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

 

 

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3 comments

  1. I´m completely agree with all the arguments you presented about “the determinants”. The only aspect I think that could be taken in consideration is that from a clinica reasoning point of view these determinants are really usefull and it fits with the ICF clasification in different domains.
    That´s why they are still used for clinical and education purposes.

    All the best from Madrid and congrats for your excellent blog!

  2. I found the discussion interesting. Perhaps not many have really asked the clinical meaning and contributions of the model. I would like to learn more about the details of the model and simulations to get a better understanding of ‘In the language of computational biomechanics it is not the model that is wrong but the simulation.’

    1. Thanks for raising this issue Anmin. I’m probably using terminology that others may not be familiar with either. In computational biomechanics the model is the description of the anatomy (and physiology in some cases). In the case of the determinants of gait this is the description of the body segments and joints which link them. The simulation is the way the model moves over a period of time in a particular context. In this case this is the pattern of movement of those segments and joints over the gait cycle during walking.

      Hope this helps

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