Happy New Year

Champagne New Year image

Apologies for not writing a more substantive post but I’ve been working hard over the last few weeks to get our new MSc in Clinical Gait Analysis by distance-learning running.  We had our first virtual classroom on Wednesday evening which was quite a milestone for me. Inevitably there were teething troubles particularly with admissions and registrations process but the final result is that we’ve ended up were we wanted to be with a good sized cohort of students embarking on the programme together. It’s large enough for a feeling of community to develop, small enough for things to feel manageable in this first year. Rather remarkably the seven students represent 4 continents which brings challenges for timetabling the virtual classrooms! We actually had applications from students on the two other continents but they came too late for us to get them on board on time.

The programme is designed for staff already working in gait analysis services. Staff currently employed in laboratories tend to take on either a “clinical” or “technical” role and one of the aims is to encourage and support individuals to develop competencies in all aspects of the role of a clinical gait analyst. It’s part of the CMAster project and next year VU Amsterdam and KU Leuven will be offering different residential options with potential for students to split their studies between any two of the three universities.

Our programme that spreads over three years of part-time study. The first two years are taught and the third gives the students a chance to undertake their master’s project. The taught programme is divided into four modules: Measuring walking, Healthy walking, Walking with pathology and Clinical interpretation. The first two modules are taught primarily by experiential learning requiring students to perform lots of gait analysis to consolidate their understanding of how to capture data and the reasons that healthy people walk the way they do. We’ve also managed to steer clear of any exams with assessment through a range of more relevant tasks.

All in all I’m pretty happy for the programme and looking forwards to it. Perhaps more importantly for this blog it is really starting to raise some issues for me that would make good topics for posts. All I need is the time to write them. So here’s looking forward to the new academic year and more on-line interaction.

Taking it slowly

Hi, I’m back, refreshed from a family holiday in France and Spain and invigorated by an excellent ESMAC conference in Glasgow. Thanks to so many people that used the opportunity to say how much they liked the blog – I suppose I better keep going. The view counter passed 10,000 earlier today so let’s see if we can keep it ticking over.

I’ve no doubt that, for clinical gait analysts, the most important paper published over the last decade is Mike Schwartz’ study on the effect of walking speed on gait variables (2008). It’s the only paper that I maintain a link to on my desktop and I rarely interpret any patient data without referring to it. If you haven’t already done so then download it now and do the same (let’s see if we can knock Tom Novacheck’s [1998] review of running biomechanics off the top of the Gait and Posture most downloaded papers table and replace it with a genuine scientific study!).

walking speed

In the study quite a lot of kids were asked to walk at a range of walking speeds. The resulting gait trials were divided up into five groups by walking speed and the average gait variables for the different groups were calculated. The darker the blue in the figure above the faster the walking with the middle trace representing self-selected walking speed. You can see that the gait traces change quite considerably with walking speed even when there is nothing wrong with the child.

We were looking at data from a patient with a rare genetic disorder today. I think if I’d looked at the same data ten years ago I’d have made all sorts of pronouncement on his gait impairments. Now I just look at Mike’s paper and can say, “Yep, he’s walking slowly”, not only that but, “he’s walking slowly in exactly the same way as anyone else would walk slowly”.  It might be worth trying to work out why he’s walking slowly but there is no evidence in the gait data of any specific impairment that is affecting his walking.

I was chatting about this in the group and talking about how we walk with different gait patterns at different speeds and one of my colleagues asked quite, “Why?”. It’s one of those simple questions that caught me completely unawares and started me thinking.

Kinematically, there is absolutely no reason why anyone shouldn’t walk more slowly by having exactly the same pattern of movement but simply going through that pattern more slowly. You could thus walk slowly in a way that gives exactly the same gait graphs (after time normalisation, see previous post on this). The answer of course is that walking is not primarily driven by the kinematics but the kinetics. The way that energy flows between the segments and the way this is mediated by muscle activity depends very much on how fast the segments move. Kinetic energy is proportional to the square of speed and forces and moments act to produce accelerations.  Walking slowly efficiently requires quite different dynamic mechanisms to walking quickly efficiently.

Although this answers the question at one level it only does so partially. What would be really interesting would to be to look at how the curves change with speed from the context of how we understand the process of normal walking and see if we can explain why the gait pattern varies the way it does. Anyone who can do this easily and comprehensively has a better understanding of normal walking than I do. I’m going to have to go away and think about this.

One thing that I think would be quite instructive would be to try and do this practically. Stick some markers on yourself and record yourself walking normally. Then try and see if you can walk slowly but in the same kinematic pattern (after time normalisation). I wouldn’t mind betting that it’s not possible. Even if it is possible to match the kinematics this will require quite different kinetics and muscle activations. You may even be able to feel which muscles you are having to use differently. I suspect there’s be  a huge amount to learn from such an exercise.

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Novacheck, T. F. (1998). The biomechanics of running. Gait Posture, 7(1), 77-95.

Schwartz, M. H., Rozumalski, A., & Trost, J. P. (2008). The effect of walking speed on the gait of typically developing children. J Biomech, 41(8), 1639-1650.

Metrology or psychometrics

Barricade

July’s over so time to move on from the Determinants of Gait. We’re starting detailed development of teaching material for our new masters degree programme in clinical gait analysis. I’m working on the measurement theory section at the moment and been reflecting how to approach this. I’ve got an engineering background and automatically assume that the language we should use to describe measurement is that of classical measurement theory which I’m going to refer to as metrology.

Modern metrology really started with the French Revolution when a political motivation emerged to standardise measurement systems across the country. Out of this emerged an international process for the standardisation of measurement which is now overseen by the Conference Generale des Poids et Mesures, still based in Paris. They publish the International Vocabulary for Metrology (IVM) which is really the international “Bible” for measurement theory.  The Vocabulary is designed to be universal including the statement that “metrology includes all theoretical and practical aspects of measurement, whatever the measurement uncertainty and field of application.”

One of the things that interests me about measurement in medicine in general and in rehabilitation in particular is that, in some respects, it is developing separately to this paradigm which is accepted almost universally in the physical and biological sciences and in engineering and chemistry. Measurement in medicine and rehabilitation is becoming increasing conceived within the framework of psychometrics. Why, if all the rest of the world is handling measurements one way does, psychology and now rehabilitation need to adopt a different approach?

Whilst its foundations can be traced back to Darwin (see Wikipedia) , psychometrics really came of age in the middle of the twentieth century and is thus a much more recent development than metrology. As the name implies it was developed by psychologists for their work studying concepts such as self-esteem or happiness or even pain which are less specifically defined than quantities in other branches of science. Over-simplifying a bit – metrology was developed to measure things that are specifically defined whereas psychometrics was developed to measure things that are not.

If the quantity you are measuring is specifically defined (e.g. someone’s height) then it is sensible to ask how accurate the measurement is (are you measuring what you claim to be measuring) and this is the fundamental challenge of metrology. If the quantity you are measuring is not specifically defined (e.g. how happy someone is) then the question of how accurate you are is rather meaningless. Psychometry thus focusses on the twin alternative questions of how reliable (repeatable) and valid measurements are.

Others may argue but I am convinced that there is a hierarchy here. If a quantity is well enough defined to determine how accurately it can be measured, then assessing repeatability and validity is second best. If you want to do the job properly you should use metrology to assess accuracy. Psychometric assessment of reliability and validity should be confined to quantities for which the superior option is not possible.

I think that the insidious onset of psychometry has made people lazy. I suspect that there would have been considerably more effort expended on improving measurements in biomechanics if the community had focussed on ensuring accuracy of measurements rather than accepting second best (and rather flattering) measures of repeatability derived from an essentially psychometric approach.

Any volunteers to man (or woman!) the barricades against the insurgence of psychometrics where it isn’t needed or wanted?

DoG IV Modelling without measurement

So what went wrong? The determinants of gait are clearly wrong from a technical perspective and it is doubtful whether they contribute anything clinically. Yet they continue to be taught across the world as a basis for the understanding of human walking. If you doubt this then Google up “determinants of gaits” and see just how many sets of slides you get from educators at a range of different institutions.

DogIV

I think the problem is in the persuasive simplicity of the models that Inman and Eberhart proposed. The pictures of simple articulated structures with neat ball joints and tidy pin joints are just so attractive. There’s  a reason for this which is that, generally, they are very good models, there’s absolutely nothing wrong with them. The problem is in the data that is used to drive the models. The hip joint is a ball and socket and could move as suggested by Inman and Eberhart it’s just that it doesn’t (during normal human walking). In the language of computational biomechanics it is not the model that is wrong but the simulation.

The fundamental problem is that no matter how convincing any simulation looks it is essentially an hypothesis that needs to be tested with experimental measurement.  One of the bizarre aspects of the determinants of gait story is that the group at Berkeley where probably the only people in the world at the time who actually had the data available to test their hypotheses. For some reason they chose not to (being fair, this was 60 years ago, when performing such an analysis would have been much more difficult than today). The result can also been seen as a failure of peer review publication – the paper should have been recognised as an untested hypothesis and rejected (again this is judging the world of sixty years ago by contemporary standards).

The take home message has implications beyond the study of walking. I suspect that simple and attractive models that have never been adequately tested are in use throughout biomechanics. I’d be interested to know if any readers can suggest widely accepted models that could do with being tested as rigorously as the Determinants of Gait have been over the last fifteen years or so. The Root  model (1977) that is the basis of modern podiatry is one example that is the subject of much current investigation although I’m not sure that it has ever been accepted particularly well outside the field of podiatry.

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Root ML, Weed JH, and Orien WP, Normal and Abnormal Function of the Foot – Clinical Biomechanics Volume II, Los Angeles: Clinical Biomechanics Corp; 1977

 

 

DoG III – a more clinical perspective

The arguments against the Determinants that I described in my last post are largely technical. It is interesting that the latest editions of three mainstream textbooks (Levine, Richards & Whittle, 2012, Kirtley, 2006  and Rose & Gamble, 2006) all print fairly damning critiques of the Determinants but choose to reproduce them anyway. Kirtley dedicates nearly four pages to describing them and then describes them in the last paragraph as “thoroughly discredited”.  Does this mean that despite the technical problems the Determinants are still useful in some way? Might they reveal some clinical truths? Let’s explore some more general issues.

One of the problems I see with the Determinants is that the basic “compass gait” (reciprocal flexion and extension of the hips) often gets overlooked. The original authors describe it quite superficially in a couple of sentences and then move on to much more extensive discussion of the Determinants. Levine, Richards and Whittle skim over it in even less detail and Kirtley doesn’t really describe it at all. The balance should really be the other way round. Reciprocal hip flexion and extension is the most fundamental characteristic (determinant?) of bipedal walking. To a large extent step length is determined by the range of motion you achieve at your hips (modified to a much lesser extent by any knee flexion at initial contact) and cadence by the rate at which you can move through this. The first thing anyone should be doing when assessing someone’s gait is to consider how effectively they are implementing this basic mechanism. If you list the Determinants, however, hip flexion and extension never appear.

Another rather disconcerting issue is how the Determinants lead you to focus on rather small movements of the pelvis in the transverse and coronal planes when there are much more significant movements at the knee and ankle in the sagittal plane. Whilst pelvic movements play an important role in the fine tuning of gait, the major sagittal plane motors acting to control hip, knee and ankle are where the action is. The fine movements of the pelvis get two Determinants to themselves and are described in precise detail whereas the knee and ankle are rolled together in one muddled paragraph (in the original paper).  Any approach to walking that distracts the focus from the hip, knee and ankle is likely to be hindering rather than helping. To this day it amazes me that when I show a video of a person walking with a really bizarre walking pattern, many people start off describing the minor imperfections in the motion of the pelvis, often concentrating on the coronal plane, before moving on to much larger aberrations of hip, knee and ankle movement in the sagittal plane.

Then finally there is the reduction of walking to achieving a single objective (walking at minimum energy cost). As Perry  (1985) and Gage (1991)  have both pointed out in different ways there are multiple objectives in walking (see my screencasts on the subject for more details). We need to support body weight against gravity, achieve toe clearance and adequate step length and achieve a smooth transition from one stride to the next whilst preserving the momentum of the passenger unit.  In pathological walking the requirement to avoid pain or maintain an adequate walking speed given some specific impairment might be more important than minimising energy cost. All of these need to be considered if we want to understand walking.

I better stop before this turns into too much of a rant but (in my opinion) the answer to my original questions are, “No, the Determinants are not useful” and, “No, they are exceedingly unlikely to reveal any further clinical insights”.  The sooner someone comes up with an alternative the better. (I’ve had a go [series of seven screencasts]  but am the first to admit that my approach lacks the elegant simplicity of the determinants even if I’d defend it as more biomechanically rigorous and clinically relevant).

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Gage, J. (1991). Gait Analysis in Cerebral Palsy. Oxford: Mac Keith Press.

Kirtley, C. (2006). Clinical gait analysis (1st ed.). Edinburgh: Elsevier.

Levine, D., Richards, J., & Whittle, M. W. (2012). Whittle’s Gait Analysis (5th ed.): Churchill Livingstone.

Perry, J. (1985). Normal and pathological gait. In W. Bunch (Ed.), Atlas of orthotics (pp. 76-111). St Louis: CV Mosby.

Rose, J., & Gamble, J. (Eds.). (2006). Human Walking (3rd ed.). Philadelphia: Lippincott Williams and Wilkins.