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Extrinsic variability of ground reaction force

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Petcu Daniel, May 10, 2012.

  1. Petcu Daniel

    Petcu Daniel Active Member

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    How do you evaluate the extrinsic variability of ground reaction force as this is considered to alter significantly the patterns of GR components ?

    ( With reference to the article "Ground reaction: intrinsic and extrinsic variability assessment and related method for artefact treatment", Marco Rabuffetti, Carlo Frigo ,Journal of Biomechanics Volume 34, Issue 3, March 2001,
    http://www.sciencedirect.com/science/article/pii/S0021929000001366 )


    Last edited: May 10, 2012
  2. Daniel, one way might be to perform intra-subject variability studies. This way you could directly measure the between trial variance. It would not allow you to differentiate the intrinsic and extrinsic components, but it would give you 95% confidence intervals about which to draw your conclusions.
  3. efuller

    efuller MVP

    The premise of the paper is that there is an ideal step and that you should reject non ideal steps when gathering data. It seems that they are proposing and algorhythm to choose which steps you reject. I don't have access to the full paper and have just read the abstract. I wonder what they meant by proper foot placement. Yes you should reject steps where the foot is half off of the measurement platform. You might want to reject steps where the patient is trying too hard to hit the platform. (e.g. taking a short step so that the foot does land fully on the platform.) (Ideally, the patient shouldn't have to be aware of where the platform is.) On the other hand the task of walking is an oscilation around an ideal. Every step is not perfect and sometimes we overcorrect and others we undercorrect, so there is "supposed" to be variation. Does the algorhythm remove too much variation?

    Another issue is does the gait cause the pain, or does the pain cause the gait. Does the algorhytm reject painful steps? You can't measure gait as a purely mechanical thing. Gait is, at some level, a behavior of the idividual and there will be variation. How you deal with the variation depends on the question that you are asking of the force platform. Sometimes, you question might be how much variation is there.

  4. Viz. you need to quantify your within subject variation.
  5. Petcu Daniel

    Petcu Daniel Active Member

    Thank you for your answers !

    In my case I would like to see how the GRF is influenced by a unilateral hallux valgus surgery [left foot].

    The experimental sessions was performed as it follows :

    - pre-operative: ten walking trials for each foot on one AMTI force platform [10 steps for left and 10 for right],

    - post-operative : same method as pre-op.

    The results has indicated a significant variation in the Fy-mediolateral normalized force for right foot = 50.6% difference (p=0.023). For the left foot [operated] the difference wasn't significant = 0.25% (p>0,05)

    I wasn't able to record the foot orientation (hoping to extract it, somehow, from COP trajectory !) and, reading the attached article, the problem of extrinsic variability was rised regarding the difference for Fy.

    As the article states that medio-lateral component and COP trajectory are the most influenced by extrinsic variability I thought it is worth to investigate this aspects. But I didn't succeed to understand the algorithm !!

    The extrinsic variability is defined in the article as the changes in the direction of walking and in the position and orientation of the striking foot relative to the platform .

  6. Petcu Daniel

    Petcu Daniel Active Member

    I have to think at intra-subject variability because I don't have experience to evaluate it. In the abstract of "Intra-subject variability of ground reaction force data in children", M.Kirkpatrick and collab, Abstracts - Gait & Posture 1993; 1: No 2, the measurements were performed in 3 days, which wasn't available in my case.

    Could you indicate to me other work about intra-subject variability, if possibly, regarding the medio-lateral component of GRF ?

    Thank you,


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