Why we walk the way we do (ug)
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This document summarizes a lecture about gait analysis and the attributes of normal walking. It states that most of the lecture material can be found on the professor's blog, and directs viewers to videos on the blog that cover the same topics as the lecture. The lecture then explains the five key attributes of walking: energy conservation, clearance in swing, appropriate step length, support of bodyweight, and smooth transitions. It uses gait graphs and diagrams to illustrate each attribute and discusses the implications for clinical practice.
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Why we walk the way we do (ug)
- 1. Most of the material in this lecture is available on my blog site www.wwRichard.net The interactive tools I use in this lecture cannot easily be embedded in this presentation. Instead use the drop down menus on the blogsie to go to “Verne”. (Note that these are written with Flash so will not work on Apple products such as i-pads and i-phones.) Go to “videos” to see a series of screencasts that cover the same material as my lecture. They are entitled “Why we walk the way we do” and the most relevant material is from screencast 4 onwards. 1
- 2. Why we walk the way we do 2 Richard Baker Professor of Clinical Gait Analysis www.wwRichard.net , http://www.youtube.com/user/WalkingWithRichard
- 4. 4
- 5. 5 75° 60° 45° 30° 15° 0° -15° Knee 1DS ESS MSS LSS 2DS ESw MSw LSw -20 0 20 40 60 80 100 120 Knee flexion (degrees) % gait cycle Left Right
- 6. Introducing e-Verne 6 www.wwRichard.net
- 7. Introducing e-Verne 7 www.wwRichard.net
- 8. Challenge • Can I describe the shape of the gait graphs in a way that all of you can understand? • Start off with simple pattern. • Introduce small steps that we understand • End up with full gait pattern. 8
- 9. Warning • I’ve prepared this material primarily because I don’t know of any text book that describes walking easily and rigorously. • Several popular theories are simply wrong (e.g. Determinants of Gait). • Some of it will be different to what many practicing physiotherapists understand. 9
- 10. Based on ideas from: Verne Inman Howard Eberhart Jacqueline Perry David Winter James Gage (Prerequisites of normal walking) 10 Attributes of walking
- 11. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 11 Attributes of walking
- 12. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 12
- 13. Walking is amazingly efficient Walking for a kilometre at comfortable speed (4km/h) 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. You could walk for 154km on the equivalent of 1 litre of petrol (3 x as far as Toyota Prius) 13
- 14. Simple Pendulum 14 5.0 2.5 0.0 -2.5 -5.0 15 10 5 0 -5 -10 -15 Total energy 0.0 1.0 2.0 Energy (J) Time (sec) Kinetic energy Horizontal velocity (m/s) Horizontal velocity Potential energy • Mass below pivot • Conserves energy • Periodic oscillation • Natural frequency • Doesn’t go anywhere
- 15. Inverted Pendulum 15 Total energy Horizontal velocity Kinetic energy Potential energy • Mass above pivot • Conserves energy • No oscillation • Moves forward 2.0 1.5 1.0 0.5 0.0 150 100 50 0 0.0 0.1 0.2 0.3 0.4 0.5 Horizontal velocity (m/s) Energy (J) Time (sec)
- 16. 16 Fierljeppen 15.55m http://www.youtube.com/watch?feature=endscreen&NR=1&v=QeMAMv6GaJQ
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- 18. 18
- 19. 19
- 20. 20
- 21. 21 60 0 70 -20 75 -15 30 -30 30 -30 Compass gait Pelvic tilt a b Hip flexion c d Knee flexion Dorsiflexion Foot angle e a) No double support so stance and swing both 50% of gait cycle. b) Pelvis tilt fixed at 14° (because PSIS above ASIS) c) Hip extends throughout stance d) Hip flexes throughout swing e) Femur movements offset from zero because of pelvic tilt.
- 22. 22 60 0 70 -20 75 -15 30 -30 30 -30 Compass gait Pelvic tilt a b c d f g h Hip flexion Knee flexion Dorsiflexion Foot angle e f) No knee movement g) Ankles mirror hips exactly h) Feet are horizontal as they scrape along floor.
- 23. Energy conservation 23 2.0 1.5 1.0 0.5 0.0 150 100 50 0 Total energy Horizontal velocity Kinetic energy Potential energy 0.0 0.1 0.2 0.3 0.4 0.5 Horizontal velocity (m/s) Energy (J) Time (sec) The energy that has been preserved through one step must be passed on to the next step as kinetic energy
- 24. 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. • Cadence and step length (and hence speed) are all determined by quality of hip movement. 24
- 25. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 25
- 26. Clearance 26
- 27. 27 60 0 70 -20 75 -15 30 -30 30 -30 Clearance i) Minimum toe clearance occurs about half way through swing j) Knee flexes to at least 50° by mid swing k) Hip must be flexed early in swing l) Ankle must be at least neutral m) Foot will follow knee (moderated by hip flexion) k l Pelvic tilt Hip flexion Knee flexion Dorsiflexion Foot angle i j m
- 28. Clinical implications • Plantarflexion and mild amounts of knee flexion both make clearance difficult. • You need a lot of knee flexion for it to be useful for clearance. • People who have difficulty with clearance can “vault” to make the other leg longe 28
- 29. Clearance 29
- 30. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 30
- 31. Step length 31
- 32. 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% 32
- 33. 33 60 0 70 -20 75 -15 30 -30 30 -30 Step length n) Step length primarily determined by difference in hip flexion-extension between opposite foot contact and foot contact. o) Require good knee extension at foot contact. p) Require some knee flexion before opposite foot off. Pelvic tilt Hip flexion Knee flexion Dorsiflexion Foot angle n p o
- 34. Clinical implications • Step length is driven by hip movement. • Obtaining hip extension on the trailing leg is just as important as obtaining hip flexion on the leading leg. 34
- 35. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 35
- 36. 36 Upward velocity • Upward velocity reduces throughout • There is a downward acceleration • Downward forces are bigger than upwards forces
- 37. 37 Bodyweight
- 38. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 38
- 39. 39
- 40. 40
- 41. 41
- 42. 42 60 0 70 -20 75 -15 30 -30 30 -30 Support of Bodyweight Pelvic tilt Hip flexion Knee flexion Dorsiflexion Foot angle 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.
- 43. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 43
- 44. Two transitions 1. Stance to swing at foot 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 44
- 45. Heel strike Foot strike Foot contact 45
- 46. David Winter “The trajectory velocity of the heel immediately prior to [foot contact] 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." 46 Winter, D. A. (1992). Foot trajectory in human gait: a precise and multifactorial motor control task. Phys Ther, 72(1), 45-53; discussion 54-46.
- 47. David Winter “Primary tasks of walking: 3) control of the foot trajectory to achieve safe ground clearance and a gentle heel or toe landing." 47 Winter DA. Biomechanics and motor control of human movement. Third Edition, John Wiley and Sons, Hoboken, New Jersey, 2004
- 48. Smooth Transitions – Foot 48
- 49. Smooth transition - Foot 49 Heel speed is less than 5% of maximum at foot contact
- 50. Horizontal- late swing 50
- 51. 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! 51
- 52. Horizontal- early stance 52
- 53. 53 60 0 70 -20 75 -15 30 -30 30 -30 Smooth transitions - Foot t) Knee flexes before initial contact and continues into early stance. u) Ankle has to be approximately neutral and plantarflexing prior to foot contact and this continues in early stance. v) Foot angle is modified by changes in knee and ankle in early stance and comes down to horizontal in early stance. Pelvic tilt Hip flexion Knee flexion Dorsiflexion Foot angle t u v
- 54. Smooth Transitions – Centre of Mass 54
- 55. Smooth transitions – Centre of mass 55 Centre of Mass moving at maximum speed 1.5 m/s.
- 56. 56 Start of late stance/swing 40%
- 57. 57 Start of late stance/swing 40%
- 58. 58 Left foot contact 50%
- 59. 59 Right foot off 60%
- 60. Trailing limb must get longer during late stance and 2nd double support 1. Plantarflexion resulting in heel rise 2. Control knee flexion (reduces leg length) 60 Smooth transitions – Centre of mass
- 61. 61 Left foot contact 50%
- 62. 62 Right foot off 60%
- 63. Leading limb must get shorter during 1st double support 1. Stance phase knee flexion 2. Some contribution from ankle 63 Smooth transitions – Centre of mass
- 64. 64 60 0 70 -20 75 -15 30 -30 30 -30 Smooth transitions - vertical w. Heel rise through double support x. Driven by plantarflexion through double support Pelvic tilt Hip flexion Knee flexion Dorsiflexion Foot angle w x
- 65. Clinical implications • Most of us avoid shock rather than absorbing it. • Achieving smooth transition from swing to stance requires a number of co-ordinated mechanisms. It is no wonder that people with disabilities find this so difficult 65
- 66. Summary • We have succeeded in explaining all the significant features of the sagittal plane gait pattern in terms of five attributes of walking. 66
- 67. Coronal plane • Not going to start again! • Not very much happens in the coronal plane during healthy walking other than small movements at the pelvis and a mild movement of the centre of mass from side to side. • The importance of both of these is greatly exaggerated in the literature and by clinicians 67
- 68. Energy conservation Clearance in swing Appropriate step length Support of bodyweight Smooth transitions 68 Attributes of walking
- 69. Thanks for listening 69
Editor's Notes
- Notes after delivery to Undergraduate physios 15/11/2013. Took about an hour and a quarter at quite a leisurely pace. Should we be building in some interactive elemensts (e-Verne for step length and clearance, something physical for pendula?). Can only turn off some aniamtion timings if I disable timed transitions on slides.
- 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).
- 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%.
- There are two major transitions in the gait cycle. One between stance and swing at foot off and the other between swing and stance at foot contact. Moving from stance into swing is relatively easy. Moving from swing to stance is much more difficult. As I put it when delivering these lectures for the first time, “It’s a lot easy to fall off a log than onto one”.
- In the old days we used to talk about “Heel strike”. Then it was recognised that many of our patient’s don’t make contact with the heel so we modified this to “Foot strike”. What I’d like to convince you of today is that the foot doesn’t actually “strike” the ground at all and that a much more appropriate term is “foot contact”. On occasions I go even further and suggest that foot is actually “placed” and that “Foot placement” may be an even better term – certainly for normal walking.
- It’s interesting to note that David Winter first commented on this as long ago as 1992. (Quote).
- 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.
- Let’s first look at the evidence for this in the horizontal plane. In this animation I’ve identified the point on the foot under which the ground reaction first appears and then plotted its trajectory. You can see that the foot comes into land smoothly (Jim Gage likens this to the landing of an aeroplane). If we plot the horizontal velocity of this point you’ll see that its top speed is 4.4m/s in mid-swing. In other words you foot in mid swing is walking almost three times as fast as you are. But by foot contact this has dropped to around 0.2 m/s or about 5% of its top speed.
- 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.