1. Biological Materials:
Chapter 9
Mechanics of
Biological Materials:
Stresses and Strains
on the body

2. Loads
The external forces that act on the body impose loads that affect the internal structures of the body.

3. Mechanics
Science concerned with the effects of forces acting on objects (body)
Rigid-body mechanics
Deformable body mechanics
Fluid mechanics
Relativistic mechanics
Quantum mechanics

4. 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

5. 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

6. 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.

8. 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

9. 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==>

10. Bite Force Human female = 81 lbs Human male = 127 lbs
Humans have 32 teeth

11. Bite Force Dogs Pit Bull = 235 lbs German Shepherd = 238 lbs
Rottweiller = 328 lbs
Wild African Dogs = 317 lbs

12. 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

13. Pressure (Comfy & Cozy)

14. A little uncomfortable

15. Somewhat painful.

16. Graphic of stress pattern when walking

17. 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)

18. PatelloFemoral Stress during the squat
Area: Huberti & Hayes (1984)
PF Force: Escamilla et al, unpublished data

19. Calculations
What is the stress on the knee when a 1000N force is exerted over a 4.0 cm2 area?

20. Knee Stress
Why are deep knee bends, squats below 90 degrees, and “duck walks” contraindicated?

21. Three principle stresses
Two are axial
normal stress or longitudinal stress
One is transverse

22. Mechanical loads on the human body:
Compression
Tension
Shear

23. Compression
pressing or squeezing force directed axially through a body

24. Compressive Loading Example

25. Tension
pulling or stretching force directed axially through a body

26. Tension
On the rack

27. Shear
Applied force tends to slide
the molecules across each other.

28. Shear
Blow to the side

29. 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.

30. Mechanical loads Torsion Combined loading
load producing twisting of a body around its longitudinal axis.
Combined loading
Simultaneous action of more than one of the pure forms of loading.

31. Stress Strain Load and Response force per unit area deformation
amount of deformation divided by original length

32. Generic Stress-Strain Relationship
Plastic Region
Elastic
Limit
(Yield Point)
Stress (load)
Elastic Region
Strain (deformation)

33. Mechanical Strength
The strength of a material has to do with the maximum stress (or strain) the material is able to withstand before failure.

34. Toughness
Mechanically, toughness is the ability to absorb energy and not fail (or before failure).

35. Strain
Strain is the quantification of the deformation of a material

36. Linear Strain
Occurs as a result of a change in the object’s length.

37. Shear Strain
Occurs with a change in orientation of adjacent molecules as a result of these molecules slipping past each other.

38. Instron
Measuring stress and strain in biological materials

39. Mechanical Properties of the Musculoskeletal System
Age and activity level affect the mechanical properties of all connective tissue.

40. Bone
Bones are strongest in compression and weakest in shear.

41. Cartilage Three kinds:
Hyaline cartilage (articular cartilage) - covers ends of long bones in joints

42. 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.

43. Cartilage
Elastic cartilage - found in the external ear and tip of the nose.

44. Cartilage
Cartilage is able to withstand compressive, tensile, and shear loads.
Articular cartilage transmits the compressive loads from bone to bone at joints

45. 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.

46. 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.

47. 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.

48. 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.

49. Ligaments and Tendons
Ligaments, tendons, and cartilage all have similarly shaped stress-strain curves due to their collagenous composition.

50. Ligaments and Tendons
Under low stresses, these materials are pliant, but as the stresses increase past a certain threshold, they become much stiffer.

51. Muscle
The mechanical properties of muscle are not as easily examined due to its contractile ability.

52. Muscle
The ultimate stress of muscle is less that that of tendon, ligament, or bone, whereas its failure strain is much greater.