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SALRE, wedges and COP

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Petcu Daniel, Sep 20, 2017.

  1. Petcu Daniel

    Petcu Daniel Active Member


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    Hello,
    Even seems to be a stupid exercise I've tried to explain myself why a varus wedge applied to a foot orthosis will shift medially the location of COP and a valgus wedge will have an opposite effect. Bart Van Gheluwe and Dananberg's work (Changes in Plantar Foot Pressure with In-Shoe Varus or Valgus Wedging, https://doi.org/10.7547/87507315-94-1-1 ) provide the evidence for this but also, based on the literature, they are referencing the opposite: "Internal wedges behave in the opposite manner from externally applied wedges". In the context of this work the internal wedges are applied on foot orthotics while the external wedges are applied on footwear sole (referenced with: https://www.ncbi.nlm.nih.gov/pubmed/10574166 )
    I would like to understand this using SALRE (and free body diagrams !) and any thought will be apreciated.
    Daniel
     
  2. efuller

    efuller MVP

    The problem with doing wedge research on the foot is that there is a brain connected to the foot. The CNS can activate muscles in response to placement of a varus wedge. When you place a varus wedge under the heel of a foot with a laterally deviated STJ axis, you can often see increased pronation motion that will tend to cause increased medial forefoot loading. You would expect for the varus wedge to cause inversion, but the CNS can cause the peroneal muscles to contract to cause a greater pronation moment than there is supination moment from the ground. This makes a net pronation moment and with that you would see pronation motion even though the intervention of the varus heel wedge caused an increase in supination moment.
     
  3. Petcu Daniel

    Petcu Daniel Active Member

    In "Effects of Laterally Wedged Insoles on Knee and Subtalar Joint Moments" , https://doi.org/10.1016/j.apmr.2004.09.033 , 2 of 13 subjects having knee OA has a behavior which can be explained as above. A description of the foot posture is not presented but a "varus deformity of the knee" is noted toghether with a femorotibial angle greater than in the case of the control group. In Bart Van Gheluwe and Dananberg's work the subjects are described as being without a history of musculoskeletal anomalies or chronic injuries of the lower extremities and for all 23 subjects the COP position was shifted to medial with varus wedging. Seems to me that when, for example, the COP is analysed a more detailed description of the subject's foot could be of help in order to better understand the conclusions.
    Daniel
     
  4. Petcu Daniel

    Petcu Daniel Active Member

    "have a behavior" - sorry!
    Another interesting article "What predicts the first peak of the knee adduction moment?" ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4268356/ ) is finding that knee adduction angle is predicting 58% of the peak knee adduction moment (KAM) variance suggesting that variable stiffness shoes has the potential to influence it. In the same time the medial-lateral COP is not significantly related to KAM which rise a question about the efficiency of the alteration of the position of COP with a valgus wedge. In this case, how a variable stiffness shoe is acting differently by a valgus wedge?
    Daniel
     
  5. When I lecture on this topic, I make the point that the moments produced by ground reaction force (GRF) are external moments caused by external forces. In addition, I also point out that the muscles, tendons and ligaments that resist these external moments from GRF cause internal forces which are internal moments. The external and internal moments must be added together for the rotational equilibrium equation to equal zero.

    There is no question that a varus heel wedge will cause an increase in external STJ supination moment (or a decrease in internal STJ pronation moment). This may be proved by modelling. The question, as Eric so nicely pointed out, is what is the central nervous system (CNS) doing about this varus heel wedge?

    Is the CNS fighting the wedge and causing more internal STJ pronation moment by increasing the efferent activity to the peroneals so that the foot doesn't move or even pronates in response to the varus wedge? Or is the CNS just letting the varus heel wedge supinate the foot without adding any extra internal STJ pronation moment in response to it?

    How and why the CNS activity alters motor activity in response to foot orthoses or other in-shoe wedges is, currently, the great unknown. We are still a long way from understanding how the CNS responds to orthoses. However, my guess, is that the primary determinants of CNS motor activity are still the same as we have long suspected. 1) optimizing the metabolic efficiency of gait and, 2) avoiding pain and injury.
     
  6. Petcu Daniel

    Petcu Daniel Active Member

    Boyer KA et al. in "Kinematic adaptations to a variable stiffness shoe: mechanisms for reducing joint loading", https://www.ncbi.nlm.nih.gov/pubmed/22541945 is discussing about an "adaptive response" or "coordinated dynamic changes" to explain the medial shift in COP position and decreasing of the medio-lateral GRF component in all 11 subjects, when variable-stiffness-shoes were tested. This make me to think that even if the COP position remain unchanged a modifying of the medio-lateral GRF component could suggest a modification of the direction of the GRF vector. Still not clear for me why a valgus wedge and a variable-stiffness shoe seems to have different action mechanisms!
    Daniel
     
  7. efuller

    efuller MVP

    Just to be clear the medial / lateral component of ground reactive force is a different thing than center of pressure. The medial lateral component is friction at the foot ground interface in the medial or lateral direction. A different concept that you see reported is a medial or lateral shift in the location of center of pressure. Apologies, if you knew that already.

    Variable stiffness shoes could mean a lot of things. One example, is the harder density midsole under the medial side of the heel. When you land on that heel with an artificial athlete, the center of pressure will be more medial with the denser midsole material. (artificial athlete is what they called the device to measure hardness of midsoles) However, the brain connected to a real foot might not like that and change the muscle activity so that the living foot might not have the same center of pressure effect as the artificial athlete. This works as long at there is enough range of motion in the joints to change the location of center of pressure.

    With a valgus wedge the foot could be everted so far that there might not be enough range of motion left to get the medial forefoot to the ground. In this case the brain cannot activate the foot muscles to move the foot into a position where the COP would be changed. This is a possible explanation of why you could not have unexpected changes in location of center of pressure with valgus wedging, but you could have unexpected changes with a dual density midsole in the heel .
     
  8. markjohconley

    markjohconley Well-Known Member

    Prof. Kirby, wouldn't, " ... varus heel wedge ... / ... an increase in external STJ supination moment ...", result in an INCREASE in internal STJ pronation moments with increase in tension of the soft tissue restraints and reflex (STJ) 'pronator' muscles response?


    Eric, thanks to you and Kevin for your informative posts, as usual, however, I having difficulty comprehending, as unless there is a forefoot varus alignment wouldn't the medial forefoot be in contact with the 'foot being everted' anyway?,
    thankyou gentlemen, mark.
     
  9. Mark:

    My response should have read as follows: "There is no question that a varus heel wedge will cause an increase in external STJ supination moment (or a decrease in external STJ pronation moment)." In other words, a varus heel wedge will always produce a medial shift in plantar heel ground reaction force which will increase external STJ supination moment or, if the STJ axis is very, very medial, the varus heel wedge will decrease the external STJ pronation moment. Hope this makes sense.

    As far as Eric's comment, I believe he means that with a large enough forefoot valgus wedge, the medial forefoot could be "hanging in the air" due to the extreme degree of forefoot valgus wedging.
     
  10. efuller

    efuller MVP

    Mark, Kevin did correctly interpret what I was saying. Just keep tilting the wedge farther and farther. At some point you will use up all the range of motion of eversion and there will only be weight on the lateral side of the foot. The amount of valgus wedge where this will occur will be smaller for a foot with more forefoot varus.
     
  11. markjohconley

    markjohconley Well-Known Member

    I'm thinking of a (TP) STJ axis, at an instantaneous point in time, so medial it is external to the foot so the wedge, wherever it be placed, would apply an external pronation moment? How would this decrease the external STJ pronation moments?, thanks

    Eric and Kevin, there's my problem i was thinking a rearfoot only valgus wedge. Makes perfect sense, thankyou again, mark

    It is depressing to know that many podiatrists do not seek to continue their education by utilising resources such as you gents (and others) provide (and other readily available sources).
     
  12. Petcu Daniel

    Petcu Daniel Active Member

    The change of the value of the medio-lateral component of the GRF vector could be an indicative of the modification of the direction of the GRF vector. If we are supposing the position of the COP is constant between interventions (valgus wedge or variable stiffness shoe compared with control), there is a possibility to modify the lever arm of the GRF vector related to knee joint (for example) through modifying of the medio-lateral component of the GRF vector ( http://www.jbiomech.com/article/S0021-9290(11)00094-7/pdf ). In all the articles which I've read related to the modifying of the knee adduction moment the variable-stifness shoe is defined as having lateral sole stiffness 13-1.5 greater than the medial one. The greater angulation of valgus (10 degree) wedge was uncomfortable for subjects so I've better understood the importance of the maximum eversion height test!
    Daniel
     
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