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Ten Biomechanical Functions of the Plantar Fascia

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Kevin Kirby, Jul 1, 2016.

  1. Petcu Daniel

    Petcu Daniel Active Member

    There is any .pdf version of this article ?

    Thank you,
  2. Petcu Daniel

    Petcu Daniel Active Member

    Maybe this is a stupid question but regarding point no.2 (Assisting In Resupination Of The Subtalar Joint During Propulsion) I'm asking if there is any situation when the orientation of the force vector from the origin of plantar fascia relative to position of STJA could generate a pronation moment around STJA ?! Here I'm thinking at Eric's explanations ( http://www.ncbi.nlm.nih.gov/pubmed/10659531 ) who is analyzing this situation in transverse plane view through the forces acting especially at the level of the talo-naviclular joint

  3. efuller

    efuller MVP

    Yes, plantar fascia tension will cause a net supination moment, the vast majority of the time. However, there are those rare feet where you will see internal leg rotation when you attempt to dorsiflex the hallux in stance. In the windlass article above I sited Jack who described feet that did this. The Jack article is where the term "push up test of Jack" came from. The push up test of Jack is another name for the hubscher maneuver. Both those names refer to attempting to dorsiflex the hallux in a standing individual. You really need to understand moments to explain how the same maneuver can cause internal leg rotation in one foot and external leg rotation in another. When tension is increased in the plantar fascia, many additional forces are created and you have to sum them all to figure out which motion will occur.

    Daniel, that was not a stupid question.

  4. Daniel:

    I will post up a pdf version of the article once I receive the magazine within a few weeks. Currently, I only have the online version.
  5. Daniel:

    The plantar fascia indirectly causes a STJ supination moment just as does a foot orthosis. A foot orthosis can not directly cause a STJ supination moment as does, for example, an ankle brace that actually crosses the STJ axis. Rather the foot orthosis indirectly causes a STJ supination by shifting ground reaction force (GRF) medially on the plantar foot.

    Likewise, the plantar fascia does not cross the STJ axis and so can not directly cause a STJ supination moment. However, the plantar fascia, by stiffening the medial column and thus increasing the GRF on the medial metatarsal heads also can indirectly cause a STJ supination moment while the heel is on the ground. During propulsion, the plantar fascia indirectly causes a STJ supination moment via the windlass mechanism.
  6. efuller

    efuller MVP

    The plantar fascia does cross the STJ and many other joints. It attaches to the medial calcaneal tubercle on the proximal side of the joint and the medial slip attaches to the sesamoids / 1st proximal phalanx on the distal side of the STJ.

    I'm not sure about the need to divide things into directly and indirectly. Tension in the plantar fascia will, in most feet, cause a supination moment from the anterior pull on the medial calcaneal tubercle and anterior to posterior push from the navicular on the talar head. (The fascia pulls the phalanx proximally, which pushes the metatarsal proximally, which pushes the cuneiform.., which pushes the navicular....) I've pulled and pushed on cadaver feet the few feet that I did this all supinated. However, if the forefoot was markedly abducted on the rearfoot, the proximal push from the navicular on the talar head could be medial to the distal pull at the attachment of the plantar fascia and this would create and internal rotation moment in the transverse plane that would tend to cause internal rotation of the rearfoot and lower leg as Jack described in his test.

  7. Eric and Colleagues:

    When I say the plantar fascia does not cross the STJ, I am referring to the fact that the plantar fascia does not pass from proximal to the STJ axis to distal to the STJ axis. The plantar fascia, therefore, remains always distal to the STJ axis and does not cross the STJ axis. Thus, the plantar fascia can not directly cause a STJ supination moment when the foot is non-weightbearing.

    Therefore, per my definition, the test for "direct" versus "indirect" ability of a structure to generate STJ moments is that in a "direct" STJ supinator, the structure can still generate a STJ supination moment if the foot is non-weightbearing. If the structure can only increase STJ supination moments while the foot is weightbearing, then I call it an "indirect" STJ supinator.

    I believe that this is an important distinction that needs to be made and may even deserve to have a paper be written on it (or at least one or two newsletters). However, I do acknowledge that there may be better terminology, rather than "direct" versus "indirect", to describe such differences in the ability of certain structures of the foot and lower extremity at generating STJ moments in both a weightbearing and non-weightbearing situation.

    Therefore, in the case of the plantar fascia, since it cannot generate a STJ supination moment while the foot is non-weightbearing, I consider the plantar fascia to be an "indirect" STJ supinator. Structures that can "directly" cause a STJ supination moment include the calcaneo-fibular ligament, tibio-calcaneal ligament, anterior tibial, extensor digitorum longus, extensor hallucis longus, peroneus tertius, posterior tibial, flexor digitorum longus, flexor hallucis longus, peroneus brevis, peroneus longus, gastrocnemius and soleus muscles and their associated tendons.

    Good discussion.:drinks
  8. efuller

    efuller MVP

    I'm still having a problem with not crossing the STJ. I don't think speaking of the joint itself or the joint axis makes much difference. I can see how if you made a transverse plane cut under the posterior facet that both ends of the plantar fascia are plantar to that transverse plane cut. However, if you make a vertical plane passing through the posterior facet the attachments of the plantar fascia are different sides of that plane. If you made a vertical plane passing through the axis the attachments of the plantar fascia would be on opposite sides of that plane.

    When a force is applied across multiple joints, the one that moves is the one with the least resistance. Just because a joint doesn't move much does not mean that moments are not created. So, take a non weight bearing foot and apply a dorsiflexion moment to the hallux. Usually what you will see is dorsiflexion of the toe and plantar flexion of the metatarsal. However, the forces that create STJ supination moment are still there. There is much less resistance to metatarsal plantar flexion so that motion occurs first. The inertia of the foot and the force of gravity acting on the foot create a greater resistance to the supination moment from tension in the fascia so, non weight bearing you don't see supination, but the moments that cause supination are still there.

  9. The plantar fascia never crosses the STJ axis. It remains plantar (i.e. inferior) to the STJ axis throughout it's course. Think of it this way, can the abductor hallucis muscle, which travels parallel to the plantar fascia, cause a STJ supination moment in a non-weightbearing setting? No, because it does not cross the STJ axis. However, all the extrinsic muscles of the foot do cross the STJ axis, since they originate proximal to the STJ axis, and insert distal to the STJ axis, and I would consider these extrinsic muscles of the foot to be able to produce "direct" STJ supination moments.

    The instant you manually place a force on the foot, then I would consider that to be a "weightbearing" load on the foot. The terminology is not perfect, but the concepts are important since it gets to the point of why foot orthoses can be so effective at causing STJ supination moments in some feet, but not so effective at causing STJ supination moments in other feet, and why foot orthoses do not actually "restrict" STJ motion. Since foot orthoses do not cross the STJ axis, they are not near as efficient at causing STJ supination/pronation moments as ankle braces, Richie Braces, Arizona Ankle Braces, and other types of AFOs that do cross the STJ axis and use a three-point force system to stabilize the STJ axis.


    Great discussion!:drinks
  10. Petcu Daniel

    Petcu Daniel Active Member

    In my understanding, the problem is not if the planar fascia crosses or not the STJ axis but a problem of the decomposition of the vector of the proximal force exerted in the plantar fascia. In a reference system including the STJ axis as one of its axis, the moment arms and the magnitudes of the components of the proximal force vector exerted in the plantar fascia from the reference plane perpendicular on the STJ axis are small. I've attached an image adapted after one of the Kevin's diagrams (hope this is Ok! I apologies if not!) This adaptation should be somehow seen in 3D... ! I think the force exerted through the first ray and navicular on talus as result of the windlass, has more potential to create supination or pronation moments than the proximal force exerted in the plantar fascia, which main role is a stabilizing one !


    Attached Files:

    Last edited: Jul 6, 2016
  11. efuller

    efuller MVP

    I think looking at only inferior to a joint is an artificial distinction. We could turn this on its side and look at the ankle joint. An analogous statement is that the gastroc and soleus muscles don't cross the ankle joint because both origin and insertion are posterior to the ankle joint. How is this different? Why should we look at anterior and posterior or inferior and superior for deciding if a structure crosses a joint?

    The talus is on one side of the STJ and calcaneus is on the other side of the STJ. One attachment of the plantar fascia is on the calcaneal side of the joint and the distal attachment of the medial slip of the plantar fascia is on the talar side of the joint. Tension in the fascia will tend to push the talus towards the calcaneus, just as tension in the Achilles will pull the calcaneus toward the tibia.

    This goes to my previous post about when a structure crosses multiple joints it will create moments at each of them. The joint that moves is the one with the least resistance. Yes if you were to take a cadaver foot and magically hang it in space and then attempt to pull either end of abdcutor hallucis closer together, you would see plantar flexion of the hallux. If the hallux was prevented from plantar flexing you would see the metatarsal plantarflex, If you prevented the metatarsal from plantarflexing you would see STJ supnation as the muscle does create a supination moment, in most feet. How do I know this? When you pronate the STJ, the distance from the metatarsal head to the medial tubercle of the calcaneus increases. (Anterior break in the cyma line with STJ pronation on x-ray) Why do you think that the abductor hallucis does not create a supination moment? (it does cross the STJ)

    The supination moment created by the abductor hallucis, or plantar fascia may be smaller at the STJ than at the plantar flexion moment at met cuneiform joint or the resistance to motion may be larger at the STJ than the met in the non weight bearing foot, so the motion that you would see first may not be STJ motion, but the moment is still created.

    I have thought a lot about how three point force systems work. A lot of it comes down to Newton's third law. When a brace applies a force to the foot, the foot applies a force to the brace. The reason that you need to "cross" a joint with a brace is that you need to hold the brace in a position with enough force to prevent the force from the foot, applied to the brace, from deforming the brace. If the brace is deformed the force it can apply to the foot will be decreased.

    The braces apply forces that would not normally be present when standing. A foot orthoic tries to alter the location of forces that already exist. Sometimes you can't move the forces far enough to get the leverage you need to reduce the stress enough on the injured structure. When you can't change the moment enough by shifting ground reaction force then you need to other external forces from, among other things, braces.

    I agree it is a great discussion :drinks

  12. Good point, Eric. I will need to spend more time on the analysis to see how we can better refine the ability of ligaments to produce subtalar joint moment. I think Daniel's drawings help me see this much better. Thanks Daniel!

    Agreed. I have considered this concept previously and think the idea of understanding the stiffness of each joint being subjected to rotational forces (i.e. moments) will best determine the movement at the joint. In other words, the stiffer the joint, the less the rotational motion per given increase in magnitude of moment. In addition, the more compliant the joint, the greater the rotational motion at the joint per given increase in moment. Another factor that would come into play, of course, is the direction, line of action, and moment arm of force vector relative to the axis of motion of each joint.

    Agreed. The three point force system of a brace relies on passive application of external forces to a dynamic structure. The brace will cause increased magnitude of external force being applied on each of the three points of the brace when larger magnitudes of "unwanted" joint motion occur and will caused only slightly increased magnitudes of external force when smaller magnitudes of "unwanted" joint motion occur. In this way, internal stresses are reduced on injured structures in much the same way that foot orthoses reduce internal loading forces and stresses.
  13. Petcu Daniel

    Petcu Daniel Active Member

    I think a good article in this sense is : Eric G. Meyer et al., "Determination of Ligament Strain During High Ankle Sprains due to Excessive External Foot Rotation" in Sports" IRCOBI Conference 2012. One reason is because it shows a situation where there is a need for an opposite effect of the flexible footwear relative to that of the three point system, introducing the notion of "rotational stiffness" of the footwear.

    Abstract. High ankle sprains represent a severe injury in sports. External foot rotation is suspected in these cases, but the mechanism of injury remains unclear. The objective of the current study was to integrate in vitro and in vivo experiments along with computational models based on rigid bone surfaces and deformable ligaments of the ankle to investigate the external foot rotation injury mechanism with different shoe constraints and ankle positioning. Injuries and the highest strains occurred in the anterior deltoid ligament (ADL) when the foot was held in neutral with athletic tape. Similarly, ADL strains were highest when a football shoe design with a high rotational stiffness was used to constrain the foot. For a flexible shoe, the anterior tibiofibular ligament (ATiFL) strain was increased and ATiFL injury occurred due to increased talar eversion. In human subjects performing a similar movement, the highest strains also occurred in the ATiFL and ADL. The models showed that ATiFL strain was positively correlated with ankle eversion, but eversion decreased strain in the ADL. Finally, the consequence of eversion on ATiFL strain was confirmed in the first cadaver study that consistently generated high ankle sprains in the laboratory.

  14. For those interested, here is the pdf of the article "Understanding Ten Key Biomechanical Functions of the Plantar Fascia" that I authored and was just published in the July 2016 issue of Podiatry Today.

    Kirby KA: Understanding ten key biomechanical function of the plantar fascia. Podiatry Today, 29(7):62-71, 2016.
  15. Ian Linane

    Ian Linane Well-Known Member

    Thank you Kevin.

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