BIOMECHANICS
PSE4U
BIOMECHANICS
 Biomechanics:
how physical forces affects
human performance
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Any discussion of biomechanics must begin with a tribute to
Sir Isaac Newton and his three “Laws of motion”!
Newton’s theories (and biomechanics) rests on two
assumptions: physical equilibrium and the conservation of
energy.
Equilibrium: when more than one force acts on a body but
the sum of the forces is zero, no change in velocity results.
Conservation of energy: energy can never be created or
destroyed, but can only be converted from one for to another.
NEWTON’S 3 LAWS
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OF
MOTION
1. The Law of Inertia: An object in motion will remain in that state
of motion unless acted on by another force
Ex. An athlete who is stationary will stay in that state until moved by his
own muscles or another player
 Ex. A skier going down a hill will need something to act on her to slow her
down
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2. The Law of Acceleration (F=ma): A force applied to an object will
cause it to accelerate in proportion to the force applied and
inversely to the mass of the object being moved.
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Example: Force applied to a baseball being thrown vs. a shot put
3. The Law of Reaction: For every action there is an equal and
opposite reaction. When one object exerts a force on a second
object it will move in the opposite direction of the applied force.
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Example: A basketball player jumping. Pushes of the floor in a downward
motion and moves inversely in an upward motion.
NEWTON’S 3 LAWS
OF
MOTION
NEWTON’S 3 LAWS
OF
MOTION
TYPES OF MOTION
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LINEAR MOTION: force is generated by the athlete’s muscles
and the resulting motion is in a straight line.
Motion usually involves acceleration
 Examples: Sprinter accelerates down a track or hockey player
quickly veers to go around another player.
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Because human movements are not in a straight line, we need to
introduce the notion of a force as a vector – in a particular
direction.
Take the example of a wide receiver running down the field. The
forward movement of the wide receiver is a combination of a
vertical force and a forward force, resulting in a vector force
somewhere in between.
TYPES OF MOTION
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ROTATIONAL MOTION: the force does not act through the
center of mass, but rather is “off-center” which results in
movement about an axis.
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Angular Acceleration – acceleration around an axis (ex. A figure
skater will increase angular velocity if they bring their arms closer
to their body.)
Moment of Inertia – Resistance to rotation. The larger the moment
of inertia the larger the moment of force needed to maintain the
same angular acceleration.
EG. After leaving the high diving board, the diver curls tightly
and then opens up just before entering the water. By opening
up before entry, the diver increases the moment of inertia
there by slowing down the angular velocity.
LEVER SYSTEM
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Every movable bone in the human body, acting alone or with
others, is part of a lever system that facilitates movement.
The HUMAN MUSCLE MACHINE: 3 classes of levers based on
the location of the fulcrum in relation to the force.
LEVER SYSTEM
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Class I Lever: Ex. Teeter-toter
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Class II Lever: Ex. Wheelbarrow
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The fulcrum (axis) is located between the force (effort) and
the resistance (load)
The resistance is between the force and the fulcrum.
Class III Lever: Ex. Snow Shoveling
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The force is between the fulcrum and resistance.
PRINCIPLES OF BIOMECHANICS
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There are seven main principles of biomechanics that can be
broken into four broader categories.
CATEGORY 1: STABILITY
Principle 1: The lower the centre of mass, the larger the base of
support, the closer the centre of mass to the base of support, and
the great the mass, the more stability increases.
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This principle has 4 sub-components
Height of the centre of mass: the point around which a persons mass is
concentrated
 Line of gravity: an imaginary line that passes down through the centre of mass
to ground
 Base of support: area between supporting limbs
 Mass: a measure of resistance to linear motion
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Ex. Sumo wrestling or football linemen
CATEGORY 2: MAXIMUM EFFORT
 Principle
2: The production of maximum effort requires
the use of all possible joint movements that contribute to
the objective. The more joints used, the more muscles will
be contracted.
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Ex. Golf swing – need to rely on more than just your arms to
swing
CATEGORY 2: MAXIMUM EFFORT
 Principle
3: The production of maximum velocity
requires the use of joints in order – from largest to
smallest. The force builds as each successive joint is put
into motion
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Ex. Slap shot – reaching maximum velocity of movement at the exact
moment of impact with the puck
CATEGORY 3: LINEAR MOTION
(MOVEMENT
IN A STRAIGHT LINE)
 Principle 4: The greater the applied impulse, the greater the increase in
velocity (also known as the principle of impulse)
 The more force needed, the longer range of motion the joints will go through.
 Can also work in reverse. To absorb force, the athlete uses a larger range of
motion to control an object.
 EX. Basketball dunk – bend your knees to get height or soccer player stopping a
ball with his foot.
CATEGORY 3: LINEAR MOTION
IN A STRAIGHT LINE)
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(MOVEMENT
Principle 5: Movement generally occurs in the direction opposite
that of the applied force (Newton’s third law).
Every action has an equal an opposite reaction.
When an athlete exerts a force, the surface pushes back with the
same force
Ex. High jump, swimming full body suits, drafting behind a cyclist
CATEGORY 4: ANGULAR MOTION
(A CIRCULAR
MOTION THAT OCCURS AROUND AN IMAGINARY LINE CALLED AN AXIS.
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Principle 6: Angular motion is produced by the application of a force acting at
some distance from an axis, that is, by torque.
Three types of torque:
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Entire body by an off center force (a force that does not go through the center
of mass) – Ex. Hockey, rugby pushing someone so they lose their balance
Body segment rotating around an axis – Ex. Pitch in baseball
Imparting a spin on a projectile – Ex. Curve ball
CATEGORY 4: ANGULAR MOTION
(A CIRCULAR
MOTION THAT OCCURS AROUND AN IMAGINARY LINE CALLED AN AXIS.
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Principle 7: Angular momentum is constant when an athlete or object
is free in the air
Ex. Diver – angular motion will remain constant if the body position is
constant. However, when the diver enters the water they open up increasing
their moment of inertia and therefore decreasing their angular
motion/velocity (tighter = increase angular velocity, looser= decrease in
angular velocity)