What is the opposite of compensatory?

I’ve recently been preparing some teaching material for a one day course on observational gait analysis which we are running this Friday. I got to the part where I normally introduce the concept of primary, secondary and tertiary abnormalities of gait. As outlined by Jim Gage (The Identification and Treatment of Gait Problems in Cerebral Palsy, 2009),   primary effects are those regarded as a the direct effects of the original brain injury. He gives loss of selective motor control, balance impairments and abnormal muscle tone as example. Secondary effects are those that are not part of the original brain injury but develop as a consequence of it. Bony torsional malalignment and muscular contractures might be seen as examples. Finally tertiary effects are those resulting from an individual’s efforts to compensate for the consequences of the injury. Vaulting, circumduction and hyperflexion of the hip are all cited as examples of compensations for a foot drop which may be a consequence of weakness of the dorsiflexors or tightness in the plantarflexors.

Although the term effects is used when first introducing these concepts the subsequent sections are headed primary gait abnormalities and secondary gait abnormalities. The terms effects and abnormalities appear to be used interchangeably in this context.

I was struggling to incorporate this into the approach that I’ve called impairment focussed interpretation. This assumes that the aim of clinical gait analysis is to identify the impairments that are most likely to be affecting the patient’s ability to walk by linking them to observed features in the gait data. This follows the WHO definition of an impairment as “a problem in body structures or functions such as significant deviation or loss” (WHO, International classification of functioning, disability and health. 2001) and my own definition of a feature as something of clinical relevance that you can see on a gait graph (or on a video).

How I wondered do my impairments and features relate to Gage’s primary, secondary and tertiary effects or abnormalities?

I’ve come to the conclusion that Gage’s primary and secondary effects are generally impairments (as the WHO has defined them). They are things that are wrong with the underlying body structures and functions and are not necessarily related to gait. His tertiary effects, by contrast, are generally features.  They are changes to the way a person walks.

It also occurs to me that the primary, secondary, tertiary terminology implies a progression from one to the next in sequence. It doesn’t take too long, however, to realise that some tertiary compensations are a direct result of a primary impairment without the requirement for a secondary intermediary.  Thus vaulting might be a direct consequence of plantarflexor spasticity, a primary impairment.

To tidy things up therefore it makes sense to me to preserve the terms primary and secondary but restrict these to impairments, and to drop the term tertiary in favour of compensation whilst restricting its use to the description gait features. The question that is now puzzling me though is, what is the best name for a feature that is the opposite of a compensation? If a compensation is an alteration to of the gait pattern in response to an impairment which makes walking easier, what do you call the an alteration to the gait pattern which is a consequence of an impairment that makes walking more difficult?

Do feel free to leave your answers as comments to this post.

Mind your language

I’ve recently heard of a new history of gait analysis being written and been given a preview of the section on language development which I’ve been given permission to share.

The first task was obviously to understand how people walked. This proved more difficult than anyone imagined and at the end of the process everyone was considerably more confused than they were at the start. To cover this up they invented a new range of words and phrases.

Someone identified six determinants of gait which everyone agreed was a good thing despite very few of them really determining gait and the one that actually did being completely over-looked. The fourth and fifth were so vague as to be virtually useless but this was cunningly disguised by describing them both in the same paragraph which then looked nearly as long as the paragraphs describing the others.

The gait cycle was divided up in such a bizarre and counter-intuitive way that everyone thought it was a joke until they found it had been published in a text book and had to start using it. Mid-stance wasn’t in the middle of stance and terminal stance wasn’t at the end. There was a pre-swing but no pre-stance (which is actually more important). Single support was divided into two phases while swing was divided into three despite being the same period of time (but for the other leg). This made it virtually impossible to talk about what one leg was doing while the other leg was doing something else. At least this made things simpler. Shock-absorption started to be used for the phase when the upward movement of the body was being speeded up and push-off of for that when its downward movement was being slowed. Heel strike was adopted for the instant (or was it a phase?) when the foot contacted the ground despite many people not using their heel and very few of them striking the ground to any appreciable extent.

The plantar flexion knee extension couple was introduced despite the fact it clearly wasn’t a couple and three rockers invented despite no-one really knowing what a rocker was. The Americans assumed it was a quaint Anglo-Saxon term whilst the English assumed it was some new-fangled American word. Speakers of English as a second language just assumed they had slept through that lesson. When it was finally established that rocker didn’t have a specific meaning a fourth was added in celebration.

There was a backlash against terms that oversimplified complex concepts and very soon a demand emerged to balance this with other terms that would overcomplicate simple ones. A double bump in the ankle plantarflexion moment thus became a widely accepted alternative to “toe walking”. One group even went as far as to suggest that treatments that resulted in toe walkers achieving a heel contact be described as having effected a biomechanical transformation.

A challenge to the hegemony emerged from a perky Canadian who asserted that it was possible to understand walking by plotting joint moments. This was immediately recognised as a threat by the establishment. If walking could be understood then there might be an obligation on them to understand it. This might compel them to learn biomechanics which was clearly a bad thing. The solution was elegantly simple. They introduced some doubt as to whether internal or external moments should be plotted. (A radical splinter group even extending this to plotting some of the graphs upside down). This essentially made it impossible to categorically distinguish between the action of an agonist and its antagonist in normal conversation and successfully curtailed any useful contribution from the new approach. The status quo was re-established and the establishment was heard to exhale a collective sigh of relief.

Everyone understood what normal walking was but then some kind person realised that this forced them to talk about their patients as being abnormal which didn’t sound very nice. There was a competition to find an alternative which several people entered but nobody won. People still seemed happy to refer to these people as subjects. This sounded even less nice to some people but after the experience with normal they were largely ignored. A small group pointed out that referring to diplegic patients “put the disability before the patient” and went around scribbling out the term whenever they saw it and replacing it with patients with diplegia. At least this kept these people occupied and prevented them doing anything more damaging. In some fields the equivalent phrases were so unwieldy that they were replaced with abbreviations such as PwPD or PwMS. The end result of this process was thus to reduce groups of people who had previously had the dignity of being described by words to the ignominy of only ever being referred to by abbreviations.

The crowning glory was in achieving universal agreement that crouch gait was the biggest enemy but universal disagreement on what the term meant. Eventually it was decided to let everyone write their own definition – problem solved.

I gather this work is still in progress and if any readers would like to contribute additional examples of linguistic development in gait analysis as comments to this post then these will all be considered for inclusion in the definitive version.

What is normal?

A couple of months ago I wrote a post entitled normative data capture Part 1. No-one has yet demanded Part II but I’ll give it anyway. The earlier post concentrated on what sort of numbers were required to determine normative ranges for any data assuming we want a reasonable estimate of both the average and the standard deviation.

Once we’ve got the numbers sorted out then the question arises “What is normal?” It might be worth starting with a paragraph addressing the political dimension here. The word “normal” is considered inappropriate in some circles because of the connotation that anyone else, our patient for example, is “abnormal” which is considered a negative term. The response from some researchers (particularly Americans?) working with children has thus been to prefer the term typically developing. This presumable implies that our patients our atypical and I’m not sure that that is any better or worse than abnormal.  What I do appreciate is that we are all abnormal in some regard. The question should not be whether the person is normal or abnormal but whether their gait pattern is. I think normative stresses this emphasis that it is the data or the pattern that is abnormal rather than the individual (but others may think differently).

But then what is a normal gait pattern?  In my dictionary there are various definitions of normal and the closest to the sense in which we are using it is not deviating from the standard. Even this though is not particularly close. I suspect that what we really mean is representative of the population. This raises the questions of how we consider people, with conditions such as  cerebral palsy  in relation to this population?  I think that conceptually they should be considered as part of the population. Thus the normal population includes people with cerebral palsy (and other gait disorders). In childhood and early adulthood at least, these conditions are quite rare (approximately 1 in 500 people is born with CP, one of the more common conditions affecting walking) so true normative ranges (calculated over a wide enough sample) would be very little affected by including or excluding them.

Of course we most often collect normative data from much smaller samples (my previous post suggested that 30 might be regarded as a reasonable number). In this case it makes sense to specifically exclude people with obvious neuromusculoskeletal pathology not because we regard them as abnormal in principle but because the statistics of the situation dictate that the normative data that we obtain by excluding them will be closer to normative data for the entire population than the data we would obtain if they were included. (Including one person with CP in a sample of 30 runs the risk of obtaining normative data that is quite different from that which would result from the one person in 500 in the general population)

A more common problem is a number of anatomical and physiological characteristics which have a wide range within the general population such as in and out-toeing, tibial torsion, femoral anteversion, flat-feet and high arches. Some health professionals will want to define an arbitrary and often subjective cut-off beyond which the individual is labelled as having an impairment and exclude them from the normative dataset. I remember hearing one story of a gait analysis service that was interested in providing normative data for a foot model and simply collected a group of individuals who, to them, had no obvious neuromusculoskeletal impairment and were entirely asymptomatic. The team was later joined by another health professional who looked at the dataset and concluded that quite a large proportion of the cohort had either flat feet or raised arches and wanted these abnormal people deleted from the dataset.

This of course raises the prospect of self-fulfilling prophecy. People with flat feet are considered as abnormal because they have data that falls outside the range of those who have been assessed as not having flat feet. This is clearly daft. Normative data ranges should ideally be generated with randomly sampled datasets from the whole population. In practical situations random sampling is extremely rare but it is inappropriate to select participants on the basis of some pre-supposition of what normal is (unless the abnormality is so rare and severe that the inclusion in a small sample risks skewing the data as described above).

There is another problem when we start to look at older populations. Iezzoni et al. (2001) suggest that over 10% of the entire population have some difficulty walking as little as 400m, rising to nearly 50% if we look at the population aged 80 and over. There is considerably potential for inclusion or exclusion of these individuals to affect normative data. If we are concerned with what data to use for comparative purposes in older populations, however, then I think the goal posts have shifted.  What we really require is not normative reference data but reference data from healthy people within a particular age range or more specifically those without any specific neuromusculoskeletal pathology. If we want a convenient shorthand then perhaps we should refer to a healthy gait pattern in these circumstances rather than a normal one.

There are the same risks here of subjective decisions as to where the healthy range ends and pathology starts which are largely unavoidable. These can be addressed to a certain extent by defining explicit and objective inclusion criteria. We might not agree with definitions but at least we will know what they are. Even these are problematic however because it is very easy to introduce sampling bias when recruiting. When selecting healthy controls for a study there will be a tendency to select the healthiest available.  All will fulfil the inclusion criteria but they may not be representative of the population of all people who fulfil those criteria.  The solution here may be to specify the characteristics of the sample that were actually recruited rather than the inclusion criteria for the study.


Iezzoni, L. I., McCarthy, E. P., Davis, R. B., & Siebens, H. (2001). Mobility difficulties are not only a problem of old age. Journal of General Internal Medicine, 16(4), 235-243.

Push off push-off

Sheila from Dundee dropped me an e-mail: 

In your meanderings around the subject of gait have you come across any definitive descriptions of push-off i.e. at what time in the cycle does it start? Or do you have any thoughts on the matter yourself?

Having replied it struck me that others may be interested in this topic.

As far as I’m aware “push-off” is only used loosely to describe a phase of the gait cycle. I’ve never seen a definition in terms of where it starts and where it ends. My preference is to describe the phases based on single and double support and swing (with single support and swing divided into three equal parts). This intentionally avoids labelling any particular phase as having any particular function (push-off, shock-absorption etc.) partly because people often get these functions wrong when describing walking and partly because patients may not achieve such functions at the same phase of the gait cycle as the able-bodied.

“Push-off” is particularly problematic. How usefully it describes the late stance phase depends both on whether you are considering the whole body or just the leg and the direction you are talking about. During late stance the centre of mass is moving downwards and forwards. The downward motion is being resisted. From this perspective late stance is a phase of deceleration and the term “push-off” is inappropriate. The segments in the limb however are moving in different directions, the foot, ankle and tibia are being “pushed up” whereas the femur is actually moving downwards with the centre of mass.

Looking in the horizontal direction both the centre of mass and the limb are being accelerated forwards. There is a relatively small acceleration of the centre of mass (but this affects a large mass) and a rapid acceleration of the limb (which has a much smaller mass). In this context “push-off”  does appear an appropriate descriptor at first.

Focussing first on the centre of mass movement though – if you model the whole body as an inverted pendulum with mass and leg length matching the human body you find that the entirely passive mechanism (no muscle activity) develops an anterior component of a ground reaction in late stance that is very similar in magnitude to that of the ground reaction at this phase of healthy walking. This force arises because of the relative alignment of the centre of mass, limb and foot and suggests that the muscles need only preserve this alignment to generate it. “Push-off” suggests something much more active and may be misleading.

If we focus on the limb – there has been a debate for nearly 200 years about whether it is being pushed forwards by the action of the plantarflexors pushing against the ground or pulled forwards by the hip flexors. I think it very likely that both are important. It’s tempting to think that some insight into this can be gained from looking as the joint power graphs. They show power generation at both hip and ankle which tends to confirm that both are important. Power, however, is a scalar quantity (it is not associated with any particular direction) representing the rate at which energy is supplied to or removed from the whole body by the muscles acting across a particular joint. Given this it is very difficult to come to any rigorous conclusions about the relationship between the power generated at the joints and the movement in a particular direction of the segments of the limb being “pushed-off”  (to say nothing of complications when power may actually be being generated by muscles spanning more than one joint). To answer the problem categorically would require some form of induced acceleration analysis as to what particular muscles are acting to accelerate the segments during late stance. I’m not aware of anyone having done this (perhaps readers can let me know if they are).

Going back to the original question. I’d maintain my suggestion that we avoid “push-off” as a term. It’s an easy label to apply that makes us think we understand something that many of us don’t (and I’d include myself in this).

Shear pedantry

I criticised a colleague the other day for using “shear force” to refer to the horizontal component of force measured by a force plate. He asked me “why?”  Apart from me being a miserable old pedant who’s got nothing better to do than be annoying, the simple answer is that someone did the same to me a long time ago (it might have been Chris Kirtley, but then again it might not).

I don’t always trust Wikipedia but think it is quite good in distinguishing between shear forces which occur when forces are unaligned and cause a shear deformation (see figure above) and compressive forces when they are aligned and lead to pure compression or elongation.  To distinguish between these you need to know how and where the balancing force is applied. The force plate only measures the ground reaction and I’d argue can’t therefore distinguish between shear and compressive forces. What it can do is resolve the overall force into components in different directions. I’d thus prefer to describe the components in terms of the direction in which they are acting rather than the assumed effect they are having on tissue.

If I was being really really pedantic I’d probably say that shear forces exist within a material rather than being applied to it. In most biomechanics it is actually the shear and compressive  stresses and strains that are more important. These are caused by the external forces exerted on the material but are conceptually quite different being within the material. Generally speaking the vertical component of the ground reaction will give rise to compressive stresses. Given the complex arrangements of soft tissues in the foot and the irregular shape of the bones, however, it will also cause some some shear stress. Similarly although horizontal forces will result primarily in shear they will exert some compressive (and occasionally tensile) stresses as well.

Or am I being too pedantic? Anyone like to defend the use of shear force to describe what a force plate measures? It’s certainly very common usage.