chapter #11 biomechanical theory &...

Chapter #11

Biomechanical

Theory & Concepts

1

What Is Biomechanics?

© 2015 Thompson Educational Publishing, Inc. 2

Biomechanics is the branch of kinesiology

concerned with understanding the behaviour

and function of the living human body when it

is acted upon by forces.

• Simply put, biomechanics is

the physics underlying

physical movement and

sport.

• Biomechanists play a key role in helping

athletes and others develop proficient

movement patterns— ones that

minimize energy expenditure while

facilitating performance.

Centre of Mass

Located at the balance point of a body; a point found in or about a body where the mass could be concentrated.

Generally, 15 cm above the groin, or approximately 55% of standing height in females and 57% in males.

Depends on the arrangement of the segments

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Centre of mass outside of body

Centre of Mass

4

Males: C of M is

typically found at

57% of standing

height in males.

Females: C of M

is typically found

at 55% of standing

height in females.

Centre of Mass

6

Centre of Mass

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What Is a “Force”?


5

A force is a push or a pull. Because

forces have both magnitude (size) and

direction, they are known as vector

quantities. Force is measured in

newtons (N). (see pg. 321)

• External forces are forces thatoriginate outside the object or bodythat we wish to study (e.g., gravity,wind resistance, or surface friction).

• Internal forces arise within thesystem we are interested instudying—in this case, the human body (e.g., when a musclecontracts, it generates a force thatresults in the movement of the boneto which it is attached).

Newton’s Accomplishments

“I can calculate the motion of heavenly bodies,

but not the madness of people.”

—Isaac Newton, Physicist and Mathematician

• Newton was a key figure in

the Scientific Revolution of

the 17th century—one of

the most influential

scientists of all time.

• Newton made major

contributions to the science

of optics—he built the first

reflecting telescope and

developed a theory of colour.

• He shares credit with

Gottfried Leibniz for the

development of calculus.


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The Meaning of “Laws” in Science


7

A scientific law is a generalized rule that describes a

body of observations.

•Typically, scientific laws take the form of a mathematical

statement—for example, Albert Einstein’s famous E = mc2.

•Scientific laws imply a cause and effect and must always apply

under the same conditions.

• Isaac Newton’s famous three laws of motion “capture the

essence” of motion and allow it to be analyzed mathematically,

over and over again, showing the same results.

•Newton’s laws are the foundation of the scientific field of

mechanics (and, therefore, biomechanics).

Newton’s Three Laws of Motion


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“If I have seen further it is by standing on the

shoulders of giants.”

—Isaac Newton, writing modestly of his

accomplishments in a letter to his fellow

scientist Robert Hooke in February, 1676.

Newton’s three laws of motion together established

the foundation for classical mechanics.

•The laws describe the relationship between an object or a

body and the forces acting upon it, and its motion in

response to these forces.

1. The Law of Inertia


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“A body in motion tends to stay in motion and a body at

rest tends to stay at rest unless acted

upon by an external force.”

Restated: A body or object either remains in a stationary

position or continues to move at a constant velocity,

unless an external force is exerted upon it.

• Inertia is the property of matter that causes an object to

resist any changes in motion.

An object will not change its

state of motion (it will

continue to be at rest or

moving with constant

velocity), unless acted upon

by a net , external force.

For example: because of

their large mass, football

linemen are difficult to move

out of the way .20

2. The Law of Acceleration


“A force applied to an object causes an acceleration of that

object of a magnitude proportional to the force and in the

direction of the force, but inversely proportional to the

object’s mass.” In other words, F = ma

Restated: Newton’s formula (F = ma) describes the relationship

between the force acting on an object (F), its mass (m), and its

acceleration (a).

• For example, as more mass is added to a blocking sled, a football

lineman must generate more force for the sled to accelerate at the

same rate.

• Proper technique and strength allow professional tennis players to

apply more force when they hit the ball, causing the ball to

accelerate faster.

For linear movements, the acceleration (a) a body

experiences is proportional to the force (F) causing it,

and takes place in the same direction as the force

F = m.a where m is the mass of the body

For angular movements, the angular acceleration of a

body is proportional to the moment of force causing it,

and takes place in the same direction as the moment of

force

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23

The greater the force applied to a soccer ball that has the same mass, the greater the ball’s acceleration.

Force, mass, and acceleration

24

As the soccer ball’s mass increases, it experiences less acceleration from a kick of the same force.

Force, mass, and acceleration cont.

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As the mass of the soccer ball is increased, greater force must be generated if the ball is to have the same acceleration

Force, mass, and acceleration cont.

3. The Law of Action-Reaction


“For every action, there is an equal and opposite

reaction.”

Restated: A reaction force arises whenever one body exerts a

force on another. The reaction force is equal and opposite in

magnitude and direction to the applied force.

•When a sprinter responds to the starter’s signal and

pushes against the starting blocks, the blocks generate a

reaction force that is equal and opposite in magnitude and

direction to the force applied by the sprinter.

•Managing ground reaction forces can help a skier

efficiently maintain control during a downhill run.

Every action has an equal and opposite reaction.

The two acting forces are equal in magnitude, but opposite in direction.

Example:-the sprinter exerts a

force onto the blocks, and Simultaneously the blocks exert an equal force back onto the sprinter.

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Examples of the First Law of Motion


The Law of Inertia

•A kettlebell resting on the floor is exerting a force on the floor

and the floor, in turn, is exerting a force on the kettlebell.

The floor exerts an equal and opposite force upward on the

kettlebell. The kettlebell and the floor are in a state of

equilibrium and no motion is observed.

•Whether an object is in a state of rest (the kettlebell) or in a

state of motion (e.g., a curling rock), the object will remain

in that state unless it is acted upon by an external force.

Examples of the Second Law of Motion


The Law of Acceleration (F = ma)

•Proper technique and strength allow professional athletes such

as tennis players, golfers, and baseball players to apply more

force when they swing their striking implements and hit the ball,

thus causing the ball to accelerate faster.

•Suppose a basketball has four times more mass than a

baseball. What happens if a force of

10 N is applied separately to each object? The acceleration of

the basketball and the baseball will differ because the two

objects differ in mass. Because the basketball has four times

more mass than the baseball, its acceleration will be one-

quarter that of the baseball.

Examples of the Third Law of Motion


The Law of Action-Reaction

•Newton’s third law of motion applies, for example, when a

basketball player jumps to make a slam dunk. The action of

pushing against the court floor leads to a reaction force—that

is, the floor pushing back—and, ultimately, the athlete’s body

leaves

the ground.

•When a volleyball player bends her knees as she prepares

to push against the floor and jump up to block a hit from an

opposing player, the floor reacts or pushes back, and the

player rises into the air.

What Are Levers?


Levers are simple machines. A machine is a device,

consisting of fixed yet interrelated parts, that is capable of

altering the direction

and magnitude of a force. Levers perform one or more of the

following functions:

•Balance two or more forces,

•Provide a force advantage, whereby less effort force is

required to overcome a greater resistance force, or

•Provide an advantage in speed of movement, whereby

the load to be moved moves farther and faster than the

effort force.

Class 1 Lever

Class 2 Lever

Class 3 Lever

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Levers in the Human Body


The machine-like configurations of the

bone- joint-muscle arrangements in our

bodies are essentially levers.

•Knowledge of these arrangements

can form the basis for developing

musculoskeletal training and

conditioning programs, as well as

rehabilitative exercise regimens.

•Knowledge of these arrangements

can also be used to re-design the

physical world in which we live and

work in order to make our movements

safer and more efficient.

Classifying Levers

There are three classes of levers. These classes of levers are defined

according to the relative positioning of the following components:

• The fulcrum or joint (the axis of rotation),

• The effort (point of application of the force), and

• The load (the mass of the object, body, or part being moved;

also known as the resistance).


Lever Configurations in the Human Body


In the case of levers in the human body, the lever

components consist of:

•The force applied through muscle contraction (the pull

on the movable bone at the attachment site) is the

“effort.”

•The joint where the bones come together is the axis

or “fulcrum.”

•The mass to be moved by the muscle is the “load.”

The Class 1 Lever


A Class 1 lever is one in which the fulcrum (the axis of

rotation) is located between the point of application of

the force (the effort) and the resistance (the load) being

moved.

Everyday examples of first class levers include:

•See-saws

•Crowbars

•Pliers

•Scissors (two first-class levers joined together)

•A hammer pulling out a nail

A Class 1 Lever in the Human Body

• An example of a Class 1 lever

in the human body is the neck

as it shifts from a position of

flexion to a position of

extension.

• The contraction of the

trapezius muscle (effort)

permits extension of the head

(resistance). The spine is the

fulcrum upon which the neck

muscles lift the head.

• A Class 1 lever is the most

versatile of all levers. It can

afford a speed and/or force

advantage.


The Class 2 Lever


A Class 2 lever is one in which the resistance, (the load) is

positioned between the point of application of the force (the

effort) and the axis of rotation (the fulcrum).

Everyday examples of second class levers include:

•Wheelbarrows

•Staplers

•Doors

•Can openers


•An example of a Class 2 lever in the

human body is the ankle joint (the

fulcrum) in combination with

contraction of the gastrocnemius

muscle (the effort).

•This lever mechanism is capable of

moving almost the entire weight of an

individual (load) during plantarflexion

(standing on one’s toes).

•A Class 2 lever affords a force

advantage: a

relatively small effort can lift a large

load.© 2015 Thompson Educational Publishing, Inc. 39

A Class 3 Lever


A Class 3 lever is one in which the point of

application of the force (the effort) is located between

the fulcrum (the axis of rotation) and the resistance

(the load).

Everyday examples of third class levers include:

•Brooms

•Rakes

•Fishing rods

•Baseball bats


• An example of a Class 3 lever in the

human body is a person performing a

biceps curl. The biceps muscle (effort)

inserts on the radius (at the end of which

is the load itself) in combination with the

elbow joint (fulcrum).

• This is the most common type of lever

found within the human body.

• A Class 3 lever provides a speed

advantage, allowing relatively light

resistance loads to be moved through a

greater range of motion.


What class lever is this?

1

Types of Motion


Physicists generally describe motion as

falling into one of two categories:

•Linear (or translational) motion, and

•Angular (or rotational) motion

This distinction allows scientists, including

biomechanists, to apply the laws of

physics and to use mathematical analysis

to understand and predict outcomes

involving the movement of bodies or parts

of bodies.

Linear (or Translational) Motion


•This type of motion is also

commonly known as

rectilinear or straightline

motion.

•Linear motion takes place

when a body or its collective

parts moves the same

distance, in the same

direction, in the same

amount of time.

Angular (or Rotational) Motion


•Angular (or rotational)

motion refers to movement

of a body about an axis.

•In contrast to linear

motion, in rotational

motion the force does not

act through the centre of

an object or body but

rather is “off-centre,” and

this results in rotation.

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Rotational Motion (Angular)

The Complexities of Human Movement


Biomechanists apply the laws of physics and an understanding of

types of motion in analyzing human movement.

•Motion in nature tends to be more or less either linear or

angular, so it is generally relatively easy to distinguish

between the two types of motion.

A ball will tend generally to continue to roll in a straight line,

other things being equal; and a wheel will tend to continue to

rotate, for example. Such movements can be analyzed

relatively easily.

•Most human movements are a complex combination of

linear and angular motion and they tend to undergo frequent

change. Thus, distinguishing between linear and angular

motion in the context of human movement is not always

completely straightforward.

Linear Motion—A Simple Example

• Consider a hockey puck. A hockey player uses a stick to apply a pushing force

to the puck.

• The state of equilibrium previously experienced by the puck is disrupted, and the

puck moves in the direction in which the force has been applied.

• The puck will move in a straight line provided that it neither spins nor encounters

any ice chips, bumps, or cracks in the ice along its path.

• Physicists can apply mathematical analysis to predict the outcome of the

applied force here.


Resultant Forces

• Typically, the linear motion of humans is a result

of the interaction of a combination of forces.

• In this photo, the

two main forces

are the ground

reaction force

exerted upward on

the player and the

player’s forward

force.

• Using trigonometry,

biomechanists

can compute the

resultant force

that causes the

player’s linear

motion.


Angular Motion: Turning on an Axis


Angular motion involves a rotation, or a turning, about an

axis.

•When a basketball player steps out to dribble or

pass, the player’s motion is linear. If the player,

however, decides to pass the ball or shoot it into the

basket, he or she often pivots on one foot, being

careful to keep the pivot foot in place.

•Pivoting is an example of angular (or rotational)

motion.

Human Movement and Rotational Motion


Human physical activities can involve rotational or angular

motion in three different ways:

•Rotations of projectiles or other objects (e.g., the swing of

a hockey stick)

•Rotations of the entire human body about one of three

axes (e.g., the tumble of a gymnast)

•Rotations of individual body segments (e.g, the throwing

motion of a softball pitcher’s arm)

In each case, the rotating object or body has been acted

upon by a force that is off-centre, causing the object to rotate

about an axis.

What Determines Which Type of

Motion Occurs?


The answer lies in the point at which the force is applied to an

object or body with respect to the centre of that object or body.

•A centric force is a force applied directly through the

centre of an object or body and it results in linear motion

only: the object or body will move linearly in the direction

of the applied force.

•An eccentric (off-centre) force is a force directed through

a point other than the centre of the object or body and it

always results in rotational motion (and sometimes linear

motion, too).

Linear motion results when the forces are applied through the centre of mass

Angular motion results when the forces are applied away from the centre of mass

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What type of motion is being caused?

54

Two Types of Motion Combined

Human movement is

typically a combination

of linear and rotational

motion.

•For example, a sprinter

dashing down the track

demonstrates linear

motion, but a great deal

of the linear movement

results from the forces

produced by the

rotational movement

of the legs and arms.


The Power of Angular Motion

•Tremendous rotational forces generated during a

hammer throw permit the object to travel a great

distance. (If the athlete simply threw the hammer,

the action would be called a shot put!)


Torque, or Turning Effect


The rotary or turning effect that is created by an

eccentric force is known as torque (also referred

to as “moment of force”).

•Torque is the turning (or twisting) effect

produced when a force is applied to a body at

some distance from an axis of rotation.

•You will learn more about torque and

rotational motion in Chapter 12.

Moment of force (torque)• The moment of force or

torque is defined as the

application of a force at

a perpendicular

distance to a joint or

point of rotation.

• The turning force

responsible for rotation

• Eg. Turn a page in a

book-it is lifted at the

edge away from the

binding.

58

Torque

Moment of force (torque)

• Off-axis forces-eccentric force and the turning effect it has on the body is called torque.

• The off-axis distance to the forces line of action is called the force arm.

• The greater this distance, the greater the torque produced when they pull on bones, and the result is rotary motion of the body segments.

• The stronger contraction of a muscle, the greater the torque.

• The greater the FA, the greater the torque.

C of MAxis

FA

Applying Biomechanics


Describing and predicting the motion of the human

body has many important applications.

•After World War II, for example, scientists at the

University of California began doing research with

wartime amputees to improve the design of lower

limb prosthetic devices.

•To build better prostheses, they needed to

understand how an average person walks with a

mature gait.

•Since then, the reach of biomechanics has

extended to many fields.

Practical Applications for Biomechanics


Some professions where a knowledge of

biomechanics has proved useful include:

•Sport coaches

•Health-care professionals,

•Product design engineers,

•Physiotherapists and other injury prevention and

recovery specialists

•Sport and para-sport equipment design

Careers Requiring a Knowlege of

Biomechanics


• Physical education teachers

• Sport coaches

• Fitness instructors and personal trainers

• Strength and conditioning specialists

• Kinesiologists

• Physiotherapists

• Athletic and occupational therapists

• Rehabilitation medical professionals

• Sport physicians and chiropractors

• Prosthetists

Workplace Design


Ergonomics is the applied science of equipment and workplace

design. The aim of ergonomics, in large part, is to maximize workers’

productivity by reducing fatigue, discomfort, and pain.

• Ergonomists match human anatomical, physiological, and

biomechanical characteristics to specific activities.

• They help design workstations and furniture that conform to

human movements.

Orthotic Devices

Orthotic devices are another widely used

product of biomechanical research.

• A subtle alteration in a movement pattern for

whatever reason can lead to a chronic injury.

• Biomechanists have

designed in-shoe orthotic

devices that help

alleviate the pain caused

by chronic conditions of

the knee, foot, and ankle.

• Biomechanists have

also made important

contributions to the

design of functional

footware and braces for

athletes and non-athletes.


Sports Injuries—Prevention and Recovery


Coaches and trainers with a knowledge of

biomechanics can help athletes avoid injury-

induced disruptions in movement patterns that

compromise skill performance.

• For example, acute (or sudden onset)

injuries such as a sprained ankle or turf toe

(sprained ligaments at the base of the big

toe) can hinder a soccer player’s

performance.

• Such injuries restrict the range of motion

of a joint and lead to a noticeable change

in the way a person walks or runs until,

given time and rehabilitation, the injury

heals and a normal range of motion

returns.

Parathletes and Biomechanical

Innovation

The equipment that parathletes use to

assist them in their chosen sport has

improved significantly, in large part owing to

biomechanical research into how the human

body performs.

Major breakthroughs

have occurred in:

• Prosthetics

• Wheelchairs

• Throwing Frames

• Archery


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