in the position of the load, fulcrum and appliedforce.
  • The names are:
          • Class I
          • Class II
          • Class III

What would happen if we

moved the axis closer to

a) the load? b) the force?

a) this lengthens the force

arm  increases power

b) this lengthens the

resistance arm increases speed

types of levers
Types of Levers



  • Class I lever
  • axis between force

and resistance

eg. see-saw


Class II lever
  • increases power  slow speed

eg. wheelbarrow




Allows person to lift

heel off ground

Class III lever
  • increases speed and range of motion  little power

eg. catapult





Allows person to lift

a weight with their hand

Levers such as racquets, bats, golf clubs and hockey sticks increase the speed and range of motion of the arm to which they are attached.
  • Why?

Because the end of a lever

moves faster than any other

point on that lever.




d 1

Time (same)



d 2

Time (same)

bernoulli s principle
Bernoulli’s Principle
  • In a moving fluid, an area of high velocity ~ relative low pressure
  • An area of low velocity has a relative high pressure.
  • The further distance the particles have to move the faster they have to move.

An aircraft’s wing is designed to give lift as it moves through the air


Applying this to Sport...

  • Note the angle of the ski-jumper’s body vs that of the aircraft’s wing
  • Spin
  • applying spin to a ball creates a pressure differential
  • Top Spin
  • High velocity on top of the ball, low velocity on the bottom  ball will lift
  • Back Spin is the reverse.

Curving and Swinging

  • The ball can be made to swing by shining one side of the ball or presenting the seam to the wind.
  • This different type of surface on either side of the ball causes a pressure differential in flight.

The ball on top is smooth and has a laminar flow. It has more drag.

This ball has a rough surface (a turbulent flow) and has less drag  travels more easily through the air

Photos: Science of Baseball (


Streamlining & Drag

  • Because water does not flow smoothly around the body, low pressure turbulence forms behind the swimmer - effectively tugging them backwards (drag)
  • streamlining decreases the turbulent flow and  the pressure differential  less drag.

Photos; AIS Swimming homepage

resources and further reading
Resources and Further Reading

- heaps of sport science activities

  • - huge science site with special sport section looking at science of baseball, ice hockey, cycling, skateboarding etc.

- great site which talks about biomechanics and kicking, momentum and impulse when tackling etc.


- the physics and science behind baseball

  • Text:Abernethy et al p.105-180