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Clarifying biomechanics?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by JasonR, Jan 6, 2013.

  1. JasonR

    JasonR Member

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    Hi All
    I would like some clarification on what may well be a simple biomechanical process to explain- but its been bugging me for a while.

    Say I have a foot with large lateral plantar GRFs, and symptoms (tissue stress) consistent with this pattern of loading. My interpretation is that the sum of internal forces are net supinatory, which results in the reaction forces noted/ measured. If I post laterally, I will generate (more lateral) external reaction forces which will seek to counter those internal moments. Whilst the intent of the lateral wedge is to alter moments, it counter-intuitively will increase another form of tissue stress in that area- compression. At what point do you shift treatment paradigms from altering moments (corrective?) to altering pressure (accommodate?). I suspect the answer depends on what tissue you are trying to 'de-stress'? It must also depend on the stiffness of the segment/ tissues in question? If a tissue cant move, does then the compression effect dominate the moment reduction effect?

    To understand how to manipulate those moments, obviously understanding axes of motion is critical. Whilst the STJA (rearfoot axis) is well represented in the transverse plane, I am not sure how obvious the primary axes are in the midfoot/ forefoot? How interdependent (constrained?) are they from the rearfoot? For eg. I will post thru to the FF based on the rearfoot projection to influence MF/FF mechanics eg peroneal tendinopathy. Do people always think of an anatomical axis, or as in inverse dynamics, is it helpful to think of (e.g. forefoot segment) in isolation and model those actions about an hypothesised centroid, for 'calculations sake'? The funny thing about individuals is that they respond very individually(!) to things like posting- and I suspect that good clinicians manage these things with a few less 'degrees of freedom'- and I am keen to improve my reasoning processes to that end.
    Thanks for any insights you may offer!
    Cheers, Jason.
  2. efuller

    efuller MVP

    Are you aware of the Coleman block test and what it is used for? There are two very different types of feet that will exhibit high lateral foot loads. There is the partially compensated varus foot and then there is the laterally positioned STJ axis foot. (actually it is possible to have a laterally positioned STJ axis foot and a partially compensated varus foot, but it is quite rare.) The Coleman block test is used to differentiate between the two (most of the time). When a block is placed under the lateral forefoot the lateral positioned axis foot will show heel eversion and the partially compensated varus foot will not show heel eversion.

    In the Partiall compensated varus foot a valgus wedge will make the lateral load higher an these feet need a forefoot varus wedge. The valgus wedge, if there is range of motion available, in the laterally positioned STJ axis foot doesn't increase pressure, it just everts the STJ and latreral fore foot.

  3. JasonR

    JasonR Member

    Thanks Eric
    That makes plenty of sense. Presumably your active eversion test tests a similar concept- 'dont evert the foot farther than it can go'.
    It looks like the block test assesses for structural FF issues- effectively insurmountable internal supinatory moments. If the internal resistance is very high, then opposing it directly with some form of wedging is a bit like riding a bike into into a brick wall. If there is some give, then there is the opportunity to oppose (balance) those internal moments and potentially redistribute GRFs more efficiently. That sounds reasonable to me, though I am intrigued by the variable and occasionally counter-intuitive physical response to wedging (eg Nigg studies). A lot of variables (muscle input, knowledge of specific axial position, magnitude of correction required, soft tissue deformation...) obviously come into play, and at the end of the day you have to remain pragmatic!
    Cheers, Jason
  4. Jason:

    High lateral forefoot loads caused by increased ground reaction force (GRF) on the lateral forefoot does not necessarily mean that there are net supination moments acting across the subtalar joint (STJ) axis. If, for example, a patient is maximally pronated in relaxed calcaneal stance position (RCSP) with their calcaneus resting 4 degrees inverted and they also have a inverted forefoot deformity, they may likely have too little pronation range of motion within their STJ to allow the medial forefoot to become completely weightbearing.

    This scenario will result in high lateral forefoot loads even though the STJ has quite large external STJ pronation moments due to high magnitudes of GRF acting on the lateral forefoot that is quite lateral to the STJ axis. However, in this same foot, there may also be relatively large magnitudes of internal STJ supination moment coming from the interosseous compression force within the sinus tarsi from the floor of the sinus tarsi of the calcaneus forcefully pushing on the lateral process of the talus in a posterior-superior direction (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989).

    As you can see, when discussing such concepts, it is important to properly identify whether the moments you are talking about are internal moments or external moments since, by definition, in equilibrium situations, they need to be exactly opposite to each other. In other words, in the above case, the large magnitude of external STJ pronation moment from GRF acting on the lateral forefoot lateral to the STJ axis needs to be counterbalanced by a large internal STJ supination moment in order to achieve rotational equilibrium, since the foot is in RCSP in a static position and, by definition, in rotational equilibrium. In the case above, I have given that the large magnitude of internal STJ supination moment results from the sinus tarsi interosseous compression force, but it could just as well have come from posterior tibial tendon tension force or Achilles tendon tension force.

    Hope this helps.
  5. efuller

    efuller MVP

    Yes, the maximum eversion height test can be used to get the same information from the Coleman block test.

    The Block test will give one result for "structural forefoot issues" and another different result for lack of eversion range of motion issues. The structural forefoot issue that we are specifically talking about was called a rigid forefoot valgus. This is a foot in which the forefoot stops STJ pronation before the STJ reaches the end of its range of motion in the direction of pronation. As Kevin mentioned, this is not really a question of internal moments. In this case, there is no net external STJ moment. The force times distance from the forces lateral to the STJ axis is equal to the force times distance from the medial side of the STJ axis. (This is the same as saying that the center of pressure is directly under the axis. So, when this foot is at rest, no internal moments are needed. The problem in this foot comes during gait when the tension in the Achilles starts to increase. The Achilles tension will usually create a supination moment at the STJ and unless some other muscle causes an increase in pronation moment then the STJ will supinate. This is the reason that I believe you see late stance phase, or early propulsive phase, pronation in gait. The peroneal muscles have add pronation moment to counteract the supination moment from the Achilles tendon.

    Yes, adding a valgus wedge to the partially compensated rearfoot varus foot (no eversion range of motion available) is, using your terminology, like riding a bike into a brick wall.


  6. JasonR

    JasonR Member

    Kevin & Eric, Thanks for your replies- crystal clear as ever :D

    Eric, I think I have sketched what you are saying re rigid FF valgus; Hmm it is attached somewhere..new to this gig.

    Kevin- I agree totally with the potential confusion in not assigning the force or moment as internal or external. Some of the initial statements about knee varus moment and MCOA were initially very confusing for me for that reason.

    Perhaps my error with the 'net (internal) supinatory' statement with high plantar FF GRFs, was that I was referring to the FF segment. Maybe internal plantarflexory (external dorsiflexory) is a more appropriate term for the FF. It would seem that FF loads are still viewed in light of their effect on the RF (via the STJA and its transverse plane representation)? Does the FF via GRFs wag the dog (RF), or the RF/ sum of proximal segments wag the tail (FF)? Given a dynamic situation, I assume the biggest moment wins (in the direction of any resultant acceleration)? I may be going a bit off key here.
    Kevin- I will go and pull out your PI books for a review now!
    Really appreciate your patience and detail.

    Cheers, Jason

    Attached Files:

  7. efuller

    efuller MVP

    The diagram is fine for the forefoot, but ignores the rearfoot. Which leads to your next question.

    Have you ever done the test Kevin described to find the position of the STJ axis? Hold the foot with the STJ in the position it is in stance. If the STJ is 1 degree from fully everted in stance hold the STJ 1 degree from fully everted and load the forefoot. Patient is in chair with bottom of foot pointed toward you. Now, with one finger push dorsally on the bottom of the foot. When you do this on the heel you will usually find parts of the heel that when you push the STJ will supinate and parts where the STJ will pronate. You will also find some points where the STJ doesn not move. The axis is directly above these points. So, the drawing you did for the forefoot can be repeated for each frontal plane section of the foot. This is essentially what the center of pressure calculation is. Where the point of average force is will determine the moment from ground reaction force.

    So, the forefoot doesn't wag the rearfoot or vice versa. Ground reaction force on both the forefoot and the rearfoot will contribute to the net moment about the STJ axis. You have to look at all of ground reaction force. (And then add in the muscle moments to get the total net moment.)

  8. Jason:

    In the accompanying illustration are two models that I think will help better explain the concept of external and internal subtalar joint (STJ) supination and pronation moments and how an internal STJ supination moment may cause excessive lateral forefoot ground reaction force (GRF).

    In the model on the left, 200 N of GRF acts on the plantar first metatarsal head and 200 N of GRF acts on the plantar 5th metatarsal head, both of which produce external STJ moments. The 200 N of GRF acting on the first metatarsal head, acting with a supination moment arm of 0.04 m, causes 8.0 Nm of external STJ supination moment. The 200 N of GRF acting on the 5th metatarsal head, acting with a pronation moment arm of 0.04 m, causes 8.0 Nm of external STJ pronation moment. Since the net external STJ moment equals the summation of all the external STJ moments, then the net external STJ moment in this example is 8.0 Nm - 8.0 Nm = 0.0 Nm. The compression force at the STJ needs to also be equal to 400 N directed inferiorly in order to produce translational equilibrium of the foot and counterbalance the GRF acting on the plantar foot.

    In the model on the right, the posterior tibial muscle is activated and produces a total of 200 N of tendon tension force which, by definition, is an internal force. This 200 N of PT tendon force, acting with a 2.0 cm supination moment arm, produces an internal STJ supination moment of 4.0 Nm and which tends to supinate the foot, but does not supinate the foot enough to lift the first metatarsal head off the ground. The mechanical effect of this 200 N of PT tendon tension is to shift GRF more laterally so that now the fifth metatarsal head GRF is increased to 250 Nm and the first metatarsal head GRF is decreased to 150 Nm. In other words, there is a lateral shift in GRF very similar to the patient you originally described, even though the foot is not maximally pronated.

    The 4.0 Nm of internal STJ supination moment from PT tendon tension force must, however, be counterbalanced by 4.0 Nm of STJ pronation moment in order to maintain the condition of rotational equilibrium and allow the foot to rest in this static position with the fifth metatarsal head receiving 66% more GRF than the first metatarsal head. This STJ pronation moment can be either generated from internal forces or external forces. In the example I have provided, the 4.0 Nm of STJ pronation moment needed to counterbalance the 4.0 Nm of STJ supination moment from PT tendon force comes from the lateral shift in GRF which causes a net 4.0 Nm of external STJ pronation moment [10 Nm of pronation moment - 6.0 Nm of supination moment = 4.0 Nm of pronation moment].

    A similar model could also be constructed demonstrating how a foot with a maximally pronated STJ could create an identical lateral shift in GRF on the forefoot, but with the internal STJ supination moment coming from interosseous compression force within the sinus tarsi.

    Now, here's a question for you: How can the STJ compression force be 600 N when the ground reaction force acting under the foot is only 400 N?
  9. Brandon Maggen

    Brandon Maggen Active Member

    Hi Jason

    Thanks for starting this thread - an overtly difficult aspect to grasp and once you're close to understanding these concepts, a pt will come along to completely challenge what you previously thought you understood :bang:

    Eric and Kevin have as usual, astutely and resolutely explained the terms of STJA rotation and tissue stresses and how GRF actively exert forces upon anatomical positions which in turn (co-dependant) and as a result (exclusively mutual) create external and/or internal forces and tissue stresses that create symptomatology congruent with pt presentation.

    But with the example you provided (large lateral GRF) it is admittedly difficult to follow Eric and Kevin. If I may be so presumptuous a lesser degree difficult example should serve much more to help me and others following to understand better rotational theory and it's co-conspirators (STJA, tissue stress etc).

    What if the patient presents with marked medial GRF with tissue stresses congruent with such for example increased 1st MPJ pressure and pain within the 1st MPJ.

    Examination reveals medial STJA position (using Kevin's (as cited by Eric) manner of locating the STJ axis). Non-weight bearing shows normal 1st MPJ range-of-motion but in weight-bearing it reveals a sagittal plane block (functional hallux limitus).

    In this 'easier' example, the deviated STJ axis position creates an net pronatory (or everted) moment around the STJ with rear-foot varus. The (as you so succinctly put it) rear-foot in this instance is wagging the fore-foots tail.

    By creating a supinatory moment (at the rear-foot) medial to the STJ axis position (i.e. inverting the calcaneuos, supinating the STJ, reducing medial Achilles tendon stress as well as post-tib tendon stress) the GRF at the 1st MPJ reduces notably.

    If however (as I used to do) one adds a fore-foot valgus pad (in the attempt to reduce the pronatory moments around the fore-foot (the painful joint)) then in most cases all we would achieve is as you said - add compression force to the 1st MPJ, successfully increase lateral forces (net supinatory moments around the fore-foot), but unsuccessfully achieve an alleviation of symptoms.

    Therefore, in this instance rear-foot control (Kirby skive) will lateral displace the STJA which in turn will reduce the GRF (and pain) in the 1st MPJ.

    ALthough, unless, as Eric said, you take muscle moments (and other soft tissue structures (ligaments especially)) into account, rear-foot control will not be enough. A 1st ray cut out (or similar) - to plantarflex the distal 1st metatarsal and alter the articulation with the proximal phalanx (to achieve sufficient movement), adequate relief will not be forth-coming.

    At this point, I would like to credit Kevin, Eric, Craig, Simon, Robert and numerous others for having in their possession the very useful and keen ability to articulate these difficult concepts into written word - able to be 'understood' by those that read them. A very difficult task indeed :craig:


  10. JasonR

    JasonR Member

    Eric, Kevin and Brandon,
    Thanks for you instruction and interest.

    Eric, I am aware of the method for finding the STJ (Rearfoot?) axis, though beyond a general appreciation of axis location I find it is harder than it looks to perform accurately. I have been fiddling around with my ipad and a free app (Uber sense) which enables you to film (foot) movement with successive frames overlaid on each other. I pen in a network of dots about the supposed entrance and exit points of the RF axis, and look for the dot which moves the least (I believe KKs work?). You never find a perfectly stationary dot, which suggests a component of accompanying translation. Probably more educational (for me!) at the moment, but its quick and easy, and kind of interesting to add the influence of active control. I should try it in WB.

    Kevin, either you have a massive library of foot-ankle FBDs, or you have gone beyond the cause. Henceforthwith, the very recognisable blue background you use shall be known as 'Kirby Cyan':drinks
    I believe the 600N of STJ compression force arises as a function of translational equilibrium in the vertical plane- the (upwardly directed) internal active input of Tib Post sums with the (upwardly directed) GRF, and is equal and opposite to the downwardly acting bodyweight generated STJ compression force.

    Brandon, thanks for input! I take what you mean, and what Eric and Kevin have emphasised re relating FF loads back to the STJA as (one of potentially many) contributors to the rotational equilibrium picture. The end ROM pronated foot with a medial deviated STJA must balance high (plantar GRF generated) external pronatory moments with internally generated supinatory moments (eg tib post tensile, sinus tarsi compressive) or, failing that, with your externally generated supinatory moment from your kirby skived orthosis. (I think the skive could contribute to a medialisation of the RF GRF and an external supinatory moment, but whether it moves the STJA laterally depends on whether it moves the foot itself- which I think it doesn't have to from a forces point of view, to be effective).
    On a side issue I cant visualise how the the plantar fascia, a likely contributing factor to your functional hallux limitus, participates to rotational equilibrium in the transverse plane (To be effective, wouldnt it need to apply a significant force perpendicular to the transverse plane?)(Its sagittal influence I get). Is this why the SRT is a less useful (predictor) in PFasciopathy (I think Craig said that?)

    Cheers to all

  11. Jason:

    I now have drawn 1,229 illustrations for the newsletters, articles and books I have published over the past 27 years. This "massive" library of illustrations therefore allows me to pick and choose a previous illustration I have done on the computer to use it, as is, or modify it further for such occasions. I use CorelDraw which saves the file in .cdr format which I then export to another file as .jpg format for publishing here for everyone's use on Podiatry Arena.

    BTW, your answer to my question is correct. The extra 200 N of STJ compression force comes from the posterior tibial muscle pulling the tibia more forcefully downward on top of the STJ.

    I am enjoying seeing your intellectual growth on this subject. Keep up the good work!:drinks
  12. efuller

    efuller MVP

    To find the stationary point you need the proper reference fram. When you put your camera on the ground, the reference point is the ground. As you move the STJ both the segment above the joint and below the joint will move relative to the ground. This is why it is impossible to find a stationary point. Even if you were to attach your camera to the tibia, ankle joint motion may cause movement that would not be subtalar joint movement. There are some interesting journal articles on the mathematics of this that try to account for both joints moving.

    He is not the only one who has a massive library of his drawings. I have a few myself and have seen many other lecturers use them as well. Thanks, Kevin.

    The way I started to move away from Root/ Weed teachings is to start thinking about how pronation is stopped. In gait, most of the time you will see pronation of the STJ soon after heel contact. The STJ will pronate until something stops it. The choices are the medial forefoot, the end of range of motion (sinus tarsi), muscle or ligament (plantar fascia and possibly lacinate ligament [tarsal tunnel syndrome]. I think you have a very good understanding of the tissue stress. I've been saying what you have just said for a long time. You don't have to change the position to change the stress. In fact a lot of feet don't change position with a rearfoot varus wedge, but it does relieve the symptoms.

    Tissue stress is in three dimensions. Have you seen my Windlass paper? In it I described how tension in the plantar fascia can create a supination moment at the STJ and why it is more difficult to get supination of the STJ in some feet and not others with dorsiflexion of the hallux in stance. (hubscher maneuver.)

    Take a medially defiated STJ axis foot. In this example, it pronates until there is a supination moment from the plantar fascia to stop the pronation. The foot has everted to a point where there is more force on the first met head than the other met heads. The first met head and hallux are still on the pronation side of (lateral to) the STJ axis, so ground readction force here is still causing a pronation moment at the STJ. The plantar fascia is attached to the base of the proximal phalanx. Through this attachment it creates the proximal push back up the first ray that creates the supination moment. However, the pull of the plantar fascia and forward push from the metatarsal head on the base of the proximal phalanx create a force couple that creates a plantar flexion moment on the proximal phalanx. This is what causes the functional limitus. So, the windlass is intimately related to rotational equilibrium about the STJ.

    Let me know if this helps.

  13. JasonR

    JasonR Member

    Thanks Eric

    I pulled your article on the windlass mechanism and I understand your descriptions of the simultaneous push- pull effects of the plantar fascia contributing to 1st met PF and to RF DF (in the case of increasing plantar fascial tension and a supinating foot).

    I don’t think I had fully appreciated that the outcome of say Jacks test (as it seems to be called down here), reflects one input in the sum of moments for the foot at a given position, load and point in time. i.e. windlass tension is a continuous variable that may or may not pass a threshold required to overcome any opposing pronatory moments…

    So if I understand correctly and the windlass does not meet that threshold, then that would be one of those feet that tests at 'heavy' (stiff) with Jacks? I would also think that more often than not, 'delayed' would accompany 'heavy'-
    Or more specifically?
    Heavy= high pronatory moments (or high internal 1st PF moments)
    Delayed= speaks more of when you cross that threshold where the sum of supinatory moments > pronatory??

    A foot with a medially deviated axis would likely mean that the equation is pre-loaded in the favour of high pronatory moments, hence greater windlass loads required to negate/ overcome this in order to demonstrate a supinating foot with Jacks.


  14. efuller

    efuller MVP

    I've seen the term "delayed" windlass before. I don't use the term myself and I'm not sure how it was defined. It seems like you understand the article quite well now.

    When you attempt to lift the hallux on a standing patient, and it is very hard to lift, it is hard to lift because of the plantar flexion moment created by tension in the fascia and compression at the MPJ joint surface. To get STJ supination with the windlass, the first met has to move proximally (pushing all the bones proximally, including the talar head.) With supination of the STJ the talar head moves proximally relative to the anterior facet of the calcaneus. This is how the proximally directed push on the talar head and distally directed pull of the fascia at the attachment on the medial tubercle of the calcaneus create supination motion at the STJ. This motion will occur when supination moment from the windlass is greater than pronation moments from other sources. So, yes, a foot with a medially positioned STJ axis is more likely to have a high STJ pronation moment from the ground that will resist the STJ supination effect of the windlass.

  15. Jason:

    Here is one of my illustrations of how the Windlass Effect of Hicks works.

    How does the Windlass Effect of Hicks actually work?

    1. Hallux dorsiflexion moment from examiner's manual force creates increased tension force within the medial fibers of the central component of the plantar aponeurosis (MFCCPF).

    2. Increase in tension within the MFCCPF causes an increase in posteriorly directed compression force at the first metatarsal head which, in turn, causes a first ray plantarflexion moment.

    3. Increase in first ray plantarflexion moment and posteriorly directed force on the talar head from increase in tension within the MFCCPF will cause an increase in subtalar joint (STJ) supination moment assuming STJ axis spatial location is not too far medially deviated.

    4. STJ supination will only occur if the net STJ supination moments created by manually generated hallux dorsiflexion moment is greater in magnitude than the net STJ pronation moments that are existing at any instant in time during the hallux dorsiflexion maneuver.

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