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Moments, net Moments, and tissue theory?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by JonathanH, May 13, 2012.

  1. JonathanH

    JonathanH Member

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    Hi Everyone,
    It has been a great challenge (and greater confusion) to integrate review case studies, anatomy, moments and all!

    Assuming an individal has a neutral/normally positioned STJ axis (neither medial nor lateral),

    1. Is the net moment generated by:
    a. flexor digitorum longus tendons = a supination moment?
    b. quadratus plantar = supination moment?
    c. plantar fascia = supination moments? (to resist pronation moments?)

    a. would factors such as ankle equinus and observed earlier heel lift, generate greater net supination moments?
    what is the net moment generated by lengthening the heel lift (e.g from posterior heel to the plantar the plantar midfoot)?

    Thank you all for your time and I appreciate all feedback and pointers!

  2. efuller

    efuller MVP

    Hi Jonathan

    Net moment vs. moment. First you need to discuss moment acting on what. A moment is applied from one object to another. When you look at an individual part all the moments acting on that part are added together to get a net moment. If the net moment is non zero then there will be an angular acceleration. If the net moment is zero there will be no acceleration and if the body was at rest it will stay at rest. So, if you see a body at rest, you know the net moment is zero, but there may still be moments acting on the body in question.

    Hicks looked at the shifting of the location of the center of pressure with pull of the tendons in a cadaver foot. He found that FDL was a close second to posterior tibial muscle in causing supination moment. This is not a net moment about the STJ. This just a moment from muscle action acting about the STJ.

    I'm not aware of any specific papers that describe the action of quadratus plantae about the STJ. It may be far enough lateral that it does not really have any affect about the STJ. The abductor hallucis is parallel with the plantar fascia and can be assumed to have the same effect as the plantar fascia.

    In a foot with an average axis, tension in the plantar fascia will cause a supination moment. There are some rare feet that when the push up test of Jack is performed you will see internal leg rotation. I explain how this effect can occur in my paper on the windlass mechanism.

    The windlass mechanism of the foot. A mechanical model to explain pathology.

    Fuller EA.

    J Am Podiatr Med Assoc. 2000 Jan;90(1):35-46.

    It depends on the location of the STJ axis. In equinus you have to competing forces with different actions. You have the pull of the Achilles tendon on the calcaneus which causes a supination moment. You also have an early heel lift wich shifts the center of pressure of ground reaction force anteriorly. The location of center of pressure, relative to the STJ axis determines the moment from ground reaction force. So, an anterior shift in the location of center of pressure will have a different effect on a medially positioned STJ axis as opposed to a laterally positioned STJ axis. In the medially positioned axis the center of pressure will be much more lateral to the axis and causing a much greater pronation moment than in the foot with a laterally positioned STJ axis.

    So, in a foot with a medially positioned STJ axis the ground will usually cause a greater pronation moment from the ground than there will be a supination moment from the direct pull of the Achilles tendon on the calcaneus. In a foot with a laterally positioned STJ axis there may be a greater supination moment from the tendon's pull on the Calcaneus than there is pronation moment from the ground. So when you add these two moments together, the net moment from these two forces will be in the direction of supination. The body is likely to notice this and add more supination moment from some other source like the peroneus brevis muscle (to prevent ankle sprains). So, when you add in this third moment you might get a net pronation moment.

    It would depend on how the lengthening effects the location of the center of pressure of ground reaction force.

    Hope this helps.

  3. David Smith

    David Smith Well-Known Member


    I'm not sure that you have a handle on what net moments are?
    Net means 'left over', like net income is what you have left over from your total or gross income after expenses and tax.:mad:

    A moment is a force about a lever but in order to produce a moment you actually need two forces acting parallel but in opposite directions to each other in the same plane but not acting directly opposite to each other on the same axis of interest, i.e. they can or tend to cause a rotation, this is often called a force couple. So a moment could be seen as a rotating or turning force. In the practical application of forces and moments then you actually require three forces to make a moment and so force couple can be a bit misleading.

    In the real world that lever might be 2 feet (60cm) long but you are interested in the moments 1 foot along it. So if you have a force acting at one end of the lever to lift it then you have another force at the other end of the lever which is the ground reaction force, the third force is gravity i.e. the weight of the lever acting in the opposite direction. (There is a another force called inertial force which is proportional to the acceleration of the body or lever but we will ignore inertial forces since the accelerations are low)

    The weight of the lever is 10kg and so the force applied to lift it just off the ground, without any acceleration, is 5kg x 9.81 = 49.05N - therefore the moments applied by your hand to lift the lever about the point 1 foot (30cm) along the lever is 49x0.3 = 14.7N/m and the moment from the grf is also 14.7N/m and so the net moment about the 30cm point of interest is 14.7 - 14.7 = zero left over.

    The net moment at 15cm is+ moments hand = 7.35N/m - Moments ground = 22.05N.m so +7.35-22.05 = Net moment of -14.7N/m (and minus might denote anticlockwise direction) left over.

    So when you have left over moments then there must be motion or you haven't accounted for some other force.

    "Aha!" I might hear you say "The lever isn't moving and yet in the second examplethere are net moments" Well that's because of the internal forces that I have not accounted for i.e. the lever is made of steel and the tendency to bend is resisted by the stiffness of the material and the thickness of its structure. If it was 0.005mm thick or it was made of warm toffee then it would bend and there would be motion.

    Does that help at all?

    Dave Smith
  4. David Smith

    David Smith Well-Known Member


    What direction do you reckon the GRF moments about the STJ in these feet are. The STJ axis is central.

    The painful condition complained of is the anterior aspect of the sinus tarsi why is that do you think?

    The STJ does not evert past vertical in gait or open chain examination.

    The jacks test (w/b hallux windlass test) is light but the supination test is heavy, why is this?

    Do you think the pressure scan indicates that the 5th ray is compliant or stiff to GRF?


    NB the white line is the CoPP (Centre of Pressure Progression) The white circle with cross is where the highest force is and the purple line is the progression of the highest force thru the stance phase.

  5. phil

    phil Active Member

    Hi Dave,

    I'm not Jonathan, but i'll have a guess, if that's ok?

    I'd guess a laterally deviated STJ axis.

    I'd also guess the STJ is at or close to pronatory end ROM throughout gait if getting sinus tarsi pain?

    The jacks test is low because there's hardly any GRF under the 1st metatarsal head?

    The supination resistance is high because the perroneals are working overtime resisting lateral instabitity from the laterally deviated STJ axis?

  6. JonathanH

    JonathanH Member

    Thanks for covering the differences between net moments and moments generated by a particular tissue. Would there be any chance you could pass me a copy of the file? my email is footmedicpodiatry@gmail.com

    !!!! hence some individuals with peroneal tendinitis almost always immediately feel the difference with a lateral forefoot wedge and show more positive clinical outcomes...it makes sense!!

    Thank you very much!! :)
  7. JonathanH

    JonathanH Member

    Hi Dave,
    Thanks for covering the physics aspects of foot moments.

    So would it be correct to say, in the foot:
    1. we have different tissues generating different moments (pronation or supination moments), with different "lever arms".

    2. And that what we see kinematically during gait analysis may not accurately reflect the interplay of forces in the foot due to other factors such as internal resistance, tissue stiffness etc.

    ie. an observed pronated foot in gait does not simply mean net pronation?

    3. mechanical treatment aims to reduce/increase the lever arms to increase/reduce pronation/supination moments in different tissues in the feet to reduce stress and optimise tissue function?

    Thanks alot for your advice, hopefully i'm in the same ball park? :)

  8. David Smith

    David Smith Well-Known Member

    Phil thanks for your input:

    You didn't notice that I wrote that the STJ axes are central.
    While it is possible that the peroneals can add pronation moments about the STJ and therefore increase potential impingement forces at the sinus tarsi, this is not the case here.

    The STJ/rearfoot does not evert past vertical in gait and symptoms indicate impingement causing compression pathology at the sinus tarsi (ST), Therefore it might be reasonable to assume that the bony compression at the ST is applying significant force about the lever (moment arm) to the STJ and so produces supinating moments that resist the STJ pronation.

    I also, deliberately, gave the info that the pain was in the anterior aspect of the sinus tarsi. This was another clue to deduce the nature of the applied forces and moments.

    I also asked you to consider the stiffness of the 5th ray. The 5th ray was stiff to GRF and low relative to the other rays. This is why the maximum force tends to be sub 5th MPJ. force applied at this position gives a long (the longest) moment arm to the STJ both in supination moments and dorsiflexion moments.

    Now before I go on ( addressing your red herring) if the peroneals were firing to resist supination due to a very lateral stj axis we might see the same high force on the 5th but it is unlikely in that case that we would also have symptoms of sinus tarsi compression since there would be some balance i.e. not net moments between the GRF supinating the foot and the peroneals pronating the foot.

    So GRF is acting sub 5th mpj to cause pronating moments about the STJ, the STJ cannot pronate past vertical and so something is stopping the rotation and this can assumed to be the sinus tarsi compression causing supination moments. Of course the sinus tarsi is umlikely to be the only structure resisting pronation but the compression forces there are sufficient to cause painful symptoms and so we need to address that problem.

    The long pronation moment arm of the 5th ray gives maximum pronation moments but it also has maximum dorsiflexion moments about the any point toward the ankle and STJ. This then will tend to dorsilflex the 5th ray, cuboid and lateral aspect of the calcaneus and so cause compression in the anterior aspect of the sinus tarsi.

    The forefoot is not varus to the rearfoot but the rearfoot does not pronate or evert past vertical and so as it approaches vertical the opposing supination moments become greater and the joint becomes stiffer to pronation moments. The 5th ray is stiff and low and the midfoot supination moment increases and so pronation resistance becomes stiffer as the forefoot approaches horizontal to the ground. The medial forefoot cannot apply much force to the ground and so the CoP remains very lateral, which means the pronation moments remain high thru the whole of stance phase and so the sinus tarsi compression is not only high it is also prolonged.

    So in summary the CoP remains lateral because the supination moments resisting pronation from the laterally positioned GRF are provided by the internal tissues and not by external forces acting on the plantar foot. This results in high External and internal moments and so requires high internal forces to maintain those moments.

    Does that cover it?

    Regards Dave Smith
  9. JonathanH

    JonathanH Member

    So it appears the GRF are lateral to the STJ axis, focused particularly bilateral 3-5th MTHs.

    Does that mean the foot has excessive supination moments (esp on the lateral MTH 3-5th) from heel strike all the way to toe off?

    would this be clinically observed as excessively supinated during heel OFF hence the high GRF observed in the diagram?

    pain due to excessive stress/compression in the sinus tarsi. is this due to:
    1. excessive internal pronation moments generated to resist the supination moments from heel strike?

    the jack's test is light force because the supination moment generated by the plantar fascia is low?

    but i am confused.....because:

    supination resistance is heavy due to excessive pronation moments that have not been resisted by pronation moments from generated tissue, eg post tib?

    i understand that, tissues with low stiffness = require less moments for movement/function??
    tissue with high stiffness = require higher moments for movement/function??

    hence the 5th ray is more stiff to GRF??

    Thank you for your time in going through and explaining these concepts through podiatry-arena. Hopefully I am not troubling you too much!!

  10. JonathanH

    JonathanH Member

    looks like i answered everything in reverse, now i jus have to understand the opposite of what i understand, how confusing !!

    thanks for the detailed explanations!!
  11. David Smith

    David Smith Well-Known Member


    Enquiring mind, hungry to learn willing to ask = fertile ground that will produce good fruit. - Happy to help like others helped me (and still do) Helpers prepare the ground and add good nutrients to allow healthy growth and then enjoy the harvest.

    EEh I'm getting all horticultural like Mr Titchmarsh, How are your begonias? !!;)

    Regards Dave
  12. efuller

    efuller MVP

    Hi Jonathan


    Newton's second law for angular motion. The net moment is equal to the angular acceleration x the moment of inertia. Therefore if you see an acceleration in one direction (the STJ pronates from heel contact to forefoot loading) then there was a net moment causing that acceleration. Then at forefoot loading the STJ pronation stopped and there was an acceleration in the direction of supination. It's not the pronation that hurts is the stopping of pronation that hurts.

    So, the observed motion must be caused by the net moment.

    Pretty much. I'd prefer the term optimize tissue stress instead of tissue function. You don't want the tissues to be placed under more load than they can handle. Or to say the same thing in engineering terms you don't want the tissue to reach its stress limit where tissue damage will occur.

  13. phil

    phil Active Member

    Hi Jonathan,

    I don't think this is right. I think an internal pronation moment would be something like perroneus brevis acting, which Dave said isn't happening.

    I think the pain in the sinus tarsi is due to the large internal supination moment acting on this area. As the SJT can't pronate enough to get the medial forefoot to the ground, most of the forefoot GRF is lateral to the STJ axis, causing high external pronation moment? And in this foot, pronation is being stopped by the anterior sinus tarsi area? Causing pain?

    Would I be right to call this an old fashioned partially compensated rearfoot varus?

    Please make sure the other guys correct me before you take what i'm saying as true Jonathan! It's pretty brain bending stuff for me. Still trying to get my head around it all too.

  14. efuller

    efuller MVP

    It sounds like you've got the concept.

    The center of pressure is (relatively far) lateral to the STJ axis and this will cause a pronation moment from ground reaction force. If the STJ is not pronating in response to the moment then there must be an equal and opposite supination moment from some source. Compressive forces between the talus and calcaneus is the most likely source of this moment. The partially compensated varus foot will look just like this. The subtalar joint doesn't know the difference between forefoot and rearfoot varus. It just knows there is not enough range of motion to get the medial forefoot fully loaded.

    Theoretically, the supination moment could come from the posterior tibial tendon, to create the pressure plots seen. But the person would have to be working pretty hard to keep the foot supinated. You can try this by walking and try to not let your first met head touch the ground.

  15. David Smith

    David Smith Well-Known Member

    Phil - some good thinking going on there

    Yes in a nutshell but here's why I don't like those quantitative descriptive terms and tend not to use them.

    My initial assessment was that the STJ/rearfoot did not pronate past vertical and because of this the medial forefoot did not tend to contact the ground i.e. classic uncompensated / partially compensated rearfoot varus.

    First off - when does partial become fully uncompensated?

    2nd My initial assessment was wrong or at least not as correct as it could have been, which highlights another problem with these terms, which is that they are subjective. Whereas identifying the tissue under stress that is identified as pathological by signs and symptoms and using logical reasoning to design a mechanical intervention is far more objective.

    When he came back today for full assessment and fitting of OTC orthoses I noted that he had a high strike angle and the tibia has a relatively highly varus attitude relative to the ground and when GRF of gait is acting on the stance foot the rearfoot is inverted relative to the tibia but vertical to the ground. Therefore, by the usual definition, this is not a partially or uncompensated rearfoot varus, effectively tho, because of the tibial varus inclination, it still is. So if you have one prescription for uncompensated rearfoot varus and another for fully compensated rearfoot varus, which this is also, then which prescription will you use? Foot posture will change depending on how much force to apply to it and in which direction that force is. Painful symptoms and signs that identify the pathological tissue directly address the thing that the patient is intersted in i.e. pain and its resolution. Using a logically designed mechanical intervention is the best way to achieve that outcome the patient expected.

    Last edited: May 16, 2012
  16. David Smith

    David Smith Well-Known Member


    Ponder on this

  17. JonathanH

    JonathanH Member

    thanks for the 1000 words diagram and explanation :)

    while i understand the other calculations in the diagram, but

    how was the value of 0.2n/m calculated from Sn x 0.04 (0.04 being distance of moment arm)?

    so to summarise and confirm:

    lateral column pain can stem from lateral CoP to STJ axis, causing excessive GRF lateral to the STJ, and are resisted by internal tissue supination moments (e.g compression of sinus tarsi pain) and external moments (e.g peroneal firing)

    2. and in medial STJ axis, and that sinus tarsi pain could also be caused by compression from excessive internal supination moments generated to resist the excessive GRF pronation moments (assuming CoP is lateral to STJ)??

    What i am confused about is the concept of GRF and internal tissue moment resistance.

    so basically, the CoP doesn't cause the problems, it is the GRF from the ground which needs to be resisted by the moments generated by the internal structures of the foot??


  18. Jonathan:

    I'm glad that Eric, Dave and Phil have made helpful comments for you. You are in good hands there.

    Let me try to anwer your question a slightly different way to see if it helps you further.

    Regardess of the spatial location of the subtalar joint (STJ) axis, the moments that are generated across the STJ axis are caused by the following forces:

    1. The magnitude, three dimensional spatial location, direction and point of application of the ground reaction force (GRF) vector on the plantar foot [the GRF vector is measurable with a force plate and will be represented on the plantar foot as the center of pressure (CoP) in a pressure mat or pressure insole system].

    2. The perpendicular distance from the GRF vector to the STJ axis (i.e. moment arm for the GRF vector to the STJ axis).

    The mathematical product of #1 and #2 (i.e. GRF x moment arm) will equal the magnitude of STJ moment and also represents the external moments acting across the STJ at any instant. Or, in other words, the external STJ moments are the moments acting across the STJ that are caused by external forces that act on the foot.

    3. The tension forces from tendons that cross the STJ axis. Those tendons include all of the extrinsic muscles of the foot. However, since none of the the plantar or dorsal instrinsic muscles of the foot cross the STJ axis, they can not directly cause a STJ moment. However, the intrinsic muscles may modify the magnitude and spatial location of the GRF vector by their contractile activity so they may indirectly influence STJ moments by altering the external moments acting across the STJ axis.

    4. The tension forces from ligaments that cross the STJ axis. This would include the deltoid ligament and calcaneo-fibular ligament since these ligaments cross the STJ axis. However, the plantar fascia, spring ligament complex and all the plantar ligaments can not directly cause STJ moments since none of them cross the STJ axis. However, tension forces within the plantar fascia, spring ligament complex and all the plantar ligaments may modify the magnitude and spatial location of the GRF vector so they may indirectly influence STJ moments by altering the external moments acting across the STJ axis.

    5. Interosseus compression forces within the sinus tarsi between the lateral process of the talus and the floor of the sinus tarsi of the calcaneus. These "bone-on-bone" compression forces restrict the pronation range of motion of the STJ by creating an internal STJ supination moment.

    The combination of #3, #4 and #5 cause the internal STJ moments, or, in other words, cause the STJ moments that result from forces acting within the body.

    The summation of all external and internal pronation and supination moments will result in the "net moment" that will pronate or supinate the STJ and cause angular acceleration of the STJ. If, however, the STJ is static (i.e. not moving) or is rotating at constant angular velocity then the special condition of rotational equilibrium is said to have become satisfied which means that there can be no net moments or, in other words, that the summation of all the internal and external supination moments exactly equal the summatino of all the internal and external pronation moments.

    I have attached my original paper on STJ rotational equilibrium that I wrote 24 years ago which may help better explain these concepts ((Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989).

    Hope this helps.
  19. efuller

    efuller MVP

    In a laterally deviated STJ axis foot, there is much more foot medial to the STJ axis (The side of the axis where ground reaction force causes supination.) If there are no muscles acting, then the moment from GRF on the lateral side of the axis has to equal the moment from GRF medial to the axis. The foot with the laterally positioned STJ axis can balance with the center of pressure from ground reaction force directly under the axis. In this case no moments internal, to the foot, are needed. However, since the axis is lateral the distnace from the lateral parts of the foot will be smaller and hence the forces will have to be higher (moment = force x distance.) This is why you may see high forces under the lateral forefoot with a laterally positioned STJ axis. In this foot type there may be high forces under the lateral forefoot even though there is eversion range of motion available. (Coleman block test)

    Sounds like you got it right.

    The CoP is GRF. From what you wrote, it seems that you are saying they are different. More specifically, the center of pressure is the point where ground reaction force can be considered to act. The relationship of this point to the STJ axis will determine the moment from ground reaction force about the STJ axis. So, once you know the moment from ground reaction force, and you know that the STJ is not moving you use the equation Net moment = moment of inertia x angular acceleration to find that there must be a supination moment from some other source. (Not moving = acceleration is zero, therefore net moment acting at the STJ is zero. There is a pronation moment from ground, so there must be a supination moment from some source that when added to the pronation moment from the ground will bring the net moment to zero.) The equation does not tell you which structures are providing the supination moment. We just assume that it is the one that hurts. At the end of its range of motion the bones of the STJ will create a supination moment.

    You usually won't get the high internal supination moment without the high external pronation moment. Theoretically, contraction of the peroneus brevis muscle could cause high supination moments in the sinus tarsi, but the body is pretty good at avoiding pain and it's unlikely that the body will hurt itself.

  20. David Smith

    David Smith Well-Known Member


    how was the value of 0.2n/m calculated from Sn x 0.04 (0.04 being distance of moment arm)?

    Sn = 5N (my handwriting)

  21. JonathanH

    JonathanH Member

    A big thank you to all the contributors of this thread, for your support, patience and time in explaining the concepts!!

    It's making much more sense now =))

    Very appreciative,

  22. davsur08

    davsur08 Active Member

    Hi Eric,
    Am bit confused here, isn't Peroneus Brevis a pronator as it is lateral to the subtalar joint axis? and Peroneus longus a supinator as its insertion point is medial to the Subtalar joint axis?

    many thanks
  23. Rob Kidd

    Rob Kidd Well-Known Member

    While in no way being critical of study (whether scientific or in a judgemental clinical manner) of subtalar axis position, there is an issue that seems to be being ignored - please correct me if I am wrong. Bone is not a lump of granite, and does change in morphology; its just that the time scale is far different from soft tissue. The issue is, is the position of the subtalar joint axis the cause of the problem, or has it been caused by the problem? I am entirely sure that joint axis willl move in response to "abnormal mechanics". Rob
  24. efuller

    efuller MVP

    In post #2 above the bolded "supination should be pronation. Sorry. Good catch Dave S.

    Last edited: May 28, 2012
  25. efuller

    efuller MVP

    Rob are you talking evolutionarily, or within an individual.

    If within an individual: Yes, it is possible for the STJ axis position to move, relative to the foot, over time. Someone may injure their posterior tibial muscle and it would no longer slow STJ pronation/ internal leg rotation. When this occurs, the rapid internal rotation of the talus, relative to the rest of the foot, will have to be stopped by something. The spring ligament is part of that something. Those high forces in the spring ligament may cause permanent elongation. When this occurs you will see increased abduction of the forefoot on the rearfoot. The position of the STJ axis in the rearfoot won't change and hence you would get a more medial position of the axis, relative to the foot, because the shape of the foot has changed. This is the classic "too many toes" sign seen with PT dysfunction.

    On the other hand there is huge variation in the position of the STJ axis across individuals. So those with a more medially positioned STJ axis (taking an average location of the center of pressure) will tend to have a larger pronation moment from the ground. This larger pronation moment will tend to increase the likelihood of injury to the posterior tibial tendon. So, the prediction is that you will see increased abduction of the forefoot on the rearfoot more often in those individuals who start with a more medially positioned STJ axis.

  26. It's probably a bit of both in reality. With contraction it will tend to create an internal STJ pronation moment, but its contraction will also tend to cause the centre of pressure to move medially which will decrease the external pronation moment (increase the external supination moment).

  27. Bob,

    In my stars I am above thee; but be afraid of deviation: some are born deviated (genetic), some achieve deviation (acquired insidious), and some have deviation thrust upon 'em (acquired traumatic)."

    Shakespeare: Twelfth Night, 1602

    The bard called it as it is. I paraphrase him, obviously.
  28. phil

    phil Active Member

    Nice. I'm totally stealing that.
  29. Ok but on reflection I'd probably replace " genetic " with " congenital"
  30. davsur08

    davsur08 Active Member

    Thanks for directing me to the thread. please help me to get this right. Contraction of the peroneus Brevis (insertion lateral to the subtlar joint axis) creates a pronation moment about the subtalar joint.
    If i consider PB in isolation the ground reaction forces which act at 90 degrees to the body will exert a vertically directed force on to the medial side of the foot. This vertical GRF creates a supinatory moment about the Subtlar joint axis (because it is a triplanar joint). For example in a partially compensated forefoot/rearfoot varus the pronation moment created by PB would allow the 2nd and 3rd metatarsal head to weightbear then would this not cause dorsiflexion of the forefoot (as the pronatory moment arm is small or/and due to lack of available eversion motion at the subtalar joint) ?. Is it fair to say that the resultant moment is subject to available motion in the involved joints and may or may not create an equal and opposit moment?
    However, if not in isolation, The posterior Tibial Muscle contraction would create a supinatory moment when the medial side of the foot contacts the ground?
    i ask this because i think Muscle timing is as important as muscle strength in the eitiology of foot pathology.

    Thanks in advance
  31. efuller

    efuller MVP

    By definition, in the partially compensated rearfoot varus foot will not get the medial forefoot to the ground. If it got to the ground it no longer would be defined as a partially compensated varus foot. Yes, with peroneus brevis tension, it will be trying to get to the ground, but it won't have range of motion to do so (assuming tibia stays at same angle to ground.) What will happen with peroneus brevis contraction, in the partially compensated varus foot, is increased compression in the floor of the sinus tarsi. In fact with sinus tarsi sydrome you can see decreased peroneal activation and an increased peroneal reaction time. The muscle may not be weak but it is inhibited.

    So, in the partially compensated varus foot, with peroneus brevis contraction, you would not see a medial shift in the center of pressure, because there is not range of motion to create the STJ supination to get the medial shift.

    Have you read Kevin's rotational equilibrium paper. He goes into this scenario perfectly. With nice pictures and everything. The posterior tibial muscle will create a supination moment at the STJ whether or not the foot is weight bearing. If there is an internal supination moment, say at the floor of the sinus tarsi there will be a pronation moment from the ground that is balanced, when at equilibrium, by the supination moment from the floor of the sinus tarsi. As the posterior tibial muscle increases tension, there will be a gradual increase in supination moment. Since the floor of the sinus tarsi can't push the talus into supination, as the tension in the tendon increases the compression in the sinus tarsi will decrease. When the force in the sinus tarsi is zero, any additional tension in the tendon will create a net supination moment and the STJ will supinate. After the supination and if the muscle tension stays constant, ground reaction force will shift a little more lateral creating a pronation moment that will be equal to the supination moment from the tendon.


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