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Plantar intrinsics: Why do we need them and what do they do?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Sep 7, 2008.


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    All,

    If we look at the EMG data for the plantar intrinsics: abductor hallucis, adductor hallucis, flexor hallucis brevis, flexor digitorum brevis, interossei and lumbricles, their phasic activity begins as the extrinsic ankle plantarflexor activity begins to die down.

    Question: why do we have short flexors as well as long flexors? i.e. why don't we just have long flexors that fire longer?

    Question: it's commonly cited that the short flexors function is to create compressive force of the phalanges against their metatarsal heads. Given that the forefoot is relatively fixed against the floor, while the rearfoot/ midfoot is off the ground when these muscles are firing, isn't the greater kinematic effect likely to be seen at the open chain end of the muscle, i.e. its origin???


    Just a bit of fun to get you thinking
     
    Last edited: Sep 7, 2008
  2. David Smith

    David Smith Well-Known Member

    Re: plantar intrinsics why do we need them and what do they do?

    Simon

    In answer to your question the first thing that occurs to me is control of foot stiffness without increasing internal ankle joint complex moments. If all the work was left to the extrinsic muscles then, as a certain ray or set of rays was required to have greater stiffness to GRF, this would neccessitate an increase in moments produced by the appropriate extrinsic and, if balanced by its antagonist, then this would also reduce the stiffening effect of the primary action. There would then also be the effect of increased joint compression and internal tendon stress and perhaps this may not be good in terms of unimpeded CoM progression over the planted foot. If there is less tension in the extrinsics then there may be less tendency to slow the CoM velocity.


    Cheers Dave Smith
     
  3. Re: plantar intrinsics why do we need them and what do they do?

    Dave,
    Thanks for taking the time to reply. The work of Carlson RE, Fleming LL, Hutton WC: The biomechanical relationship between the tendoachilles, plantar fascia and metatarsophalangeal joint dorsiflexion angle. Foot Ankle Intl., 21:18-25, 2000 and Erdimir A, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA: Dynamic loading of the plantar aponeurosis in walking. JBJS, 86A:546-552, 2004, suggests that plantar fascial tension is directly influenced by achilles tendon tension. This makes mechanical sense since they both attach to the calcaneus. Since many of the plantar intrinsics also have attachments at the calcaneus, it would seem reasonable to hypothesise that the tension in the plantar intrinsics is capable of influencing the tension in the achilles tendon also and vice versa. Therefore, it may be argued that many of these muscles will influence ankle joint complex moments too.

    I'm not sure I follow your logic here, Dave. Surely the same argument can be applied to the intrinsics: flexor digitorum brevis and extensor digitorum brevis?

    Given that the function of the intrinsics is often cited as being to increase joint compression, I'm not sure I follow your argument re: compression and CoM progression. Why should the continued ankle plantarflexion from the extrinsics decelerate the CoM?
     
    Last edited: Sep 8, 2008
  4. More things to think :cool:
    What are the actions of the following muscles at the subtalar joint:
    Abductor hallucis
    Flexor digitorum brevis
    abductor digiti minimi
    Quadratus plantae
     
  5. David Smith

    David Smith Well-Known Member

    Re: plantar intrinsics why do we need them and what do they do?

    [​IMG]

    Just thinking on my feet so feel free to blast away ( as if you needed permission)

    Cheers Dave:drinks
     
  6. Dave,
    You lost me at the outset when you said:

    "During stance phase the achilles can have no tension"

    Please explain, then I'll come back to the rest of your post.

    P.S. while I find your scribbled diagrams "interesting", I have no idea what you are attempting to demonstrate with this one! ;-)
    Moreover, I don't understand why you believe that Abductor hallucis, Flexor digitorum brevis, Abductor digiti minimi and Quadratus plantae only produce moments Lisfrancs joint
     
    Last edited: Sep 8, 2008
  7. David Smith

    David Smith Well-Known Member

  8. pretty much- they acting like strain gauges
     
    Last edited: Sep 8, 2008
  9. David Smith

    David Smith Well-Known Member

    What I mean is for the sake of this argument AT can have no tension because the plantar vault requires no AT tension to maintain its integrity. When body weight is balanced by GRF at each end of the arch it only requires a tensor across the base for stability. In reality the AT has tension but this may or may not be relative or correlated to the plantar intrinsic tension. The AT tension is only neccessary to enable useful moments that raise the CoM and increase its potential energy and give ground clearance for the swing leg and then add some propulsion along with other muscles.

    [​IMG]

    Cheers Dave
     
  10. David Smith

    David Smith Well-Known Member

    Simon

    P.S. while I find your scribbled diagrams "interesting", I have no idea what you are attempting to demonstrate with this one! ;-)
    Moreover, I don't understand why you believe that Abductor hallucis, Flexor digitorum brevis, Abductor digiti minimi and Quadratus plantae only produce moments Lisfrancs joint

    Of course moments are produced at any point of interest but for this model I only consider moments about that joint. How in depth can I be with a hastily scribbled drawing. I can only consider a simple model as an example to help the argument.

    What I am trying to convey with my diagram is the three articular segments of this model I am using. I.E. Unit 1 the rearfoot segments that are all held fairly rigidly by the stiff conjoining ligaments. Unit 2 the forefooot segments that rotate about the Nav-cuneiform joint that is not substantially tied to the reafoot by ligaments that restrict its compliance. Unit 3 is the entire foot about the ankle joint (TC) and is only possible when the other two units are stable in relation to each other and less compliant than unit 3.

    Dave
     
  11. But the evidence suggests that achilles tension is correlated to plantar vault tension. So going back to your original point would additional extrinsic plantar flexor moment decelerate the CoM? http://www.asbweb.org/conferences/2004/pdf/146.pdf
     
  12. David Smith

    David Smith Well-Known Member

    Simon

    That seems pretty cool and a good answer to your original question.

    If the intrinsics were soley slave controlers based on stress strain or force length relationships, then would they need to have intrinsic ability to adjust their application of force. They must have an active part to play as well I would have thought.


    Intrinsics in the hand increase dexterity that the larger more gross actions of the extrinsics could not deliver. This may have applied to the foot when it was more like a hand, as in apes. The adaption to weight bearing bipedal ambulation may have been the introdution of the stretch sensors. I have read that evolution only evolves to the point of good enough to survive for the time and not to optimum.
    Zebras didn't evolve with machine guns to kill loins, was the example given, they just were able to run and kick enough for most of them to survive.

    Therefore now we have a foot with intrinsics that can do fine control but are more useful as slave contoler units. We could have evolved to use just the extrinsics for joint control and the intrinsics for control sensors but what we have is good enough.

    What do you make of that?

    Cheers Dave
     
  13. Dave, what you said was:
    This just isn't true.
     
  14. Of course throughout this thread I have been arguing the point I do not strictly believe, I'm sure there is an expression for this in philosophy, but I could not tell you what that is. I think the role of the intrinsics is poorly understood, hence this thread. I think you are right that the plantar intrinsics act to moderate the stiffness of the foot, yet their role in controlling the extrinsics is important and not recognised by most.

    Cheers Dave, thanks for your time and brain.:drinks
     
  15. David Smith

    David Smith Well-Known Member

    Simon

    I obviously agree that that AT tension is generally shown to correlate with Plantar intrinsics tension. If there were no tension then you would fall flat on your face after a couple of steps. The answer to whether increased plantar flexor moments decreases CoM velocity is that it depends where the position of the CoM is when you measure the plantar flexor moment.

    I've read the paper cited here and its pretty standard as far as I can see.
    Got to go home now but I'll get back to this tommorrow. I need some sustenance.
    (don't have internet at home otherwise I'de never go to bed)

    Dave
     
  16. That's why I cited this paper. Note the reduction in plantarflexion moment early in stance in the acceleration trials. "Good night Dave.."
     
  17. efuller

    efuller MVP

    Re: plantar intrinsics why do we need them and what do they do?

    Simon and Dave,

    Be careful with corelation and causation. Hicks described quite well how the Achilles tendon shifted the center of pressure forward and that forward shift of center of pressure increased bending moment on the midfoot. The increased bending moment has to be resisted to prevent arch collapse and the plantar fascia can resist that bending moment. So an increase in Achilles tendon tension will cause an increase in plantar fascial tension. But an increase in plantar fascial tension does not necessarily lead to an increase in Achilles tendon tension.

    I think I see Dave's point about zero achilles tendon tension. Say you had a cut Achilles tendon and you dorsiflex the ankle (rotate the tibia forward at the ankle joint) At some point there will be resitance to further motion. Certainly this resistnace could come from the neck of the talus hitting the anterior plantar aspect of the tibia. Further rotation of the tibia will create a plantar flexion moment on the foot, an anterior shift in center of pressure, and increased tension in the fascia.

    So, we can look at the research and see a corelation, but that does not give us a mechanical explanation.

    Cheers,

    Eric Fuller
     
  18. Re: plantar intrinsics why do we need them and what do they do?

    Eric,
    I take your point. The key difference between the plantar fascia and the intrinsics is that the intrinsics are contractile tissues and thus are not reliant upon an external force generator.
     
  19. efuller

    efuller MVP

    Interesting question Simon,

    Some musings on intrinsics.

    The lumbricales are a swing phase muscle that helps in toe clearance in barefoot walking. Non barefoot walking they are not used/ needed as much and toes are not straightened and perhaps could be responsible for hammertoes/dorsal mpj contractures in folks who wear shoes more often. Again just a musing

    The intrinsics would help individual toes bear more weight compared to their neighbors. My kids love walking barefoot and are able to do it on surfaces my tender feet cannot handle. There has to be some learned adaptations to this ability to walk on pebbles barefoot. If the intrinsics are more active after the heel lifts off the ground, as was implied by the above EMG data, then it is plausable that the intrinsics are acting to better distribute weight in an unshod foot.

    The abductor hallucis essentially has the same attachments as the plantar fascia so a mechanical analysis of the plantar fascia would be pretty much be the same as the abductor hallucis. My paper on the windlass describes how the windlass can create a supination moment in some feet. Therefore the abductor hallucis could do the same.

    On the reversal of the origin and insertion: The mass of the muscles is so small relative to the inertia of the body that it seems that it would be improbable that the intrinsics could dramatically change the course of the body.


    On the idea that the role of the intrinsics is to increase compressive force at the metatarsal phalangeal joints: I've heard this logic and it does not make sense to me. Why is this a good use of energy by the body? Most muscle contractions increase compressive force at joints, but usually there is a force couple involved that will generate a moment at the joint.

    Just some thoughts,
    Eric
     
  20. David Smith

    David Smith Well-Known Member

    Simon

    I did some analysis this morning of a multi segmented (3 joints and 4 segments) limb with flexion and extension components in terms of joint position. The model I used was a finger type construction where the IPJ's are flexed and the proximal Joint (MCJ) was extended. I found that, where there is both flexion and extension components, it is impossible to balance moments across all joints with one flexor extensor set. Of course where there is only flexion in all joints it is possible to balance moments with one set. (This is how an arch maintains integrity ie like the foot plantar vault can maintain its integrity with just the intrinsics)

    Infact you require one set for the distal joint, two sets for the mid joint and two sets for the proximal joint, one of which overlays the mid joint. In other words there are three flexor / extensor sets required to balance the moments about each joint in the model.

    It is possible for two of these sets to be intrinsic and only one set extrinsic to the whole limb. It may also be possible that all the sets could be extrinsic but it is not possible to calculate that since there are to many unknowns and not enough equations to find them all.

    However its would seem to me that containing all the extensor flexor (EF) sets in the limb extrinsic to the joints and segments of interest may be a waste of space and tissue and mechanically difficult to design. The EF sets that control distal joint only require low tensional force since the external forces only have short levers to those joints and the more distal the joint is to the force applied at the distal segment then, generally the tensional force required for moment balance will be greater.

    Therefore having smal short intrinsic muscle tendon units controling short levers at the distal segments and long large muscle tendon units controling large moments balances at the proximal joints would kind of make sense intuitively.

    In this way it is also possible to reduce internal stress in the distal tendons that are required to be thinner because the whole distal segment is required to be smaller. If you had a large extrinsic muscle ending in a very thin tendon at th edistal segment it (the tendon) would potentially be subject to very high stresses if that muscle worked nearer to its max force value. Whereas a small muscle intrinsically positioned could not develop these potentially pathological stress levels. As you know it is stress, not force that ultimately causes mechanical failure of a material.

    If we also consider that the small intrinsics were already there ready for use from when the foot was more hand like then it also make s evolutionary sense that these muscles are retained as is, but used slightly differently. If we then factor in the slave controller element from the research that you cited earlier then this complete model looks quite viable and reasonable.

    What do you think?

    All the best Dave
     
  21. I think this is a good analysis of the situation.
     
  22. I guess a good way of analysing what the intrinsic muscles do is to look at the data Erin Ward has produced with his cadaver walking simulator- here the feet mounted in the simulator have no input forces to the intrinsics (N.B. it still walks!). Perhaps comparing the data produced using these walking simulators with in-vivo data will reveal exactly what the intrinsics are adding to the party.

    http://www.isb2005.org/proceedings/abstracts/0546.pdf
     
    Last edited: Sep 9, 2008
  23. David Smith

    David Smith Well-Known Member

    Simon

    Just so you can't accuse me of always being a scribbler and also so you (and Eric)can check out my analysis to see if you think it still makes sense, I have drawn up and attached a PDF of the analysis I previously mentioned.:dizzy:


    LoL Dave
     

    Attached Files:

  24. Dave, thanks for taking the time to do this, it is much easier to read. I've got diagram 1 trying to get my head around no2. I'm not sure about the arrangement of the tendons in relation to the foot. I'm too tired to think about it now but I'll try to take the time tomorrow.
     
  25. Dave, I've been looking again at your second diagram this morning and confess I am unable to see how this relates to the arrangement of intrinsic and extrinsic muscles and the joints of the foot. I appreciate that this is a hypothetical model, but in attempting to relate it to the previous discussion, I am unable to work out why you have arranged the tendons in the way that you have. Perhaps you could explain?

    Lets go back a step or two for those who don't like algebra and acronyms, my original question:
    Why do we have short flexors as well as long flexors? i.e. why don't we just have long flexors that fire longer?

    What is the actions of flexor hallucis longus at the following joints:

    inter-phalangeal joint?
    metatarsophalangeal joint?
    1st metatarsal cuneiform joint?
    medial cuneiform- navicular joint?
    talonavicular joint?
    subtalar joint?
    ankle joint?

    What is the actions of flexor hallucis brevis at the following joints:

    inter-phalangeal joint?
    metatarsophalangeal joint?
    1st metatarsal cuneiform joint?
    medial cuneiform- navicular joint?
    talonavicular joint?
    subtalar joint?
    ankle joint?
     
    Last edited: Sep 11, 2008
  26. David Smith

    David Smith Well-Known Member

    Simon

    I'll answer your question in two seperate posts.

    First you wrote

    Ok so as you appreciate I can't analyse the intimate configuration of the foot structure without a very sophisticated Finite Element modeling system.
    So what I am trying to show is that where the foot already had intrinsics it is possible and often desirable to use them to balance moments about a multi jointed limb where the applied and reaction forces are well defined and uncomplicated. The hand on the otherhand has loads and reactions applied from muti directions due to the way it is used, therefore intrinsics are obviously very useful for fine contol of balancing forces.

    The foot has huge gross loads that would break the hand if applied. Therefore more robust structures are likely to be required. Introduce the Plantar fascia (PF) spring ligament complex (SLC) plus some heavy weight extrinsics like FHL and EHL.
    The Tibialis Anterior and Achilles Tendon could represented in my diagram 2 by the red tensors Ec and Fc. E and F are extensor and Flexor and if transfered to diagram 1 woulld appear in the same place ie above and below the proximal segment of Joint Nc.

    If you refer to the diagram below this shows a new configuration of the four segments of diagram 1. In this diagram here the calcaneous is included and the AT inserted at the base. how ever the Spring ligament complex ties the calc rigidly back to the segment distal to joint Nc. These beome similar then in action to the diagram 1 tensors Ec, Fc.

    With the addition of calc, SLC and PF this will signifcantly change the balancing properties of the intrinsics relative to the actions of these additions. Therefore it is possible that to accomodate these changes more intrinsics and extrinsics are required.

    The original simple model was however viable and reasonable under certain circumstances and allowed consideration of forces and their actions and assumption made about their mechanical properties and limitations.

    [​IMG]

    As we add extra tensors (and compressive units like the calcaneous) to the model we still keep the original configuration but increase the range of operating capacity to suit the loading requirements.

    I would expect that if we made a robot with human like feet, that walked on a flat laboratory floor, we could use only extrinsics to balance the model. However when walking in bare feet on natural ground and doing other things besides walking, eg climbing trees or cliffs or using as a hand substitute, the intrinsics become much more valuable and useful in their actions. Keeping the intrinsics in the model may therefore be useful and unavoidable in evolutionary terms. Again if they are also used as slave contoller signaling units for the firing of extrinsics then this adds to their usefulness.

    Cheers Dave
     
  27. David Smith

    David Smith Well-Known Member

    Simon

    You wrote
    Now Simon, as you are obviously trying to push in this direction, it is clear that this follows on nicely from my last post. The 2D analysis indicates problems of 2D balance of moments ablout a certain joint of interest. Now if we move to a 3D view we can see that there are 3Dimensional considerations to the problem of control of joint action. The intrinsics in Simon's example above indicate that EHL and EHB are synergistically antagonistic in their actions about the joints of the forefoot. Therefore due to the anatomical tracking of each muscle, a balance of structural integrity in the fore foot is allowed. Therefore it may be assumed the 3D integrity of the whole foot could rely on the synergistic actions of the intrinsics and extrinsics.

    It is not only convenient for muscles to work in this way but also neccessary. This system builds in redundancy, whereby if one muscle tendon unit fails or works sub optimally then due to the oblique nature of the action of other muscle tendon sets they can take over some of the work done by the damaged unit. This however may result in observable kinematic and morphological changes and internal stress levels that may become pathological within the substituting muscle tendon sets.

    I think the above discussion that Simon started in his deliberately thought provoking style, indicates how, while robust and reliable in normal circumstances the internal forces are well balanced within the foot, in pathology they become tenuous and unpredictable. However with the use of orthoses, physical therapy and manipulative techiques it may be possible to restore the integrity of a failed or weak unit and / or substitute external forces via and orthosis that will reduce stress on the substituting muscle tendon sets.

    Cheers Dave
     
    Last edited: Sep 11, 2008
  28. David Smith

    David Smith Well-Known Member

  29. Thank you Dave,

    But the gist of what you have said is were I is at. What I wanted to explore here was the similarities and differences between the long and short flexors. Lets assume that Dave has the correct action for the muscles at the joints I listed so lets spot the differences in Dave's list:


    1st metatarsal cuneiform joint? FHL adduction / FHB ext rotation, abduction

    medial cuneiform- navicular joint? FHL adduction internal rotation / FHB ext rotation, abduction

    talonavicular joint? FHL flexion adduction intenal rotation / FHB abduction

    ankle joint? FHL plantarflexion (extension) / FHB none

    I'm not 100% sure about the action Dave lists for FHB at the IPJ: "flexion" as it doesn't generally cross the IPJ- are you counting an action here through the sesamoid apparatus???

    so perhaps we could add:
    inter-phalangeal joint? FHL flexion / FHB None

    What is the functional significance of these differences during gait? So what would happen if the FHB didn't fire and the FHL fired to the end of contact? Would the differences be cancelled out by the other intrinsics?
     
  30. David Smith

    David Smith Well-Known Member

    Simon

    You wrote
    Oh no my mistake FHB probably doesn't do anything to the hallux IPJ.

    Dave
     
  31. David Smith

    David Smith Well-Known Member

    Simon

    You wrote
    1) Medial displacement and external rotation of the 1st ray similar to that seen in HAV.

    2) Possibly they could be balanced by -

    A) Adductor hallucis = adduction or lateral displacement in terms of body midline plus some internal rotation.

    B) Ext Hallucis brevis lateral displacement and some internal rotation. but with unwanted extension.

    C) Plantar fascia to balance unwanted extra extension.

    Dave
     
  32. Simon, Eric and David:

    Sorry I haven't been able to contribute to this interesting thread earlier. I've been setting up a new computer.:bang:

    The plantar intrinsics obviously have important functions for the foot, but certainly they don't have the muscle strength to contribute greatly to gait function as do the extrinsic muscles of the foot. I consider the plantar intrinsics, as a whole, to play an assistive role for reducing the passive tensile forces within the plantar ligaments and plantar fascia, for resisting longitudinal arch flattening (i.e. increasing the forefoot dorsiflexion stiffness), for maintaining digital purchase and for preventing digital deformities.

    Together the deep flexors and peroneus longus are the only extrinsic muscles of the foot that cause a forefoot plantarflexion moment which will not only resist longitudinal arch flattening but will also increase forefoot dorsiflexion stiffness. The plantar intrinsics, plantar ligaments and plantar fascia all work together synergistically, along with the deep flexor and peroneus longus muscles, to perform these same functions.

    Can the foot function without the plantar intrinsics? Yes, an individual could probably walk and even run for some time without plantar intrinsic muscles. However, the loss of the function of the plantar intrinsic muscles would tend to, over time, cause flattening of the longitudinal arches, would tend to lead to digital deformities and would tend to make the subtalar joint more pronated during weightbearing activities.

    It must be remembered, when we observe a foot that has a normal appearance and has good gait function, this is due to a delicate balance of all the muscles, ligaments and bones of the foot and lower extremity interacting in harmony with each other and the central and peripheral nervous systems to produce and maintain this normal structure and function. Certainly, the loss of muscles even as small and seemingly insignificant as the plantar intrinsic muscles will upset this harmonious balance within the body and increase the risk of painful pathology, deformity and abnormal gait function occurring over time.
     
  33. Ben

    Ben Member

    I have been thinking about this for some time in regards to symptoms at the anterior calcaneal tubercles. Simon you mentioned that the intrinsics are contractile tissue and when the extrinsics are not functioning correctly we will get an overload of these intrinsics. This should then increase its contractile force at its origins, and in my mind makes a lot more sense that this is the cause of the common plantar heel pain and not so much as the origin of the plantar fascia. I also think that when these intrinsics are functioning normally, they are going to contribute to the windlass mechanism extensively and mainting and increasing the MLA height during propulsion.

    Hope that makes sense... in a hurry.

    Ben
     
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