Why we walk (2014 kinetics)

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Why we walk (2014 kinetics)

  1. 1. Kinetics Richard Baker Professor of Clinical Gait Analysis Blog: wwRichard.net 1
  2. 2. Aims • Ground reaction – What is it? – What can we see in the gait graphs? • Joint moments – What are they? – What can we see in the gait graphs?b 2
  3. 3. Ground Reaction Force What is it? 3
  4. 4. Forces (in general) • If no forces act on an object it will continue to move in a straight line at constant speed. • This is often hidden because of: – Friction – Air resistance
  5. 5. Forces in different directions • Any force will only cause a change of speed in the direction in which it is acting so we can think about forces acting in different directions separately. • We’ll look first at a simple case of how forces act in a horizontal direction.
  6. 6. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. No forces acting Ball stays still (for ever)
  7. 7. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Apply a force Ball moves in direction of force
  8. 8. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Stop applying force Ball continues to move at same speed (for ever)
  9. 9. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Apply another force Ball starts to move faster (acceleration)
  10. 10. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Stop applying force Ball continues to move at same speed (for ever)
  11. 11. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. No forces acting Ball stays still (for ever)
  12. 12. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Apply a force Ball moves in direction of force
  13. 13. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Stop applying force Ball continues to move at same speed (for ever)
  14. 14. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Apply another force Ball starts to move faster (acceleration)
  15. 15. Forces (in general) • If you want to change the speed at which an object moves you have to apply a force. Stop applying force Ball continues to move at same speed (for ever)
  16. 16. Forces • When a force is applied the object changes the speed of movement – it accelerates • Speed of movement does not change unless a force is acting.
  17. 17. Forces (in general) • A force is also required if you want to slow an object down No forces acting Ball moves at constant speed
  18. 18. Forces (in general) • A force is also required if you want to slow an object down Force against motion Ball slows down (deceleration)
  19. 19. Forces (in general) • A force is also required if you want to slow an object down Remove force Ball continues at slower speed
  20. 20. Forces (in general) • A force is also required if you want to slow an object down Force against motion Ball stops
  21. 21. Acceleration and Deceleration • If you apply a force in the direction that the object is already moving its speed will increase (acceleration) • If you apply a force opposite to the direction in which the object is already moving its speed will decrease (deceleration) • There is no difference between these forces (apart from the direction in which they are acting)
  22. 22. Acceleration and Deceleration • The change of speed is proportional to the force: the bigger the force the more the speed changes. • It is inversely proportional to the mass of the object: the lighter the object the more the speed will change (for the same force)
  23. 23. Take home • Objects continue to move at constant speed unless a force acts. • The bigger the force the bigger the change in speed.
  24. 24. Vertical forces and gravity • Gravity acts on all objects. • It always acts downwards. • Unless another force is acting on an object then the object will accelerate downwards
  25. 25. Vertical forces and gravity Gravity always acts downwards The ball doesn’t move An equal force must be acting upwards This force is called a “reaction”
  26. 26. Vertical forces and gravity Remove support No reaction Ball will accelerate downwards
  27. 27. Vertical forces and gravity Still no support Still no reaction Ball will accelerate downwards more
  28. 28. “Components of force” • Forces very rarely act purely horizontally or vertically.
  29. 29. “Components of force” • We can always split the force into components: one horizontal and one vertical Vertical component Horizontal component
  30. 30. “Components of force” • We can treat the components as if they are two different forces. Vertical component Horizontal component will accelerate/decelerate object vertically will accelerate/decelerate object horizontally
  31. 31. Ground reaction in walking Measured with force plate
  32. 32. Ground reaction in walking Vertical component Horizontal component
  33. 33. Ground reaction in walking • Vertical component can only act upwards • Horizontal force can act backwards (here)
  34. 34. Ground reaction in walking • Vertical component can only act upwards • Horizontal force can act backwards (here) • Or forwards (here)
  35. 35. Ground Reaction What can we see in the gait graphs? 35
  36. 36. Ground reaction in walking • Components of ground reaction can be displayed exactly as joint angles • Both vertical (top) and horizontal (bottom) component grapsh have characteristic shapes (like joint angles) Forwards Backwards Up
  37. 37. Vertical component 37
  38. 38. Trajectory of centre of mass 38
  39. 39. Upward acceleration 39 A B BA Warning: This is not what you read in many text books!
  40. 40. 40 A B C BA C Upward deceleration
  41. 41. Downward acceleration 41 A B C D BA C D
  42. 42. Downward deceleration 42 A B C D E BA C D E Warning: This is not what you read in many text books!
  43. 43. Horizontal component 43
  44. 44. Trajectory of centre of mass 44
  45. 45. Forwards acceleration 45 BA C A B C
  46. 46. Forwards acceleration 46 BA C D E A B C D E
  47. 47. 47 Fore-aft component of ground reaction largely a consequence of alignment of segments (requires little more muscle activity than that required to maintain that alignment)
  48. 48. Cyclic walking 48
  49. 49. Cyclic walking 49 Average vertical component (on both feet) must be equal to bodyweight Average horizontal component (on both feet) must be equal to zero
  50. 50. Joint moments What are they? 50
  51. 51. Moment – Angular “force” 51
  52. 52. Moment arising from force 52
  53. 53. Moment arising from force 53 Moment is proportional to size of force
  54. 54. Moment arising from force 54 Moment is proportional to perpendicular distance from pivot
  55. 55. Moment arising from force 55 Moment independent of direction of force
  56. 56. Moment arising from force 56 Force acting in opposite sense will give opposite moment
  57. 57. 57 Forwards Know forces and moments What are accelerations? Know forces and accelerations What are moments? Inverse
  58. 58. 58 Moment tells us which muscle group is dominant Only tell us which group is dominant Tell us nothing about antagonistic activity
  59. 59. Calculating moments 59
  60. 60. Calculating moments 60
  61. 61. Calculating moments 61 • Ounpuu, O., R. Davis, and P. Deluca, Joint kinetics: Methods, interpretation and treatment decision- making in children with cerebral palsy and myelomeningocele. Gait and Posture, 1996. 4: p. 62-78.
  62. 62. Ankle moment 62 AM1
  63. 63. Ankle moment 63 AM2
  64. 64. Ankle moment 64 AM3
  65. 65. Ankle moment 65 AM2AM1 AM3
  66. 66. Knee moment 66 KM1
  67. 67. Knee moment 67 KM2
  68. 68. Knee moment 68 KM3
  69. 69. Knee moment 69 KM4
  70. 70. Knee moment KM5
  71. 71. Knee moment 71 KM6
  72. 72. Knee moment 72 KM6KM5KM4KM3KM2KM1
  73. 73. Hip moment 73 HM4
  74. 74. Hip moment 74 HM4
  75. 75. Hip moment 75 HM4
  76. 76. Hip moment 76 HM4
  77. 77. Hip moment 77 HM2 HM5HM4HM1 HM2
  78. 78. Hip moment 78 HM2 HM5HM4HM1 HM2
  79. 79. 79
  80. 80. 80
  81. 81. Thanks for listening Richard Baker Professor of Clinical Gait Analysis Blog: wwRichard.net 81

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