Cga ifa 2015 7 why we walk the way we do
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Cga ifa 2015 7 why we walk the way we do
- 1. 1 blog: www.wwRichard.net Why we walk the way we do Richard Baker Professor of Clinical Gait Analysis
- 2. Aim • To develop a theory of walking that is biomechanically rigorous and clinically meaningful. • A clinically meaningful model is one that provides insights into how we can help our patients to walk more easily. 2
- 3. 3 “ … it is obvious that any improvement - either in surgical and physiotherapeutic procedures or in braces and prostheses - must rest upon an accurate knowledge of the functional characteristics of the normal locomotor system.” Eberhart, Inman and Bresler Human Limbs and their Substitutes (1954)
- 4. Motor control • Biomechanics can tell us why we walk the way we do • Motor control tells us how we achieve this • This lecture, following, my expertise, will focus on the biomechanics 4
- 5. Learning from history: The determinants of gait 5
- 6. Hip flexion Pelvic rotation Stance phase knee flexion Swing phase knee flexion Compass gait Successively smoothing the trajectory of the centre of mass
- 7. “Translation of the body in a straight line with the least expenditure of energy may be achieved mechanically by means of a wheel but it is quite impossible by means of bipedal gait. The next most economical method would be translation of the body through a sinusoidal pathway of low amplitude in which the deflections are gradual.”
- 8. • Inspired • Elegant • Persuasive • Repeated in nearly all the major text books. • Wrong!
- 9. Gard, S., & Childress, D. (1997). The effect of pelvic list on the vertical displacement of the trunk during normal walking. Gait and Posture, 5:233-238. Gard, S., & Childress, D. (1999). The influence of stance-phase knee flexion on the vertical displacement of the trunk during normal walking. Arch Phys Med Rehabil, 80:26-32. Kerrigan, D. C., Della Croce, U., Marciello, M., & Riley, P. O. (2000). A refined view of the determinants of gait: significance of heel rise. Arch Phys Med Rehabil, 81(8), 1077-1080. Gard, S., & Childress, D. (2001). What determines the vertical displacement of the body during normal walking? Journal of Prosthetics and Orthotics, 13, 64-67. Kerrigan, D., Riley, P., Lelas, J., & Della Croce, U. (2001). Quantification of pelvic rotation as a determinant of gait. Arch Phys Med Rehabil, 82, 217-220. Ortega, J. D., & Farley, C. T. (2005). Minimizing center of mass vertical movement increases metabolic cost in walking. J Appl Physiol, 99(6), 2099-2107 Kuo, A. D. (2007). The six determinants of gait and the inverted pendulum analogy: A dynamic walking perspective. Hum Mov Sci, 26(4), 617-656. Gordon, K. E., Ferris, D. P., & Kuo, A. D. (2009). Metabolic and mechanical energy costs of reducing vertical center of mass movement during gait. Arch Phys Med Rehabil, 90(1): 136- 144.
- 10. What went wrong? • Cannot explain all of the features of human gait on basis of one criteria (smooth the trajectory of centre of mass). • Inman and friends only ever thought about the problem - they never tried to match their conjectures to any data. 10
- 11. New approach 1. Identify the Requirements for walking 2. Start off with a simple model 3. Add in complexity that we understand 4. Test against our data 5. Keep adding complexity until we understand the major features of human walking Work in progress 11
- 12. New approach • Not quite as simple • Not quite as elegant • Not quite as wrong! 12
- 13. What are the requirements of functional human walking? 13
- 14. Requisites of walking Continuing ground reaction forces that support the body Periodic movement of each foot from one position of support to the next in the direction of progression 14 Inman V, Ralston H, & Todd F (1981). Human Locomotion. ?
- 15. Locomotor functions Propulsion Stance phase stability Shock absorption Energy consumption 15 Perry, J. (1992). Gait Analysis.
- 16. Major motor functions Maintenance of support Maintenance of upright posture Control of foot to achieve safe clearance and a gentle heel or toe contact Generation of energy to maintain the present forward velocity or accelerate Absorption of mechanical energy for shock absorption and stability or decelerate 16 Winter D.A. (1991). Biomechanics of Human Gait ? ?
- 17. Pre-requisites of normal gait Stability in stance Clearance in swing Pre-positioning in foot in terminal swing Adequate step length Energy conservation 17 Gage, J. (1991). Gait Analysis in Cerebral Palsy. ?
- 18. Requirements of functional walking Energy conservation Clearance in swing Adequate step length Support of bodyweight Smooth transitions 18 Baker, R. (2009). Melbourne Gait Courses.
- 19. Terminology – gait cycle 19
- 20. Conventional terminology Loading response Initialcontact Mid-stance Terminal stance Pre-swing Initial swing Mid-swing Terminal swing 0% 2% 10% 30% 50% 60% 73% 87% 100% Timings only applicable for healthy walking
- 21. Loading response Initialcontact Mid-stance Terminal stance Pre-swing Initial swing Mid-swing Terminal swing 0% 2% 10% 30% 50% 60% 73% 87% 100% Initial contact isn’t a phase Loading occurs throughout first single support Why not pre-stance? Single support and swing are divided into a different number of phases Mid-stance isn’t in the middle of stance Terminal stance isn’t at the end of stance Pre-swing emphasises continuity of gait cycle Conventional terminology Vertical component of ground reaction
- 22. Loading response Initialcontact Mid-stance Terminal stance Pre-swing Initial swing Mid-swing Terminal swing 0% 2% 10% 30% 50% 60% 73% 87% 100% New proposal First double support 1DS Second Double Support 2DS Early single support ESS 37%23% Mid- single support MSS Late single support LSS Early swing ESw Mid- swing MSw Late swing LSw
- 23. Modelling 23
- 24. Modelling • Making simplifications and assumptions to be able to understand something that is very complex • Stating explicitly what those assumptions are and knowing the limitations of our conclusions. • Making predictions on the basis of the model • Testing those predictions against experimental data 24
- 25. Assumption 1: We are working in two dimensionsAssumption 2: The human body can be modelled as a number of rigid segments – Head/Arms/Trunk (HAT), Femurs, Tibias and feet Introducing our model: e-Verne 25 Assumption 3: The segments are linked by simple jointsAssumption 4: The movement about each joint can be specified by a simple joint angle
- 26. 26 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transistions Requirements of walking Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transistions
- 27. Efficiency of walking Walking for a kilometre at comfortable speed (4kmh) uses up the energy in two teaspoons of sugar. A healthy child has to walk for over an hour to work off the energy contained in a can of coke. 27
- 28. Simple Pendulum 28 -5.0 -2.5 0.0 2.5 5.0 -15 -10 -5 0 5 10 15 0.0 1.0 2.0 Energy(J) Time (sec) Total energy Kinetic energy Horizontalvelocity(m/s) Horizontal velocity Potential energy • Mass below pivot • Conserves energy • Periodic oscillation • Natural frequency
- 29. What is an inverted pendulum? Interactive poll 29
- 30. Inverted Pendulum 30 • Mass above pivot • Conserves energy • No oscillation • Moves forward 0.0 0.5 1.0 1.5 2.0 0 50 100 150 0.0 0.1 0.2 0.3 0.4 0.5 Horizontalvelocity(m/s) Energy(J) Time (sec) Total energy Kinetic energy Horizontal velocity Potential energy
- 32. 32
- 33. 33
- 34. Validation 10 50 1DS 2DS 60 1000 34 McGrath, M., Howard, D., & Baker, R. (2014). The strengths and weaknesses of inverted pendulum models of human walking. Gait and Posture [elecrtronic publication]
- 35. Inverted pendulum • Models horizontal components of velocity well throughout cycle • Models vertical component well through single support (but not double support) • Faster you walk the less good the inverted pendulum model is. 35
- 36. Hip moment 36 Largely explained as moment required to keep trunk vertical (particularly at natural and slow speeds)
- 37. 37
- 38. 38
- 40. Compass gait a) No double support so stance and swing both 50% of gait cycle. b) Can only see angles for one leg because in walking is symmetrical. c) Pelvis tilt fixed at 14° (because PSIS above ASIS) d) Hip extends throughout stance e) Hip flexes throughout swing f) Femur movements are symetrical about vertical but offset because of pelvis. g) No knee movement h) Ankles mirror hips exactly i) Feet are horizontal as they scrape along floor. f c d e g h a i
- 42. Clinical implications • Walking is a dynamic activity requiring preservation of kinetic energy from step to step. • It can’t be taught (re-taught) as a sequence of static postures. 42
- 43. 43 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight
- 44. 44 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transistions Requirements of walking Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transistions
- 45. What is the minimum knee flexion required for clearance with a plantigrade ankle? Interactive poll 45
- 46. Clearance 46
- 47. Knee flexion 47
- 49. Limit dorsiflexion in middle swing 49
- 50. Clearance j) Minimum toe clearance occurs about half way through swing k) Knee flexes to at least 50° by mid swing l) Hip must be flexed early in swing m) Ankle must be at least neutral in mid-swiing n) Foot will follow knee (moderated by hip flexion) j k l m n
- 51. Clinical implications • If you are going to use kinee flexion to clear the leg you need a lot of it • Small amounts make things worse • Modifications of hip and ankle movement are required for knee flexion to be effective 51
- 52. 52 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight
- 53. What are the four most effective joints for increasing step length Interactive poll 53
- 54. Step length 54
- 55. Adequate step length A 10° change in joint angle will increase step length by: Femur-femur angle +21° Leading knee flexion -13° Trailing knee flexion +13° Trailing heel rise +5° Pelvic rotation +5° Trailing dorsiflexion 0° 55
- 57. Knee flexion 57
- 58. Heel rise to facilitate knee flexion 58
- 59. Step length o) Step length primarily determined by difference in hip flexion-extension between opposite foot contact and foot contact. p) Require good leading knee extension at foot contact. q) Require some trailing knee flexion before opposite foot off. r) Requires a little heel rise to facilitate knee flexion p o q r
- 60. Clinical implications • Step length is fundamentally determined by the range of hip movement • Extension of the leading knee and flexion of the trailing hip are also important • Ankle movement and heel rise play a minimal role in determining step length 60
- 61. 61 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight
- 62. 62 Upward velocity • Upward velocity reduces throughout • There is a downward acceleration • The ground reaction must be less than gravity
- 63. 63 Bodyweight
- 64. An inverted pendulum does not support its own weight. 64
- 65. 65
- 66. The modified ground reaction on its own is not sufficient to support bodyweight 66
- 67. 67
- 68. 68
- 69. Support of Bodyweight The inverted pendulum motion requires a double support phase. r. Stance must thus be longer than swing. s. Opposite foot contact and opposite foot off become meaningful.
- 70. 70 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight
- 71. Two transitions 1. Stance to swing at for off 2. Swing to stance at foot contact “It’s a lot easier to fall off a log than onto one” Richard Baker – August 2009 The swing to stance transition is by far the more difficult 71
- 72. Heel strike Foot strike Foot contact 72
- 73. 73 Winter DA. Foot trajectory in human gait: a precise and multifactorial motor control task. Phys Ther. 1992;72(1):45-53; “The trajectory velocity of the heel immediately prior to HC is virtually zero vertically … … and low in the horizontal direction; such findings raise the question as to why many researchers refer to this initial contact as ‘heel-strike’”
- 74. David Winter “Primary tasks of walking: 3) control of the foot trajectory to achieve safe ground clearance and a gentle heel or toe landing." 74 Winter DA. Biomechanics and motor control of human movement. Third Edition, John Wiley and Sons, Hoboken, New Jersey, 2004
- 75. Smooth transition - horizontal 75 Heel speed is less than 5% of maximum at foot contact (Winter exaggerated this by measuring ankle speed) Swing Stance Pongmala et al. Is foot contact a collision? GCMAS 2015 Slow, normal and fast speeds
- 76. Inter-subject and speed variablility 76 Pongmala et al. Is foot contact a collision? GCMAS 2015
- 78. Horizontal – late swing Achieved through swing limb mechanisms: 1. Knee flexion before foot contact 2. Plantarflexion before foot contact You don’t read this in the text books! 78
- 80. Horizontal – early stance Late swing motion is continued 1. Knee continues to flex 2. Ankle continues to plantarflex Knee flexion in late swing and early stance serves to avoid “shock” not to absorb it. 80
- 81. 81 Smooth transitions - horizontal r) Knee flexes before initial contact and continues into early stance. s) Ankle has to be approximately neutral and plantarflexing prior to foot contact and this continues in early stance. t) Foot angle is modified by changes in knee and ankle in late swing and comes down to horizontal in early stance. rr s s t t
- 82. Smooth transitions – Centre of mass 82 Centre of Mass moving at maximum speed 1.5 m/s. Horizontal velocity
- 83. 83 40%Start of late stance/swing
- 84. 84 40%Start of late stance/swing
- 87. Trailing limb must get longer during late stance and 2nd double support 1. Plantarflexion 2. Controlled knee flexion (reduces leg length) 87 Smooth transitions – Centre of mass
- 90. Leading limb must get shorter during 1st double support 1. Stance phase knee flexion 90 Smooth transitions – Centre of mass
- 91. Smooth transitions - vertical U. Limit dorsiflexion in late single support V. Plantarflexion through double support (to maintain length of limb as knee flexes) 91 u v
- 92. Clinical implications • Knee flexion in late swing is essential to avoid “shock” at contact • Prosthetic limbs have no mechanism for this and hence heavy damping of impact is required. • Excellent motor control is required to avoid shock. Toe walking may be a much simpler mechanism if this is absent. 92
- 93. Which bump does what? 93 Maxvelocity Zerodvelocity The second peak of the ground reaction slows the body down in a vertical direction – “push-off” is an extremely mis-leading term Dedeleration
- 94. Which bump does what? 94 Maxvelocity Zerodvelocity The first peak of the ground reaction is when the body is accelerated upwards. Acceleration
- 95. Which bump does what? 95
- 96. Which bump does what? • In the horizontal direction the opposite happens: – Early stance is a deceleration phase – Late stance is an acceleration phase 96
- 97. 97 blog: www.wwRichard.net Requirements of walking Energy conservation Clearance in swing Appropriate step length Smooth transistions Support of bodyweight
- 98. Original aim 1. Identify the Requirements for walking 2. Start off with a simple model 3. Add in complexity that we understand 4. Test against our data 5. Keep adding complexity until we understand the major features of human walking 98
- 99. 99 Energy conservation (compass gait) Clearance Adequate step length Support of bodyweight Smooth transitions
- 101. 101 Requirements of walking Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transistions Can we apply these principles to understand walking with pathology?
- 102. Thanks for listening 102 blog: www.wwRichard.net
Editor's Notes
- Translation of the body in a straight line with the least expenditure of energy may be achieved mechanically by means of a wheel but it is quite impossible by means of bipedal gait The next most economical method would be translation of the body through a sinusoidal pathway of low amplitude in which the deflections are gradual.
- Import Excel graph as Enhanced metafile, group and convert to Microsoft graphic object. (Make sure that the horizontal axis is not leading to cropping of series data).
- Height 540 Width 620 Left 60
- Height 540 Width 620 Left 60
- These are the graphs of how the horizontal (top) and vertical (bottom) components of the speed of the centre of mass change as the inverted pendulum operates. The CM has to be travelling reasonably quickly in a forwards direction at the (1) start of the movement because it is going to slow down as gets higher and kinetic energy is converted to potential energy. It will be going slowest in the (2) middle of the movement when the mass is highest after which it picks up (3) speed again as the potential energy is converted back to kinetic. We can also think about what is happening in a vertical direction. At the start of the (4) movement the centre of mass is moving upwards. As it moves around the circular arc that’s dictated by the hip joint rotating about the ankle then two things happen. In the first half of the movement the body is losing kinetic energy so the vertical as well as the horizontal component of velocity must reduce, but also the direction of movement flattens out so a smaller part of that movement is in the upwards direction. The two effects reinforce each other so that the vertical velocity reduces through the first half of the movement. It is actually zero as the pendulum (5) passes over its highest point because, just for a split second, the centre of mass is actually travelling exactly horizontally. We can put this a different way and say that the centre of mass is decelerating in a vertical direction over the first half of the movement. After passing the mid-point then the same two factors act in the opposite sens (6) e. The body gains kinetic energy so starts moving faster and as it moves round the arc a greater proportion of its velocity is directed downwards. Both factors combine and the vertical component of velocity becomes increasingly negative (it is accelerating downwards).
- (1) Now we said we were going to talk about forces and indeed we already are because Newton’s laws tell us that if accelerations are occurring then forces must be acting. (2) Let’s think about what those forces must be. We’ll look at the vertical component of the ground reaction on this graph with (3) bodyweight as our reference. Early on (4) the body is travelling upwards (positive vertical component) but getting slower – it’s decelerating. The overall force on the body must be acting downwards. That is the (5) vertical ground reaction acting upwards must thus be less than bodyweight acting downwards. You can see that the slope of the velocity graph is getting a little more gradual with time so the ground reaction must be getting closer to bodyweight with time. In the (6) middle of the movement the body goes over the top of the arc and from moving upwards to moving downwards so it is still accelerating downwards. (7) The ground reaction must still be less than bodyweight. The slope is at its gentlest at this point though so the reaction will be closest to bodyweight at this point. In the (8) later part of the movement the centre of mass is accelerating downwards so again the (9) ground reaction must be less than bodyweight and as the slope gets steeper it must the ground reaction must be getting smaller. You can see that the ground reaction under an inverted pendulum is always less than gravity. I haven’t been able to trace where but I think Jacquelin Perry once referred to walking as a process of continuous falling – I don’t know if she knew quite how biomechanically accurate.
- Most of us will know that the ground reaction doesn’t actually look like that under the inverted pendulum, it’s got two (1) characteristic bumps. These certainly increase the ground reaction quite considerably above that generated by the inverted pendulum (we’ll discuss the mechanisms for this in a later screen cast) but look closely at the graph in relation to bodyweight. The ground reaction is only (2) above bodyweight is only above bodyweight for a fairly short period and by not very much. If we contrast this with the amount of time that the ground reaction is less than bodyweight and the size of the deficit you’ll see that on average the ground reaction generated is considerably less than bodyweight. We are still not supporting bodyweight through stance.
- Here is the data plotted for left and right limbs you’ll see that the average vertical component of the ground reaction (the horizontal green line) is a little over 80% of bodyweight. If we allow a short period of double support however …
- … you’ll see that the ground reaction under the left and right limbs combines and that the greatest total ground reaction acting on the body is actually just a little before the middle of double support when two relatively modest individual ground reactions combine. The result of this is that the average value of the ground reaction is thus increased. We thus reach the conclusion that if we are going to rely on the inverted pendulum as the essential mechanism for moving the body forwards then we will have to have a period of double support to ensure that bodyweight is supported. If the ground reaction was purely that under an inverted pendulum you can work out that the double support period would have to be about 15% of the gait cycle. With the modified form of the ground reaction we find in normal walking it is about 10%.
- The only experimental work I am aware of that has sought to quantify the extent to which foot contact can be considered a collision is David Winter’s work from over 20 years ago. He measured a “virtually zero” [1] component of velocity in the vertical direction and a low velocity [2] in the horizontal direction which led him to conclude that use of the term “heel-stike” was inappropriate. In a later work he described five major motor tasks of which the third was “Control of the foot trajectory to achieve safe ground clearance and a gentle heel or toe landing."
- He also framed five primary tasks of walking of which the third included controlling the foot trajectory to give a gentle heel or toe landing.
- As we commented in an earlier screen cast the centre of mass has to be travelling at maximum speed at the same time in order that the kinetic energy from one gait cycle can be carried over to the next gait cycle whilst the potential energy is at a minimum. It’s thus travelling at around 1.5m/s at this point.