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Does limb length discrepancy affect supination resistance test?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by alwinliew, Aug 30, 2012.

  1. alwinliew

    alwinliew Member

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    Hi pods,

    I had a 20-yr old male who came in to my office yesterday for orthotic therapy. He has had orthoses in the past. He tore a ligament (he forgot which one) in his left foot when he was 7 yr old (by stepping on a gum nut?). Since then, he has been getting diffuse aches and pain in b/f, esp around the heels and plantar fascia bands.

    O/e, L/foot very pronated and R/foot mildly cavus during RCSP, STJ axis location medially deviated on L/foot, laterally deviated R/foot. LLDx noted with R>L by abt 1cm. Interesting enough, supination resistance test = HIGH FORCE bilaterally ( although STJ axis laterally deviated R/foot!!!!!!)

    Im wondering if LLDx has an effect on Supination resistance test/Force? Should I still do a lateral heel skive on the R/foot?
    Feedback and ideas much appreciated.!:bang::bang::bang:

    ALWIN -Perth
  2. Great question. If limb length discrepancy influences centre of pressure position during relaxed standing as suggested here: http://www.ncbi.nlm.nih.gov/pubmed/4074116 then supination resistance should be influenced by leg length discrepancy too.

    In relaxed bipedal standing the centre of pressure should normally fall somewhere between the two feet. The authors in the above study noted that the centre of pressure was shifted toward the longer limb in a simulated limb length discrepancy. This should shorten the lever arm component of the external subtalar joint supination moment, which should increase supination resistance in the long side compared to the shorter side, all other factors being equal.
  3. Admin2

    Admin2 Administrator Staff Member

  4. efuller

    efuller MVP

    Postural sway studies are usually done with both feet on the same platform. So, when there is a shift to the long leg in the study, that is a shift of the average of both feet. It will not give the change in location of the center of pressure under each foot. One could hypothesize that the long leg has more weight, but the center of pressure under the foot could be in the same location. This will still effect the magnitude of moment from ground reaction force about the STJ axis because moment is force times distance. So, to answer the question, is the center of pressure under each foot changed, the study would have to be repeated with separate platforms for each foot.

  5. efuller

    efuller MVP

    The ground is not the only source of pronation moment. Someone with a laterally deviated STJ axis could be constantly contracting their peroneal muscles to keep the foot plantargrade. The peroneal contraction would give the result of high force for the supination resistance test. As noted above, if there was higher force on the more laterally positioned STJ axis leg then you could have equal pronation moment. However, the study that Simon cited the min limb length difference was 1 cm. That is quite high. Were you watching for frontal plane body lean when you did the supination resistance test? People can lean and choose to redistribute the weight on their feet.

  6. Agreed.
  7. RobinP

    RobinP Well-Known Member

    I think this is much more common than is regularly talked about in our line of work. It is one reason that I think slow motion video footage is a bit of a must in cases like this - to watch the timing of the pronation movement. It's timing within the stance phase will give a clue as to whether it is an "active" pronation and that will likely determine prescription.

    I make the point to all patients referred to me for assessment that may involve LLD that it is one of the most difficult things to understand and measure for clinical significance yet on the face of it would appear to be one of the easiest

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