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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 )

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Chapter 9

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chapter 9
Chapter 9

Mechanics of

Biological Materials:

Stresses and Strains

on the body

  • The external forces that act on the body impose loads that affect the internal structures of the body.
  • 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
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
  • 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
Pressure or Mechanical Stress
  • Mechanical stress (pressure) is the internal force divided by the cross-sectional area of the surface on which the internal force acts.
pressure p f a
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 stress1
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
Bite Force
  • Human female = 81 lbs
  • Human male = 127 lbs
  • Humans have 32 teeth
bite force1
Bite Force
  • Dogs
    • Pit Bull = 235 lbs
    • German Shepherd = 238 lbs
    • Rottweiller = 328 lbs
    • Wild African Dogs = 317 lbs
bite force2
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

Graphic of stress pattern when walking

units of stress or pressure
Units of Stress or Pressure
  • Metric system (SI)
    • N/m2 (one Pa (Pascal))
      • Mpa = Mega Pascal (106 Pa)
  • English system
    • lbs/in2 (pounds per square inch or psi)
patellofemoral stress during the squat
PatelloFemoral Stress during the squat

Area: Huberti & Hayes (1984)

PF Force: Escamilla et al, unpublished data

  • What is the stress on the knee when a 1000N force is exerted over a 4.0 cm2 area?
knee stress
Knee Stress
  • Why are deep knee bends, squats below 90 degrees, and “duck walks” contraindicated?
three principle stresses
Three principle stresses
  • Two are axial
    • normal stress or longitudinal stress
  • One is transverse
  • pressing or squeezing force directed axially through a body
  • pulling or stretching force directed axially through a body
  • On the rack

Applied force tends to slide

the molecules across each other.

  • Blow to the side
mechanical loads on the human body1
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
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

force per unit area



amount of deformation divided by original length

Load and Response
generic stress strain relationship
Generic Stress-Strain Relationship

Plastic Region



(Yield Point)

Stress (load)

Elastic Region

Strain (deformation)

mechanical strength
Mechanical Strength
  • The strength of a material has to do with the maximum stress (or strain) the material is able to withstand before failure.
  • Mechanically, toughness is the ability to absorb energy and not fail (or before failure).
  • Strain is the quantification of the deformation of a material
linear strain
Linear Strain
  • Occurs as a result of a change in the object’s length.
shear strain
Shear Strain
  • Occurs with a change in orientation of adjacent molecules as a result of these molecules slipping past each other.

Measuring stress and strain in biological materials

mechanical properties of the musculoskeletal system
Mechanical Properties of the Musculoskeletal System
  • Age and activity level affect the mechanical properties of all connective tissue.
  • Bones are strongest in compression and weakest in shear.

Three kinds:

  • Hyaline cartilage (articular cartilage) - covers ends of long bones in joints
  • 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.
  • Elastic cartilage - found in the external ear and tip of the nose.
  • Cartilage is able to withstand compressive, tensile, and shear loads.
  • Articular cartilage transmits the compressive loads from bone to bone at joints
  • 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.
  • 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
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 tissue1
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 and Tendons
  • Ligaments, tendons, and cartilage all have similarly shaped stress-strain curves due to their collagenous composition.
ligaments and tendons1
Ligaments and Tendons
  • Under low stresses, these materials are pliant, but as the stresses increase past a certain threshold, they become much stiffer.
  • The mechanical properties of muscle are not as easily examined due to its contractile ability.
  • The ultimate stress of muscle is less that that of tendon, ligament, or bone, whereas its failure strain is much greater.