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Chapter #12

© 2015 Thompson Educational Publishing, Inc. 1

Static Systems


The seven principles of biomechanics are best understood

in the context of static and dynamic systems.

Statics is the branch of mechanics that deals with objects

or bodies in a state of constant, unchanging motion.

• In static systems, the rate of change of motion of an object or

body is unchanging over time (e.g., a gymnast holding a

stationary pose on a balance beam or a high diver in free

fall).

• If an external force is applied to a body and the rate at

which the body is moving changes, the system is now said

to be dynamic in nature.

Dynamic Systems


The branch of mechanics that studies changes in the

motion of objects or bodies as a result of the actions of

forces is known as dynamics.

•A dynamic system is one that experiences a change in the

rate at which it is moving as a result of forces applied to it

(e.g., a rugby player weaving his or her way down the field).

•Changes in our movement patterns are the product of

multiple internal as well as external forces.

•The seven biomechanical principles involve the

interactions of static and dynamic systems.

The Seven Principles


•1: Stability

•2: Production of Maximum Force

•3: Production of Maximum Velocity

•4: Impulse-Momentum Relationship

•5: Direction of Application of the Applied Force

•6: Production of Angular Motion (Torque)

•7: Conservation of Angular Momentum

Grouping the Principles


These seven principles can be grouped into four

broad categories (for easy recall) as follows:

•Stability (Principle 1)

•Maximum Effort (Principles 2 and 3)

•Linear Motion (Principles 4 and 5)

•Angular Motion (Principles 6 and 7)

Overview Chart


THE SEVEN PRINCIPLES OF BIOMECHANICS

Concept Biomechanical Principle

Stability 1. Stability

Maximum

Effort

2. Production of Maximum Force

3. Production of Maxiumum Velocity

Linear

Motion

4.Impulse-Momentum Relationship

5.Direction of Application of the Applied

Force

Angular

Motion

6. Production of Angular Motion (Torque)

7. Conservation of Angular Momentum

Biomechanical Principle 1


Stability“The greater the mass, the lower the centre of mass to the

base of support, the larger the base of support, and the

closer the centre of mass

is positioned to the base of support, the more stability

increases.”

Key Concepts


•Stability. The quality, state, or degree of being stable and

capable of resisting a change in motion.

•Balance. An even distribution of mass enabling someone or

something to remain steady.

•Mass. The quantity of matter contained within an object or

body.

•Centre of mass. The imaginary middle point around which

the mass of an object or person is balanced.

•Base of support. The supporting area beneath an object or

body; its limits are defined by the points of contact with the

supporting surface.

The Concept of “Mass”

Because football linemen have large mass, and therefore more

inertia, it is more difficult for their opponents to push or pull

them out of position (i.e, destabilize them).


The Concept of “Centre of Mass”

The centre of mass is the imaginary

point around which an individual’s

or object’s mass is

concentrated.

•When an individual stands upright,

with their arms hanging at their sides,

the centre of mass is located in the

middle of the body at about the level

of the navel.

•The concept of the centre of mass is

important in the context of resistance

to rotation.


The Concept of the “Base of Support”

In a sports context, the base of support refers to the

supporting area beneath the limbs of an athlete.

•The stability of gymnasts is

enhanced when they

broaden their base of

support.

•Likewise, the stability of a

football lineman is enhanced

when he broadens his base

of support.


Intentional Instability

In some sports, an athlete intentionally puts himself or

herself in an unstable or potentially unstable position.

• The narrow base of

support in combination with

the high centre of mass of

the

football player in front tells us

that this is not a very stable

situation at all.

• A slight shift in the person’s

centre of mass will lead to

complete instability and likely

a fall—which is what the

player intends to happen.


Principles Related to Maximum Effort


In many activities, we must exert maximum effort, or “go

all out” in order to accomplish a specific task. Two

biomechanical principles are related to maximum effort:

•Principle 2

“The Production of Maximum Force,” and

•Principle 3

“The Production of Maximum Velocity by

Sequencing of Joint Rotation”

Principle 2


THE PRODUCTION OF MAXIMUM FORCE

“The production of maximum force requires the use of all possible joint movements that

contribute to the task’s objective.”

Interpreting Principle 2


When people lift heavy objects, or perform other such tasks,

they must make slow, controlled, and simultaneous high-intensity

movements.

•These movements are best produced by

sequenced joint rotations.

• If the full joint range of motion (ROM) is restricted at any

one of the joints involved in the movement, perhaps due to

injury or disease (e.g., arthritis), fewer muscles are able to

contribute to the movement and therefore less force is

produced.

Example of Principle 2 in Action


When we attempt to run as fast as we can, we are

demonstrating biomechanical principle 2.

•When we run, we necessarily rely on joint rotation at

the ankle, knee, and hip joints.

•Full rotation at each joint is achieved through the

contraction of multiple muscles.

•These movements begin at the ankles and are

followed by similar sequenced joint rotations at the

knees and hips.



Another example of biomechanical principle 2 is the

awkward action of a four-year-old

T.ball player’s swing of a bat (with that of a professional

baseball player).

•A young T-ball player will often stand very upright, with

feet planted, and swing the bat using only the arms to

make contact with the ball.

•The player has the potential to use more joints during the

swing, but may not do so due to lack of experience.

•Over time, with practice and good coaching, the T-baller

will become more proficient at this task.



Professional baseball player hitting a ball also demonstrate

principle 2 in action.

•They typically flex their knees and hips while waiting for the

ball to be delivered by the pitcher.

•As the ball is released by the pitcher, the batter will step

toward the oncoming ball, extending previously flexed hip

and knee joints, while rotating the hips (the core/trunk

remains stiff).

•At the same time, the batter swings the bat fully using the

shoulders, arms, and wrists.

Principle 3


PRODUCTION OF MAXIMUM VELOCITY

“The production of maximum velocity

requires the use of joints in order—from

largest to smallest.”


Activities requiring the

production of maximum

velocity (e.g., tennis serve,

golfing, or pitching a baseball)

are performed most

successfully if the larger,

slower joints begin the

movement, and the smaller

joints come into action later.




When a baseball is thrown, the player’s joint actions are

sequenced.

• Joint movement in the legs is followed closely by rotation of

the hips.

•Rotation of the hips is followed by rotations of the arms, the

elbows, and the wrists.

•By engaging more muscles and joints in a pitching motion,

and sequencing them correctly, a professional baseball

pitcher is able to generate maximum velocity. (See next slide.)



Sequencing of joint rotation is particularly important when

performing activities in which an object is being thrown or being

struck by an implement.

•For example, a fly fisher can cast her line more

effectively by using sequenced joint rotations.

•She first rotates at the trunk, followed by the shoulder,

then the elbow, and finally the wrist.

• If her movements are sequenced correctly, the fly fisher

will be able to cast her line with the attached fly a fair

distance downstream.


A proficient golfer relies on

biomechanical principle 3.

•An experienced golfer

performs a precisely

sequenced swing.

•Leg, hip, and arm action

are sequenced to produce

a slower, more controlled

swing of the club.


Principles Related to Linear Motion


Two biomechanical principles are related to linear (or

translational) motion:

•Principle 4

“The Impulse-Momentum Relationship,” and

•Principle 5

“The Direction in Which Movement Usually

Occurs”

Principle 4


THE IMPULSE-MOMENTUM RELATIONSHIP

“The greater the applied impulse, the greater

the increase in velocity.”



When an object such as a cricket ball, field hockey ball, or

tennis ball is in motion, it is said to have momentum. The

momentum of the ball or any other object in motion is equal to

its mass multiplied by its velocity.

•To get a ball moving, a cricket, field hockey, or tennis player

will use a striking implement to apply a pushing force to the

ball over a period of time.

•The greater the pushing force, and the greater the amount

of time over which it is applied to the ball, the greater the

impulse. This is a restatement of biomechanical principle

4.


Imparting High Velocity

to a Cricket Ball



Elite athletes and their coaches often rely on biomechanical

principle 4 to improve their techniques and performance.

•For example, today’s high jumpers commonly use a technique

called the Fosbury Flop.

•As jumpers near the bar, they arch their neck and back and

push against the ground to create a powerful impulse force.

•An equal and opposite ground reaction force is generated,

which propels the high jumper into the air.



The “jump serve” in volleyball provides another good

example of biomechanical principle 4.

•Players begin well back behind the service line, lob the ball

forward, and run and jump into the air in order to “spike” the

ball to the opposing team.

•The forward running motion of the server’s body transfers

momentum to the ball, making it move through the air at a

high velocity.

•This increase in velocity, combined with a high flight path,

makes it difficult for the ball to be returned by the opposing

team.


Applying Impulse in a Bobsled Race

Principle 5


THE DIRECTION OF APPLICATION OF THE APPLIED FORCE

“Movement usually occurs in the

direction opposite that of the applied

force.”



The fifth biomechanical principle is closely related to

Newton’s third law of motion, which states that for every

action there is an equal and opposite reaction.

•People at work and at play rely on this principle

constantly.

•For example, when a person sitting in an armchair

stands up, the individual will place his or her hands on

the armrests and push down. A reaction force that is

equal in magnitude but opposite in direction will be

generated by the chair arms.

Principle 5 and Aquatic Events


Biomechanical principle 5 is evident in many aquatic events.

•When completing a length of a pool, for example, free-style

swimmers turn and push against the wall of the pool with

their legs.

•The swimmers’ bodies are propelled forward—in the

direction opposite that of the applied force.



Biomechanical principle 5 can be seen in action in many team

sports.

• In making a cut, for example, an ultimate player or a soccer

player will push his or her foot against the ground to make a

change in direction away from an opponent.

•Similarly, an ice hockey player will push off using the edge of

the skate blade to make the same type of movement to either

avoid a hit or make one.

Principles Related to Angular Motion


Two biomechanical principles are related to angular (or

rotational) motion:

• Principle 6

“Production of Angular Motion (Torque),” and

• Principle 7

“The Conservation of Angular Momentum”

Principle 6


PRODUCTION OF ANGULAR MOTION (TORQUE)

“Angular motion is produced by the applicationof a force acting at some distance from an axis;

that is, by torque.”



If an eccentric (or “off-centre”) force is applied to a body,

the force tends to make the body rotate about its axis.

•This “turning effect” is known as torque.

•The magnitude (size) of the torque depends on

three factors.

Factors That Affect the Amount of Torque

The amount of toque that is generated is

affected by three factors:

•The applied force,

•The length of the lever arm, and

•The angle of application of

the force, as shown in the

diagram.




In the generation of torque, the length of the lever arm and the

angle at which the force is applied are very important.

•As might be expected, it is easiest to initiate rotation when the

force is applied as far away as possible from the axis.

• It is also easiest to initiate rotation when the force is applied

perpendicularly to the lever arm (e.g., when unfastening a bolt

with a socket wrench).

Generation of Torque at Human Joints


Principle 7


THE CONSERVATION OF ANGULAR MOMENTUM

“Angular momentum is constant when an individual or object is free in the air.”

(Angular momentum is the quantity of motion contained within an object or a body.)



Many physical activities—trampoline, gymnastics, tumbling,

aerial skiing, aerial snowboarding, and diving—require

individuals to be airborne and in a state of free fall.

•Angular momentum is the product of the rate at which

the athlete is rotating—or her angular

velocity—and the extent to which her body resists angular

motion.

•This resistance to angular motion is known as the “moment

of inertia.”

•The farther a body’s distribution of mass from the axis of

rotation, the greater is the body’s moment of inertia.

Adjusting the Moment of Inertia


Successful rotational maneuvers in many sports involve

adjusting (i.e., either minimizing or increasing) the moment of

inertia.

•The moment of inertia is minimized, for example, when a

trampolinist’s arms and legs are brought close to the

athlete’s axis of rotation in what is commonly referred to

as a tuck position.

• In this position, the trampolinist rotates rapidly.

•To slow the rate of rotation, she simply needs to extend

her arms and legs away from her axis of rotation.

• In other words, she can adjust her moment of inertia

by controlling how far her mass is distributed from

her axis of rotation.

Examples of Principle 7 in Action


A high diver and a figure skater can adjust her moment of inertia

by controlling how far her mass is distributed from her axis of

rotation.

•By pulling her arms and legs close to her body, the diver can

decrease her moment of inertia.

•As the moment of inertia changes, angular velocity also

changes—by speeding up.

•As the diver approaches the water, she straightens out, which

reduces the rate of rotation just before entry into the water.

•The same angular forces are at play in the case of a figure

skater’s spin.

The Law of Conservation of AngularMomentum


The principle underlying the angular forces at play in a diver’s rotation or a figure skater’s spin is known as thelaw of conservation of angular momentum. This law states:

“THE TOTAL ANGULAR MOMENTUM OF A

ROTATING BODY REMAINS CONSTANT IF THE

NET TORQUE ACTING ON IT IS ZERO.”

•A rigid spinning object continues to spin at a constant rateand with a fixed orientation unless influenced by theapplication of an external torque.

Example of the Law of Conservation of Angular Momentum


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