Terminology

What’s in a name?

We’ve recently advertised for a “Clinical Gait Analyst”. Perhaps I shouldn’t have been surprised but we’ve had expressions of interest from all sorts of people that obviously have quite a different idea of what clinical gait analysis is to the one that I’ve got. To me a clinical gait analyst is someone who works in a clinical gait analysis service. They capture data using a 3-d optoelectronic measuring system (or equivalent) which may incorporate synchronous force plate or EMG measurements. Many also provide an interpretation of this, generally drawing on additional information from a quantitative physical examination. If clinically qualified they may provide clinical recommendations based on the analysis.

“Gait analysis” is, quite appropriately, used in many other contexts. Google up “gait analysis” and there is a good chance that the first hits will refer to a combination of video recording of running and expert advice to help you choose an expensive pair of running shoes. Another group of gait analysts will look at your running and suggests ways of improving your style to improve your times or prevent injury. Getting more clinical many orthotists, prosthetists, podiatrists and physiotherapists base much of their working lives on observational gait analysis. Some will take video recordings but many will simply look at how their patients are walking as a basis for clinical recommendations. On the more technical side there are a number of people interested in gait for a variety of reasons with little or know interest in clinical applications. There is another group of people who perform gait analysis for clinical research. They perform a variety of analyses on grouped data to try and learn more about a disease condition or intervention but don’t offer any results or interpretation for individual patients.  Gait analysis is also proposed as a biometric technique for security purposes. It’s not restricted to humans – Google up “canine” or “equine gait analysis” and you might be surprised by the number of hits.

None of us has a monopoly of such a generic term as “gait analysis” or even “clinical gait analysis” but I do think there is a need for something that refers specifically to what I do (perhaps as far as most readers of this blog are concerned to what we do). Trying to claim that only someone that does what I do is involved in gait analysis is ridiculous and mildly insulting to other practitioners. Perhaps we need a more specific term for what we do.

Some people use “3-d gait analysis” but taking a coronal and sagittal plane video, or even just watching someone walk from different angles is three dimensional. “Instrumented gait analysis” has also been used  but there are a wide range of instruments – a single force plate for example. The best I can come up with while writing this article is “Comprehensive Clinical Gait Analysis” (CCGA). To me this captures the aim of getting a reasonably complete picture of the way someone is walking (even if its rare that anything like a complete picture actually emerges!). Anyone have any other ideas?

What is an inverted pendulum?

“Inverted pendulum” is one of those terms that seems to have crept up on me over my time in biomechanics. I don’t remember it being commonly used or taught when I was a student but now it seems to be everywhere. I suspect it is one of those terms that is not understood anywhere nearly as well as it should be. I’m not aware, for instance, of any biomechanics text book that properly explains what an inverted pendulum is or what its mechanical characteristics are. This is particularly important because in mechanics the “inverted pendulum” is more often studied as a classic example of dynamics and control theory (see the Wikipedia article for example). Anyone looking at these descriptions but wanting insight into the biomechanics of walking is going to end up very confused.

An ordinary pendulum is one with the pivot at the top and the mass at the bottom. An inverted pendulum is the opposite way round. The pivot is at the bottom and the mass is on top. Fierljeppen (canal vaulting) is the best example I’ve got of an inverted pendulum (see video below). The pole rotates about its foot (at the bottom of the canal) and transports the vaulter from one side of the canal to the other. “Transports” is the key word here. The inverted pendulum is a mechanism for carrying an object form one place to another and this is how it functions during walking. The “passenger unit” as Perry would call it is carried forward by the outstretched leg as it pivots over the foot.

It should be noted that there are important differences between the two types of pendulum. The inverted pendulum only carries an object in one direction, it doesn’t swing backward and forward like the ordinary pendulum. Another difference is that the inverted pendulum does not have a characteristic frequency like an ordinary pendulum – it would be absolutely useless inside a grandfather clock.

The earliest use of the term as a model of the stance phase of walking that I am aware of was by Cavagna et al. (1976). Earlier workers have used different terms for essentially the same concept. The “compass gait” of the much aligned Saunders, Inman and Eberhart (1953) is essentially a description of the inverted pendulum. A decade later Elftman (1966) suggested that “the body moves forwards as if vaulting on a pole” and a further decade on Alexander used the term “stiff-legged gait” (1976). It is probably the more recent work of the dynamic walking group (best summarised by Kuo, 2007) that has really popularised the use of the term.

Some papers refer to Cavagna as having tested the hypothesis that the leg behaves like an inverted pendulum (e.g. Kuo, 2007, page 619). I’ve never found any evidence of this in Cavagna’s writing or anywhere else. He certainly commented that changes in kinetic and potential energy of the centre of mass correlate so that the total energy remains approximately constant throughout the gait cycle but there are an infinite number of ways this can occur without requiring an inverted pendulum mechanism (I might write more about this in a later post).

“Proving” that walking is based on the inverted pendulum is problematic in that at a very broad level it is obvious that walking involves a similar mechanism. The foot is clearly planted and the passenger unit is carried over it by the outstretched leg. On the other hand it is equally clear that the mechanism is not a simple inverted pendulum. The trunk remains upright, there is stance phase knee flexion and the pivot with the floor changes position and anatomical location through stance (Perry’s rockers). Any study attempting to establish whether stance is like an inverted pendulum will inevitably conclude that it is a bit like one but not exactly. Forming a sensible research question to “prove” the importance of this mechanism is quite a challenge.

Anderson and Pandy (2003) reported briefly on the dynamics of the inverted pendulum as a model of stance phase and Buczek and his team in more detail (2006). Both these papers are worth reading and held a couple of surprises for me but I’ll keep those for a later post.

Alexander, M. (1976). Mechanics of bipedal locomotion. In P. Davis (Ed.), Perspectives in experimental biology (pp. 493-504). Oxford: Pergamon.

Anderson, F. C., & Pandy, M. G. (2003). Individual muscle contributions to support in normal walking. Gait Posture, 17(2), 159-169.

Buczek, F. L., Cooney, K. M., Walker, M. R., Rainbow, M. J., Concha, M. C., & Sanders, J. O. (2006). Performance of an inverted pendulum model directly applied to normal human gait. Clin Biomech (Bristol, Avon), 21(3), 288-296.

Cavagna, G. A., Thys, H., & Zamboni, A. (1976). The sources of external work in level walking and running. J Physiol, 262(3), 639-657.

Elftman, H. (1966). Biomechanics of muscle with particular application to studies of gait. J Bone Joint Surg Am, 48(2), 363-377.

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.

Saunders, J. D. M., Inman, V. T., & Eberhart, H. D. (1953). The major determinants in normal and pathological gait. Journal of Bone and Joint Surgery, 35A(3), 543-728.

Recycling terminology

Do other people struggle with the conventional terminology for describing the phases of the gait cycle? I’m not sure how this arose but the earliest reference I know of is the first edition of Jacquelin Perry’s book [1].

The first phase is initial contact and lasts for just 2% of the gait cycle. It should really be described as an instant in the gait cycle not a phase. Loading response is also a misnomer. Any response is what happens after an event and even the most cursory glance at the vertical component of the ground reaction shows that loading is not complete until well after the end of the loading response phase.

Mid stance isn’t in the middle of stance – as Perry describes it  it is the phase leading up to the middle of stance. Terminal stance isn’t at the end of stance – it ends at opposite foot contact. I quite like the term pre-swing as it emphasizes the continuity of the gait cycle and how the final requirement of stance is to prepare for swing. Following this logic, however, there is a much stronger case for labelling the phase prior to foot contact as pre-stance as preparing the limb for loading is extremely important in late swing.

Another puzzle is why single support and swing, which are the same phases of gait but viewed from the perspective of one limb or the other are divided into a different number of sub-phases. Single support is divided into two phases whereas swing is divided into three. This makes any attempt to describe gait taking into accounting for the interaction between the limbs is unnecessarily complicated.

Some of the technical delineations for the phases are also based on the characteristics of normal walking and it may not be clear how to delineate the phases for different types of pathological gait. Terminal stance, for example, starts with heel rise. Where does terminal stance start for a patient who walks on their toes and never has their heel down in the first place.  Mid-swing is defined as ending when the tibia is vertical. Patients walking with a crouched gait pattern, however, may not have a vertical tibia at any stage of the gait cycle.

I’m not really a fan of trying to re-define the terms to be more logical. This only leads to confusion. Try looking at the literature on crouch gait. There are at least four well accepted definitions of crouch gait and thus, unless there is additional clarification, it is actually impossible to know what any given individual is referring to when they use the term.

Phases of gait

The terminology I prefer is illustrated above. Single support and swing are both divided  into three or equal duration phases. The terms early, middle and late  are different to the original initial, mid and terminal which helps avoid confusion. The three letter abbreviations can be useful where brevity is an asset.

1.   Perry, J., Gait Analysis1992, Thorofare: SLACK.