Chapter 9
Mechanics of
Biological Materials:
Stresses and Strains
on the body
Loads
 The
external forces that act on the body
impose loads that affect the internal
structures of the body.
Mechanics
 Science
concerned with the effects of
forces acting on objects (body)
 Rigid-body
mechanics
 Deformable body mechanics
 Fluid mechanics
 Relativistic mechanics
 Quantum mechanics
Rigid Body Mechanics
 Acceptable
for analyzing gross
movements
 Assumptions
 body
does not deform by bending,
stretching or compressing
 segments are rigid links joined by frictionless
hinges at joints
Free body diagram
 Free
body diagram - sketch that shows
a defined system in isolation with all
the force vectors acting on the system
 defined
system: the body of interest
 vector: arrow to represent a force
 length:
size of the force
 tip: indicates direction
 location: point of application
Pressure or Mechanical Stress
 Mechanical
stress (pressure) is the
internal force divided by the crosssectional area of the surface on which
the internal force acts.
Pressure (P = F/a)
 Pressure
- is the force per unit area.
 When forces are sustained by the
human body, the smaller the area over
which the force is distributed, the
greater the likelihood of injury.
 Scalpel
vs butter knife example
 Stiletto heel vs moccasin
Pressure or Mechanical Stress
 Force

per unit area.
P = Force / area
 For
a similar force
 increase
area==>
 decrease area==>
 For
a similar area
 increase
force==>
 decrease force==>
Bite Force
 Human
female = 81 lbs
 Human male = 127 lbs
 Humans
have 32 teeth
Bite Force
 Dogs
 Pit
Bull = 235 lbs
 German Shepherd = 238 lbs
 Rottweiller = 328 lbs
 Wild African Dogs = 317 lbs
Bite Force
 Wild
Animals
 African
Lion = 691 lbs
 Great White Shark = 669 lbs
 Hyena = 1000 lbs
 Alligator snapping turtle = 1004 lbs
 16 ft Nile Crocodile = 2500 lbs
Pressure (Comfy & Cozy)
A little uncomfortable
Somewhat painful.
Graphic of stress pattern when walking
http://www.uni-essen.de/%7Eqpd800/anim1.html
Units of Stress or Pressure
Metric
system (SI)
N/m2
Mpa
English
(one Pa (Pascal))
= Mega Pascal (106 Pa)
system
lbs/in2
or psi)
(pounds per square inch
PatelloFemoral Stress
during the squat
Degrees
Knee
Flexion
20
30
50
90
PF Contact
Area
(cm^2)
2.6
3.1
3.9
PF
Force
(N)
238
615
4186
Area: Huberti & Hayes (1984)
PF Force: Escamilla et al, unpublished data
PF
Stress
(Mpa)
0.92
7.00
10.21
Calculations
 What
is the stress on the knee when a
1000N force is exerted over a 4.0 cm2
area?
Knee Stress
 Why
are deep knee bends, squats below
90 degrees, and “duck walks”
contraindicated?
Three principle stresses
Two
are axial
normal
stress or
longitudinal stress
One
is transverse
Mechanical loads on the human body:
 Compression
 Tension
 Shear
Compression
pressing
or
squeezing
force
directed
axially
through
a body
Compressive Loading
Example
Tension
pulling
or
stretching
force
directed
axially
through
a body
Tension
 On
the
rack
Shear
Applied force tends to slide
the molecules across each other.
Shear
 Blow
to
the side
Mechanical loads on the human body:
 Bending
- asymmetric loading
 produces
tension on one side of the
longitudinal axis and compression on the
other side
 Axial
- directed along the longitudinal
axis of a body.
Mechanical loads
 Torsion
 load
producing twisting of a body around
its longitudinal axis.
 Combined

loading
Simultaneous action of more than one of
the pure forms of loading.
Load and Response
Stress
force
per
unit area
Strain
deformation
amount
of
deformation
divided by
original length
Generic Stress-Strain
Relationship
Elastic
Limit
(Yield Point)
Strain (deformation)
Mechanical Strength
 The
strength of a material has to do
with the maximum stress (or strain) the
material is able to withstand before
failure.
Toughness
 Mechanically,
toughness is the ability to
absorb energy and not fail (or before
failure).
Strain
 Strain
is the quantification of the
deformation of a material
Linear Strain
 Occurs
as a result of a change in the
object’s length.
Shear Strain
 Occurs
with a change in orientation of
adjacent molecules as a result of these
molecules slipping past each other.
Instron
Measuring
stress and strain
in biological
materials
Mechanical Properties of the
Musculoskeletal System
 Age
and activity level affect the
mechanical properties of all connective
tissue.
Bone
 Bones
are strongest in compression and
weakest in shear.
Cartilage
Three kinds:
 Hyaline
cartilage (articular cartilage) covers ends of long bones in joints
Cartilage
 Fibrous
cartilage - found within some
joint cavities (the menisci of the knee),
the intervertebral discs, at the edges of
some joint cavities, and at the insertions
of tendons and ligaments into bones.
Cartilage
 Elastic
cartilage - found in the external
ear and tip of the nose.
Cartilage
 Cartilage
is able to withstand
compressive, tensile, and shear loads.
 Articular
cartilage transmits the
compressive loads from bone to bone at
joints
Cartilage
 Articular
cartilage - serves two
purposes:
 Spreads
loads over a wide area so that the
amount of stress at any contact point
between the bones is reduced.
 It allows movement of the articulating
bones at the joint with minimal friction and
wear.
Cartilage
 Function
may include distribution of loads
over the joint surfaces, improvement of the
fit of the articulating surfaces, limitation of
translation or slip of one bone with respect
to another, protection of the periphery of
the articulation, lubrication, and shock
absorption.
Articular Connective
Tissue:
 Tendons
- connect muscles to bones.
 Ligaments - connect bone to bone.
 Both are composed primarily of
collagen and elastin fibers.
 Do not have the ability to contract, but
they are slightly extensible.
Articular Connective
Tissue:
 These
tissues are elastic and will return
to their original length after being
stretched, unless they are stretched
beyond their elastic limits.

Can only be fixed with surgery.
Ligaments and Tendons
 Ligaments,
tendons, and cartilage all
have similarly shaped stress-strain
curves due to their collagenous
composition.
Ligaments and Tendons
 Under
low stresses, these materials are
pliant, but as the stresses increase past a
certain threshold, they become much
stiffer.
Muscle
 The
mechanical properties of muscle
are not as easily examined due to its
contractile ability.
Muscle
 The
ultimate stress of muscle is less that
that of tendon, ligament, or bone,
whereas its failure strain is much
greater.