Principles of
Biomechanics
Reaction
Action
Lesson Aim:
• To critically examine the
biomechanical principles which are
important in Physical activity & Sport.
• To be able to state why these
principles are important to consider
in terms of athletics.
Outline:
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Newton’s Laws
Balance and Stability
Levers
Force Summation
Projectile Motion
Newton’s First Law of Motion
• every body continues in it state of rest or
motion in a straight line unless
compelled to change that state by
external forces exerted on it.
(conservation of momentum)
This external force
will act to change the
vertical motion, but
not the horizontal
Newton’s Second Law of Motion
• the rate of change of momentum
(acceleration) of a object is
proportional to the force causing it
and the change takes place in the
direction in which the force acts.
Newton’s Third Law of Motion
• 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 (action reaction)
–
Balance
Balance C-o-M
• The point about which the body’s
mass is evenly distributed is referred
to as the Centre of Mass (CoM)
• This is also sometimes referred to as
the Centre of Gravity, but this
actually refers only to the vertical
direction.
Balance C-o-M
• The location of the
CoM depends on the
location of the body
segments:
– Raise your arms above
your head and your
CoM rises within your
body
– Lift one arm and one
leg out to the side,
and the CoM will move
towards that side
Balance C-o-M
• Understand the effect of
positioning/moving limb segments while in
the air…
– if both of these people are the same height,
have the same build and jump the same height,
which one is going to hit the volleyball at the
greatest height? Why?
Balance Base of support
• The area that a body is in contact
with a solid surface
• Base of support can be between to or
more points.
Balance unbalanced
• A body becomes unbalanced when the
centre of mass moves outside the
base of support.
• sometimes it is more valuable in
athletics to be off balance, when
why?
LEVERS
Levers
• “a bar, hinged/pivoted at one point, to which
forces are applied at two other points”
– Fulcrum/axis
– Weight/resistance
– Force
3rd
1st
2nd
F
F
F
R
R (BW)
R
Levers – functional aspects
Increase
Speed
Increase
Force
First Class
F
R
F
R
F
R
F
Second Class
R
F
Third Class
R
Force arm <
Force arm =
Force arm >
Resistance arm Resistance arm Resistance arm
Most levers in the body are 3rd class
M
R
3kg
• …… we’re built for speed, not force!
4kg
Force Summation
Inter-segment Sequencing
• when the second segment reaches its
maximum angular velocity (zero
acceleration) during the midrange of its
motion, it too decelerates and transfers
angular momentum to the next adjacent
distal segment in the chain.
• such a sequence occurs, link by link, from
large to small, from base to end (proximal
to distal) until the end of the chain is
reached.
Force Summation
W
H
FA
E
rUA
S
UA
VS
angular
velocity
Force Summation
time
Projectile Motion
Projectiles
We typically look at
Release height
Release angle
Release speed
to evaluate projectile motion
What effect does air
resistance have on flight?
What are the
characteristics of a high
jumper as a projectile?
What effect does varying
the height, angle or speed
of release have on the
distance achieved?
• Release angle: there is an optimum angle
depending on the release height:
Release ht = landing ht:  = 45
Release ht > landing ht:  < 45
Release ht < landing ht:  > 45
• Release speed:
an increase in speed = increased range
• Release height:
increased height = increased range
Magnus Effect
• if a ball spins in flight, it will drag some of the air
close to the surface with it. This creates an area
of high pressure and an area of low pressure on
opposite sides of the ball
• this pressure imbalance will make the ball curve in
flight
Ignoring air resistance, the flight of a
projectile will take a parabolic path
• Horizontal velocity will be constant
• Vertical velocity will change by -9.8 m/s/s