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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
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Daniel -
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 -
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.-
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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 -
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 .-
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thankyou gentlemen, mark. -
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.-
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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). -
Daniel
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