FW275 Biomechanics

© 2010 McGraw-Hill Higher Education. All rights reserved.
Chapter 6
Biomechanics

Exploring Biomechanics
 Biomechanics: bio means “life”
 Study of the actions of forces
 Term first used 1970’s
 Includes both internal forces produced by
muscles and external forces that act on the body
 Biomechanics human movement is one sub
discipline of exercise science

Exploring Biomechanics
 Biomechanics
 Academic backgrounds: zoology, medicine, engineering,
physical therapy
 Common interest in biomechanical aspects of the structure
and function of living things
 Undergraduate course
 Usually requires prerequisite courses
 Focusing on forces acting on the human body
 Biomechanics commonly a quantitative field of study
 Strong math, computer applications and problem solving

History of Biomechanics
 Aristotle (384-322 B.C.)
 Archimedes (287-212
B.C.)
 Galen (131-201 A.D.)
 Leonardo da Vinci
(1452-1519)
 Galileo (1564-1642)
 Giovanni Alfonso Borelli
(1608-1679)
 Isaac Newton (1643-1727)
 Eadweard Muybridge
(1831-1904)
 Julius Wolff (1836-1902)
 Christian Wilhelm Braune
(1831-1892) & Otto
Fischer (1861-1971)

Basic Biomechanics--Kinematics
 Kinematics
 The form, pattern, or sequencing of movement with
respect to time
 Qualitative
 Involving nonnumeric description of quality
 Quantitative
 Involving the use of numbers
 Kinematic analysis often requires the use of both
visual description and quantitative measurement

 Kinematic quantities: distance, displacement,
speed, velocity, acceleration
 Vector quantities have both size and direction
 Displacement, velocity, acceleration

 Linear displacement
 Change in linear position

 Angular displacement
 Change in the angular position or orientation of a
line segment

 Angular--most human movement requires
angular quantities
 Rotation around a central line or point
 Primarily rotational motion occurs at joints
 Linear velocity
 Rate of change in linear position
 Angular velocity
 Angular displacement divided by the time interval
which displacement occurs

 Acceleration
 Angular acceleration
 Does not take into account particular
circumstances or rules when faced with a moral
problem
 With every moral situation, there is a blank slate
and each case must be viewed independently

 Biomechanics use quantitative kinematics to
research differences between skilled performers
and non-athletes. Examples:
 Runners stride length & frequency
 Swimmer’s stroke rate & length
 Track events velocity of takeoff
 High jumpers vertical displacement
 Basketball angle of projection, angle of release
 Throwing events aerodynamics of flight path
 Swinging (bat, racquet) timing, project speed and
angle

 Skilled performers move body segments at high
rate of angular velocity
 Most biomechanical studies of human kinematics
involve non-athletes:
 Infant through maternity developmental stages
 Adapted P.E. kinematic patterns with motor disorders
 Therapeutic uses to aid in recovery of
injuries/surgeries

 Quantitative kinematic analyses often use:
 High-speed cinematography
 videography
 Reflective markers
 Digitizing

 Kinetics
 Study of the action of forces
 Force
 The product of mass and acceleration
 Push or pull acting on a body
 Vector quantity: magnitude, direction & point
of application to a body
 Free body diagram
 First step used when analyzing actions
 Body, body segment or object of interest
Basic Biomechanics--Kinetics

Force rarely acts in isolation
 Net force
 Overall effect of many forces acting on a body
 Vector sum of all the acting forces
 Net force zero = acting forces balanced in
magnitude and direction
 Net force present = body moves in the direction
of the net force with acceleration

 Torque
 The rotary effect of a force
 Torque quantified
 The product of force (F) and the perpendicular
distance (d┴) from the force’s line of action to the
axis of rotation
 T = F d┴
 The greater the force, the greater the tendency for
rotation

 Inertia--Sir Isaac Newton
 Tendency of a body to resist a change in its
state of motion
 Resistance to acceleration
 Inertia has no units of measurement
 Amount of inertia a body possesses is
directly proportional to its mass
 More massive an object is, the more it tends
to maintain its current state of motion

 1st
law of inertia
 A Body will maintain a state of rest or
constant velocity unless acted on by an
external force that changes the state
 A motionless object will remain motionless
unless there is a net force acting on it

 Inertia is influenced by mass (m) and
distribution of mass in the body
 Mass distribution is characterized by the
radius of gyration (k)

 Newton’s 2nd
law--law of acceleration
 A force applied to a body causes an acceleration of that body of a
magnitude proportional to the force, in the direction of the force, and
inversely proportional to the body’s mass
 The greater the amount of force applied, the
greater the speed
 F=ma
 Angular motion, the law becomes:
 When a torque acts on a body, the resulting angular
acceleration is in the direction of the torque and of a
magnitude inversely proportional to its moment of
inertia

 Newton’s 3rd
law--law of reaction
 For every action, there is an equal and opposite
reaction
 In terms of forces:
 When one body exerts a force on a second, the
second body exerts a reaction force that is equal in
magnitude and opposite in direction on the first body

 Ground reaction forces
 Are measured during gait, each contact the foot
makes with floor is analyzed
 Walking, running patterns throughout development
stages using force platforms
 Magnitude of the vertical component of the GRF
during running is generally two to three times the
runner’s body weight
 Runner’s are classified as rearfoot, midfoot, or
forefoot strikers

 GRF is an external force acting on the human
body
 Magnitude and direction have implications for
performance in many sporting events
 High jumper’s
 Baseball pitching
 Golf

 Friction is a force that acts at the interface of
surfaces in contact in the direction opposite
the direction of motion or impending motion
 Units of force (N)
 The size of the friction force is the product of
the coefficient of friction (µ) and the normal
(perpendicular) reaction force (R).

 Factors influencing the value of friction µ
 Relative roughness and hardness of the surfaces
 Type of molecular interaction between surfaces
 Greater the molecular interaction, the greater
is the value of µ

 Normal reaction force (R)
 Sum of all acting vertical forces (usually body’s
weight)
 Altered magnitude R
 Increased or decreased
 When friction is altered, how does this change
motion?

 Altering the coefficient between two
surfaces can change friction
 Use of gloves, thin wax…
 Important daily influence to prevent
slippage

 Momentum (M) is a vector quantity
 The quantity of motion that an object possesses
 mechanical quantity important in collisions
 Linear momentum
 Product of an object’s mass and its velocity
 Body with zero velocity has no momentum
 What should the momentum be during most
weight training exercises?

 Impulse
 Product of a force and the time interval over which
the force acts
 Impulse = Ft
 Impulse can change momentum
 Impulse and momentum
 Changes in momentum depend not only on the
magnitude of the acting external forces but also on
the length of time over which each force acts
 When impulse acts on system, the result is a change
in the system’s total momentum

 Angular momentum (H)
 Quantity of angular motion possessed by a body, measured as
the product of moment of inertia and angular velocity
 H product of angular inertial property (moment of inertia) and
angular velocity
 Linear momentum
 Product of the linear inertial property (mass) and liner velocity
 Liner motion: M – mv
Angular motion: H = Iw
or: H = mk2

 Factors affect magnitude of a body’s angular
momentum
 Mass (m)
 Distribution of that mass with respect to axis of rotation
 Angular velocity of the body (w)
 If body has no angular velocity; it has no angular
momentum
 Mass or angular velocity increases; angular
momentum increases proportionally

 Factor that most dramatically influences angular
momentum
 Distribution of mass with respect to the axis of rotation
 Why?
 Angular momentum is proportional to the momentum
result from multiplying units of mass, units of length
squared, and units of angular velocity
 Kg ·m2
/s

 Angular impulse
 Change in angular momentum equal to the
product of torque and time interval over which the
torque acts
 Angular impulse generates a change in angular
momentum

Basic Biomechanics--Fluid Mechanics
 Fluid (interchangeably with liquid)
 Is any substance that tends to flow or continuously
deform when acted on by a shear force
 Both gases & liquids are fluids
 Archimedes Principle
 The magnitude of the buoyant force is equal to the
weight of the fluid displaced by the body
 If magnitude of weight is greater than buoyant force,
the body sinks, moving downward in the direction of
the net force

Basic Biomechanics--Fluid Mechanics
 Buoyancy is studied in relation to floatation of
human body in water
 Difference in floatability is function of body
density
 Density of bone and muscle is greater than the
density of fat
 What changes with floatation when a person
holds inspired air in the lungs?

Basic Biomechanics--Fluid
Mechanics
 Orientation of the body as it floats in
water is determined by the relative
position of the totally body center of
gravity relative to the total body center of
volume
 What is center of gravity?

Mechanics
 COG is the point around which the body’s
weight is balanced in all directions
 Center of volume and Center of gravity vary
depending on body shape and size

Mechanics
 Drag
 Generally, a resistance force:
 A force that slows the motion of a body moving
through a fluid
 Theoretical square law
 The magnitude of the drag force increases approx
with the square of velocity
 Effect seen with high velocity sports: cycling, speed
skating, downhill skiing, bobsled, luge
 3 types of drag
 Skin friction/surface drag
 Form drag/profile drag
 Wave drag

Mechanics
 Form drag

Mechanics
 Wave drag

Mechanics
 Force that acts perpendicular to fluid flow is
lift
 Lift can assume any direction (not just
vertically upward)
 Determined by direction of fluid flow and
orientation of body
 Can be generated by foil
 Shape capable of generating lift in the presence of a
fluid flow
 Human hand during swimming

Basic Biomechanics
 The study of biomechanics covers
many other topics that help us to
better understand human
movement in relation to forces
acting on the body, joint
movements and forms of motion

Professional Organizations
 Advantageous for students to consider
student memberships with professional
organizations in their chosen field of sport
science study
 Biomechanics has many supported journals
 Memberships include discounts: journal
subscriptions, conference fees and
opportunities for students to interact with
professionals

Professional Organizations
 American Society of Biomechanics
 Canadian Society of Biomechanics
 European Society of Biomechanics
 International Society of Biomechanics
 International Society of Biomechanics in Sport
 Mulit-disciplinary organizations
 American College of Sports Medicine
 American Alliance for Health, Physical Education,
Recreation and Dance

Careers
 What are some potential career opportunities
in biomechanics?

FW275 Biomechanics

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