1. Joint mechanics
Lennard Funk

2. Joint mechanics
Hundreds of articulations in the human
body
Many injuries occur to these joint
structures
No two joints are structurally identical

3. Joint Lubrication
Synovial fluid
– Reduction of friction
– Distribution of force
– Nutrition for tissues
Injury implication: joint wear

4. Joint

5. Joint Lubrication
Synovial fluid produced by
Synoviocytes
Arthritis => damaged synoviocytes

6. Articular Cartilage
Type II collagen
Different fibres
orientation
– Shear forces
– tensile resistance
to swelling
Creep
– constant load
– compression load
Cyclic loading
– Benefits vs. damage

7. Articular Cartilage
lubrication
Synovial joints
– Low coefficients of friction .
01-.04
molecules
Theories of lubrication
– Boundary
• "lubricin" from synovial fluid
– Fluid film
• hydrodynamic (non
deformable) Fluid
• Squeeze Film
– right angle movement
– short duration
– Mixed

8. Articular Cartilage:
Adaptation
Active loading &
unloading
Degenerative changes
(OA)
Aging
– ↓ water content
– ↓ PG
– ↓ collagen content

9. Joint Mobility and
Mobility sometimes has very distinct
endpoints
– Elbow or knee hyperextension
In other cases variable soft tissue
properties and other factors limit ROM
Some see stability as the joint’s ability to
resist dislocation.
Stability

10. Joint Mobility and
Mobility sometimes has very distinct
endpoints
– Elbow or knee hyperextension
In other cases variable soft tissue
properties and other factors limit ROM
Some see stability as the joint’s ability to
resist dislocation.
Stability
hi p!
pt the
e
Exc

11. Lever Systems
Most motion at the major joints results
from the body’s structures acting as a
system of levers
– Multiple “classes” of lever systems
Functions:
– Increase the effect of an applied force
• Moment arms
– Increase the effective velocity of
movement
• v=rω

12. Levers

13. Levers
• Levers are used to alter the resulting
direction of the applied force

14. Levers
• Levers are used to alter the resulting
direction of the applied force
• A lever is a rigid bar (bone) that turns
about an axis of rotation or fulcrum (joint)

15. Levers
• Levers are used to alter the resulting
direction of the applied force
• A lever is a rigid bar (bone) that turns
about an axis of rotation or fulcrum (joint)
• The lever rotates about the axis as a result
of a force (from muscle contraction)

16. Levers
• Levers are used to alter the resulting
direction of the applied force
• A lever is a rigid bar (bone) that turns
about an axis of rotation or fulcrum (joint)
• The lever rotates about the axis as a result
of a force (from muscle contraction)
• The force acts against a resistance
(weight, gravity, opponent, etc.)

17. Levers

18. Levers
The relationship of the points
determines the type of lever

19. Levers
The relationship of the points
determines the type of lever
The axis (joint), force (muscle
insertion point), and the resistance
(weight, etc.)

20. First Class
F R
A
F
A
R

21. First Class

22. First Class
A
R
F

23. Neck extension
First Class
A
R
F

24. Neck extension
First Class Erector spinae
and Splenius
A
R
F

25. First Class

26. First Class
F
A
R

27. First Class
Elbow extension
F
A
R

28. First Class
Elbow extension
Triceps
F
A
R

29. First Class
F R
A A

30. First Class
Designed for speed and range of motion
when the axis is closer to the force
F R
A A

31. First Class
Designed for speed and range of motion
when the axis is closer to the force
Designed for strength when the axis is
closer to the resistance
F R
A A

32. Second Class
R F
A
A
R
F

33. Second Class

34. Second Class
R
F
A

35. Second Class
Plantar flexion
R
F
A

36. Second Class
Plantar flexion
Gastrocnemius
and Soleus
R
F
A

37. Second Class

38. Second Class
Designed more for force

39. Third Class
F R
A
A
F
R

40. Third Class

41. Third Class
F
A
R

42. Elbow flexion
Third Class
F
A
R

43. Elbow flexion
Third Class Biceps brachii and
Brachialis
F
A
R

44. Third Class

45. FUNCTIONAL RELATIONSHIP PRACTICAL HUMAN
CLASS ARRANGEMENT ARM MOVEMENT DESIGN TO AXIS EXAMPLE EXAMPLE
1ST F-A-R Resistance arm Balanced Axis near Seesaw Erector
and force arm movements middle spinae neck
in opposite extension
direction
Speed and Axis near Scissors Triceps
range of force
motion
Force Axis near Crow bar
(Strength) resistance
2ND A-R-F Resistance arm Force Axis near Wheel Gatroc and
and force arm (Strength) resistance barrow, soleus
in same nutcracker
direction
3RD A-F-R Resistance arm Speed and Axis near Shoveling Biceps
and force arm range of force dirt, catapult brachii
in same motion
direction

46. Factors In Use of
Anatomical Levers
F

47. Factors In Use of
Anatomical Levers
A lever system
can be balanced
if the F and FA
equal the R and
RA F

48. Factors In Use of
Anatomical Levers
A lever system
can be balanced
if the F and FA
equal the R and
RA F

49. Factors In Use of
Anatomical Levers
A lever system
can be balanced
if the F and FA
equal the R and
RA F
(A = Arm - distance from A)

50. Balanced
Force Arm Resistance Arm
R
F
A

51. Balance with More
Force Arm Resistance Arm
R
F
A

52. Balanced with Less
Force Arm Resistance Arm
R
F
A

53. Factors In Use of
Anatomical Levers

54. Factors In Use of
Anatomical Levers
A lever system can become unbalance when
enough torque is produced

55. Factors In Use of
Anatomical Levers
A lever system can become unbalance when
enough torque is produced
Torque is the turning effect of a force; inside
the body it caused rotation around a joint.

56. Factors In Use of
Anatomical Levers
A lever system can become unbalance when
enough torque is produced
Torque is the turning effect of a force; inside
the body it caused rotation around a joint.
Torque = Force (from the muscle) x Force
Arm (distance from muscle insertion from
the joint)

57. Practical Application
Resistance
Force

58. Practical Application
Force is produced by the
muscle
Resistance
Force

59. Practical Application
Force is produced by the
muscle
FA the distance from
Resistance
Force
joint (i.e. axis or
folcrum) to insertion of
the force

60. Practical Application
Force is produced by the
muscle
FA the distance from
Resistance
Force
joint (i.e. axis or
folcrum) to insertion of
the force
Resistance could be a
weight, gravity, etc.

61. Practical Application
Force is produced by the
muscle
FA the distance from
Resistance
Force
joint (i.e. axis or
folcrum) to insertion of
the force
Resistance could be a
weight, gravity, etc.
RA the distance from
joint to the center of
the resistance

62. Examples
Resistance
Force

63. Examples
1. How much torque needs
to be produced to move
45 kg when the RA is 0.25
m and the FA is 0.1
Resistance
Force
meters?

64. Examples
1. How much torque needs
to be produced to move
45 kg when the RA is 0.25
m and the FA is 0.1
Resistance
Force
meters?
Use the formula F x FA = R
x RA

65. Examples
1. How much torque needs
to be produced to move
45 kg when the RA is 0.25
m and the FA is 0.1
Resistance
Force
meters?
Use the formula F x FA = R
x RA
Note: A Newton is the unit of force
required to accelerate a mass of one
kilogram one meter per second per
second.

66. Example 1
RA = 0.25
FA = 0.1
?
45
A

67. Example 1
F x 0.1 meters = 45 Kg x 0.25 meters
RA = 0.25
FA = 0.1
?
45
A

68. Example 1
F x 0.1 meters = 45 Kg x 0.25 meters
F x 0.1 kg = 11.25 Kg-meters
RA = 0.25
FA = 0.1
?
45
A

69. Example 1
F x 0.1 meters = 45 Kg x 0.25 meters
F x 0.1 kg = 11.25 Kg-meters
F = 112.5 Kg
RA = 0.25
FA = 0.1
?
45
A

70. Example 2: Increasing the
FA
RA = 0.25
FA = 0.15
?
45
A

71. Example 2: Increasing the
FA
2. What if the FA was increased to 0.15 meters?
RA = 0.25
FA = 0.15
?
45
A

72. Example 2: Increasing the
FA
2. What if the FA was increased to 0.15 meters?
F x 0.15 meters = 45 Kg x 0.25 meters
RA = 0.25
FA = 0.15
?
45
A

73. Example 2: Increasing the
FA
2. What if the FA was increased to 0.15 meters?
F x 0.15 meters = 45 Kg x 0.25 meters
F x 0.15 = 11.25 Kg-meters
RA = 0.25
FA = 0.15
?
45
A

74. Example 2: Increasing the
FA
2. What if the FA was increased to 0.15 meters?
F x 0.15 meters = 45 Kg x 0.25 meters
F x 0.15 = 11.25 Kg-meters
F = 75 Kg
RA = 0.25
FA = 0.15
?
45
A

75. Example 3: Decreasing the
RA
RA = 0.2
FA = 0.1
?
45
A

76. Example 3: Decreasing the
RA
3. What if the RA was decreased to 0.2 meters?
RA = 0.2
FA = 0.1
?
45
A

77. Example 3: Decreasing the
RA
3. What if the RA was decreased to 0.2 meters?
F x 0.1 meters = 45 Kg x 0.2 meters
RA = 0.2
FA = 0.1
?
45
A

78. Example 3: Decreasing the
RA
3. What if the RA was decreased to 0.2 meters?
F x 0.1 meters = 45 Kg x 0.2 meters
F x 0.1 = 9 Kg-meters
RA = 0.2
FA = 0.1
?
45
A

79. Example 3: Decreasing the
RA
3. What if the RA was decreased to 0.2 meters?
F x 0.1 meters = 45 Kg x 0.2 meters
F x 0.1 = 9 Kg-meters
F = 90 Kg
RA = 0.2
FA = 0.1
?
45
A

80. Summary

81. Summary
• The actual torque needed to move a
given resistance depends on the
length of the FA and RA

82. Summary
• The actual torque needed to move a
given resistance depends on the
length of the FA and RA
• As the FA increases or RA
decreases, the required torque
decreases.

83. Summary
• The actual torque needed to move a
given resistance depends on the
length of the FA and RA
• As the FA increases or RA
decreases, the required torque
decreases.
• As the FA decreases or RA
increases, the required torque

84. Levers Continued

85. Levers Continued
Inside the body, several joints can be
“added” together to increase
leverage (e.g. shoulder, elbow, and
wrist.

86. Levers Continued
Inside the body, several joints can be
“added” together to increase
leverage (e.g. shoulder, elbow, and
wrist.
An increase in leverage can increase
velocity

87. Lever Length
Z’
S’
S Z

88. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z

89. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z

90. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z

91. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z

92. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z

93. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z

94. Lever Length
Where is the velocity or speed the
greatest; at S’ or Z’? Z’
S’
S Z
How can this principle be applied to
tennis?

95. Lever
Length

96. Lever
Length
A longer lever would
increase speed at
the end of the
racquet unless the
extra weight was
too great. Then the
speed may actually
be slower.

97. Wheels and Axles
R = 3”
R = 1”

98. Wheels and Axles
Wheels and axles can
enhance speed and
R = 3”
range of motion
R = 1”

99. Wheels and Axles
Wheels and axles can
enhance speed and
R = 3”
range of motion
They function as a form of
lever
R = 1”

100. Wheels and Axles
Wheels and axles can
enhance speed and
R = 3”
range of motion
They function as a form of
lever
Mechanical advantage
= radius of wheel /
radius of axle
R = 1”

101. Wheels and Axles
H

102. Wheels and Axles
Consider the humerus as an
axle and the forearm/hand
as the wheel
H

103. Wheels and Axles
Consider the humerus as an
axle and the forearm/hand
as the wheel
The rotator cuff muscles
inward rotate the humerus
a small amount
H

104. Wheels and Axles
Consider the humerus as an
axle and the forearm/hand
as the wheel
The rotator cuff muscles
inward rotate the humerus
a small amount
The hand will travel a large
amount
H

105. Wheels and Axles
Consider the humerus as an
axle and the forearm/hand
as the wheel
The rotator cuff muscles
inward rotate the humerus
a small amount
The hand will travel a large
amount
A little effort to rotate the
humerus, results in a
significant amount of
movement at the hand
H

106. Joints and
moments
Note, as a joint moves through its
ROM, two things change:
– Instantaneous Center of Rotation
• Rotation
• Sliding
• Rolling
– Muscle Line of Action
These combine to change the moment
arm

107. lenfunk@shoulderdoc.co.uk