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Intrinsic muscles and pes planus

Discussion in 'Biomechanics, Sports and Foot orthoses' started by salihangin, Apr 5, 2012.

  1. salihangin

    salihangin Member

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    Hi all,

    I am wondering if any updated knowledge about morphological changes of intrinsic muscles in foot with pes planus.

    Many thanks,
  2. Craig Payne

    Craig Payne Moderator

  3. salihangin

    salihangin Member

    Thanks Craig. Since I am very new in here, I have been reading all posts related to pes planus / overpronation / hyperpronation, and discussions about orthotics that strengthen the intrinsic muscles. This must be the mechanism that foot orthotics align all segments back to their optimal position, thus lever arm and pulling angles are restored to normal and muscle may work efficiently.

    Do intrinsic muscles morphologically atrophic, or weak (insufficient force exertion) in the foot with pes planus?


    (Apology for language error I might made because of I am not a native English speaker)
  4. Craig Payne

    Craig Payne Moderator

    The rhetoric and propaganda is that they do, but those that claim this need to explain the following:

    1. During gait, the intrinsic muscles do not fire until around the moment that the heel comes off the ground, so in static stance they generally are not active, YET, there was the study on pes planus/flat feet (sorry, forgot the reference) from a zillion years ago that using EMG showed the intrinsic muscles are very active during static stance --> this implies that the intrinsic muscles are working very hard in the flat foot during the whole stance phase and not just during propulsion, so how do they atrophy and become weaker? They must actually be stronger as they are working so hard!

    2. A weakness of the intrinsic muscles in some pathological conditions (eg the 'intrinsic minus foot' in diabetes and the initial intrinsic muscles weakness that occurs in CMT) actually leads to a higher arched foot or at least the appearance of a higher arch foot due to the wasting of the intrinsic muscles.

    I keep asking those who parrot the propaganda and rhetoric to explain or answer the above two issues. They never do. They go away and parrot the same propaganda and rhetoric somewhere else!!!
  5. The intrinsic muscles are active during midstance and are less active after heel off.
  6. Craig Payne

    Craig Payne Moderator

    Thats not my reading of that chart. Most of them are starting to fire late midstance, just as load is coming off the heel into the propulsive phase.

    BTW - by "heel unweighting", I did not mean "heel off", I meant just after the tibia has gone past vertical to start heel "unweighting"

    Also look how much more active they are in the flat foot! How can they be weaker if they are having to work that harder over a longer period of time? I which those touting the propaganda and rhetoric could come along and explain this ????
  7. pod29

    pod29 Active Member

    Hi All

    I may be add some additions to this discussion as we have only just finished collected a bunch of data looking at the recruitment of these muscles under varying loading conditions, with reference to foot kinematics (using a multi-segment foot model). I haven't analysed the data yet, but looking at it subjectively it appears that activity in these muscles may be related more with mid-foot deformation/ mid-foot dorsiflexion (possibly due to a stretch reflex?) than whether the foot is planus or cavus to begin with. Another thing that was quite obvious is that these muscles won't be activated until after the ground reaction force is anterior to the ankle joint, which also adds to the argument that these muscles play a significant role in applying a plantar-flexion moment to the midfoot.

    Very general observations, will hopefully have some more meaningful results in the not too distant future.

    Last edited: Apr 6, 2012
  8. I tend to agree
  9. Craig:

    Here is what you wrote: "During gait, the intrinsic muscles do not fire until around the moment that the heel comes off the ground."

    I think that most reasonable people would interpret this sentence as meaning that "during gait, the intrinsic muscle do not show electromyographic activity until the propulsive phase of gait, after heel off".

    A more accurate way of stating the true electromyographic activity of the intrinsic muscles, given our available data, is to say they are active throughout the late midstance phase and into early propulsive phases of gait.
  10. salihangin

    salihangin Member

    Hi all,
    Since intrinsic foot muscles (IFM) more active in pes planus compared to normal foot, they would have already been reached to steady level of their strength in long term. There would not any space been left for further strengthening by exercising or foot orthotics. Wouldn't we expext some morphological changes such as hypertophy of the muscles and increase in volume and number in muscle fiber in pes planus? This is a kind of rope pulling game, team with more men wins.

    Many thanks,
  11. Craig Payne

    Craig Payne Moderator

    OK ... its was a typo and not what I meant ... it was probably the middle of the night. I meant "when load starts to come off the heel" rather than "heel off"
    ...which is when "when load starts to come off the heel"

    In the absence of evidence, I try to apply 3 criteria:
    1) Its theoretically coherent
    2) Its biologically plausible
    3) Its consistent with the available evidence

    ....some of the paradoxes in this (ie intrinsic muscles working hard in the flat foot, so can't be weak) and the evidence that foot orthoses strengthen the intrinsic muscles (ie how can they if the muscles already strong?) are problematic.
  12. Rob Kidd

    Rob Kidd Well-Known Member

    I have not read Mann and Inman since I was a PhD student, about a squillion years ago. My memory is that the take home message was that the intrinsics worked longer and harder in "flat" feet than in "normal" feet. From where I am sitting (a campervan in the Barossa Valley), that is entirely logical. Rob
  13. Craig:

    Rather than indicating that a muscle group is firing "when load starts to come off the heel", which, during walking, could be anywhere from the initiation of midstance to the termination of midstance, why not use more precision to indicate when a muscle group is active? I think if you simply stated that the plantar instinsics started to fire in the middle of midstance, this would be much more precise and less ambiguous than saying "when load starts to come off the heel".:drinks
  14. drsha

    drsha Banned

    My take on this relates to Dr Dananberg's concept of muscle engine inhibition or exhaustion and relates to compensatory threshold.
    Theoretically, I believe that when tested from rest the intrinsics are seen to be very active, they exhaust rather quickly and force compensatory muscles to overwork and which become stronger (triceps, EDL, for example)
    My biological plausibility is that if flatfooted subjects were tested in a similar manner as the Mann and Inman Study at the start of gait, then retested at lets say 15 minute intervals for 1 hour without rest, I predict these muscles would show reduced and possibly markedly reduced function causing them to be weaker from disuse during daily activity.
  15. salihangin

    salihangin Member

    Thanks Dennis, I agree with you. We would make an assumption that prolonged activity of the intrinsic muscles during gait does not simulate a strengthening exercise, which is essential to perform within the pattern of primary function of the muscle to be strengthen. However, seems, we have yet to have the evidence to prove whether intrinsic muscles in pes planus become weak and atrophic or strong and hypertrophic.

  16. Dennis:

    I believe you are probably right here. Nice reply!

    Much of the plantar intrinsic muscle fatigue that would be expected in people with flatter arch contours likely results from their decreased moment arm by which to produce longitudinal arch raising moments. In theory, some flat footed individuals may have stronger plantar intrinsics but may also have, due to the reduced arch-raising moment arm, less ability to raise the arch of the foot with their plantar intrinsics. At some point, these small muscles probably do fatigue and I saw at least two patients this week who have moderate pes planus deformity, complained of a vague aching sensation in the arches with prolonged standing and walking, which I interpreted as plantar intrinsic muscle fatigue.

    In addition, how do we adequately test the isolated muscle strength of a plantar intrinsic muscle? Certainly not by flexing the the MPJs, PIPJs and DIPJs together since the long flexor tendons are much more powerful. Maybe these muscles may be tested by just flexing the proximal phalanges of the digits? Probably the only test that would isolate out the intrinsics from the extrinsics would be to have the patient "fan out" their toes against resistance.

    Good discussion on our little, and often-forgotten friends, the plantar intrinsics.
  17. salihangin

    salihangin Member

    I may put in an other way, if one has limited knee extension, preventing the knee from further flexion, Quadriceps muscle activity is expected to be higher than those who have normal knee . Does it mean that Quadriceps is stronger in knee with extension limitation while it never reach to its position with full extension? I think the answer is "No" Because, force exerted by muscle is always depend on certain angle within the full range of motion, i.e. force exerted by Quadriceps when knee joint in 90 degree flexion is different than the force exerted in the position of 45 degree flexion. Actually this was the basics for isokinetic contraction and exercise.

    If we transfer this to the foot with pes planus, IFM might not able to exert force within their optimal position and also within their primary function. Because they have certainly been displaced by abnormal skeletal structure in pes planus. Therefore overactivity or disuse probably leads weakness.

  18. efuller

    efuller MVP

    We are talking about basic physiology of muscles. With maximum stimulation of the muscle a given amount of force can be produced at each muscle length. Within the excursion that the muscle goes through within the body, the force produced by the muscles is pretty close to the same throughout its range of motion. When you take the muscle out of the body you can test it at lengths shorter and longer than its normal functional range. At these lengths you will see a decrease in force produced by the muscle. I really doubt that a flat foot will put the muscle outside of its optimal length and it will be able to produce its normal amount of force when not fatigued.

    I recall one article that found that a muscle that was transfered to a location where it needed a longer excursion actually adapted to that distance. Again, if the foot has been flat for all the person's life, the muscle will probably be in its optimal length for producing force. Kevin's point about the muscle in the flat foot not having as much leverage as the muscle in a high arched foot so that when the muscle, in the flat foot, produces the same amount of force, it does not produce the same amount of moment.

    As for the muscle being weak. Yes it will be weak when it is fatigued, but if it is often brought to the point of fatigue it oght to be quite strong when not fatigued.

  19. drsha

    drsha Banned

    Speaking theoretically,
    What if we take this flexible and repairable flat foot whose muscles have the ability to produce force but because they lack leverage become fatigued and weak (Dananberg's inhibition) and structurally place it in a more vaulted position so that its leverage arm would immediately improve. and

    If these muscles maintain their ability to produce force, they will fatigue and weaken less and if trained from this position, as you put it they would not be allowed to get to the point of fatigue, would Wolf's and Davis's Laws produce a more efficient, more stable structure from which to exert ORF's (apply TS)?
    Intrapersonally, practitioners would gain skill in approaching this "Optimal Functional Position" instead of utilizing the subtalar joint neutral position of the flat foot (which hardly changes the leverage of the system)?

  20. salihangin

    salihangin Member

    Then how would we explain Kevin's point that plantar intrinsic muscles do not produce sufficient moment to support MLA? Muscle need to exert higher amount of force to produce sufficient moment, i.e. in symptomatic flat foot or acquired adult flat foot, which, both have altered muscle - joint relationship. Overactivity of these muscles during relatively long distance walk, or daily activities in a long period of time causes fatigue that may boost the effect of altered moment arm . (Fatigue manifest as decreased isometric force production, reduced shortening speed, altered force–velocity relationship, and slowed relaxation. The combination of decreased force production and slowed shortening results in a decreased power output. Westerblad H, et al. 2010: Skeletal muscle: Energy metabolism, fiber types, fatigue and adaptability, Experimental Cell Research, Volume 316, Issue 18, 3093-9).

    Thanks for keep boosting this discussion.


  21. efuller

    efuller MVP

    Are you defining muscle weakness as ability to produce moment or ability to produce force. To hold up the arch you need to look at moment. However, if the muscle has little leverage, it can be quite strong, produce a lot of force, yet not produce sufficient moment. (Moment = force x distance.) That is how a muscle can be strong and not produce sufficient moment.

    When muscle is active it will tend to get stronger.

  22. salihangin

    salihangin Member

    Each activity pattern in our body is consist of a series of rotational movemets that muscles always need lever arm to produce them. I am defining not force alone, rather focusing how amount of moment is produced by a muscle to move a joint in a pre-defined trajectory or hold the joint in a certain position. With a very basic knowledge of mine, M=Fxd equation represents force-lever arm relationship that produce moment. If "d" is become smaller "F" must be increased to keep "M" constant. Plantar intrinsics muscles may basically be strong, however not able to produce sufficient momnent as produced in normal foot, herefore to fulfill their own function, they have to work harder and longer.

    Here there are two points I must bring forward:

    1- Since these muscles work harder and longer, they would expected to be stronger, then what is the point of attempting to strengthen these muscles that already strong? (Although, it is suspicious that whether a muscle get stronger as it works out of its capacity, especially with a same nutritional condition).

    2-Contrary to first statement, It would be expected that when a muscle work harder and longer within out of its capacity, fatigue will most probably be developed earlier than that of the muscles in a normal foot. Muscle may continue to work, but with fatigued,and oftenly exhausted and painful conditions with diminished force generation capacity.(I aggree with Dennis in this)

    These paradoxal (stated by Craig Payne in his earlier post here) situations need to be explained for optimal management of pes planus.

  23. salihangin

    salihangin Member

    Couple of research papers for muscle pain - muscle inhibition mechanism.

  24. Plantar intrinsic muscle function, including their susceptibility to fatique and their apparent "strength" is further complicated by what I call the medial longitudinal arch load-sharing system. The plantar intrinsic muscles function along with passive tension within the plantar ligaments and plantar fascia and from extrinisic musles of the plantar foot (posterior tibial, flexor hallucis longus, flexor digitorum longus and peroneus longus) to produce their arch-raising moments. Therefore, a loss of arch-raising moments from any one of the other passive or active components of the medial longitudinal arch load sharing system will produce an increase in strain and/or stress within the plantar intrinsic muscles.

    Here are the first two paragraphs from my recent Precision Intricast Newsletter from March 2012 titled "Load Sharing System of the Medial Longitudinal Arch-Volume I". In Volume II, for April 2012, I discuss how Achilles tendon directly affects the passive tension forces within the plantar ligaments and plantar fascia during gait.

    Longitudinal arch height also has a very significant effect on the tension forces within the plantar fascia and the force required by the plantar intrinsics to produce arch-raising moments. Those of you who are following along may want to check out my Thought Experiment #1: Tie Tensile Force in Loaded Arch from over six years ago to see the mathematics behind calculating tie tension forces within the arch structures of different heights.
  25. pod29

    pod29 Active Member

    Hi Kevin

    I'd love a copy of those two letters when they are available, if possible.:drinks

    The more I learn about the function of the intrinsics, the more I come the importance of their role in active regulation of of mid-foot stiffness. IMHO this process occurs primarily as a mechanism to maximise the efficiency of the achilles catapault. ie providing more efficient load transfer from the achilles to the ground. So I'm really looking forward to reading part 2 of your newsletter.

  26. dragon_v723

    dragon_v723 Active Member

    I have read through the threads so if I am not mistaken that the practice of intrinsic muscle group strengthening in pes planus pts wont actually help as there is a physiologically limit as to how strong the muscles can be n that these intrinsic muscles are amost likely to be already in their maximum/optimum strength when pts start to feel the symptom?
  27. salihangin

    salihangin Member

    Below abtract was the tread from NewsBot, and addressed by Craig Payne in his firt post here.

    Would it be speculative, if we say as these plantar intrinsics have some capacity to be strengthen, or, overactivity and prolonged work in a foot with pes planus seems not strengthen intrinsic muscles?

    On the other hand, saying that orthosis strengthen the intrinsic muscles in reference to this research study, actually orthosis alone did not make big difference as it was expected according to the same study.


    Last edited: Apr 11, 2012
  28. Foot orthoses may play a role in plantar intrinsic muscle strengthening by the following mechanisms:

    1. If the patient is simply able to walk and/or run longer and farther every day due to wearing orthoses, then its likely that all the lower extremity muscles, including the plantar intrinsics, will become stronger.

    2. The foot orthoses may be placing the plantar intrinsics at a more optimal length during gait in order to allow optimal strengthening to occur. Changes in muscle length have been shown to directly affect muscle strength so that if the muscle is too short or too long, optimal muscle strength may not be achieved. With foot orthoses in a pes planus deformity, I would expect foot orthoses to prevent the plantar intrinsics from being abnormally lengthened by the excessive longitudinal arch flattening during gait.

    3. By foot orthoses allowing more normal gait function (kinematics), more normal phasic activity of the plantar intrinsic muscles during gait may occur (kinetics) which may increase plantar intrinsic muscle strength over time.

    Further research will be necessary to see if these hypotheses are accurate or not.
  29. salihangin

    salihangin Member


    What impress me here is you and the people in your kind who are spending huge amount of effort to keep this learning environment up to date, and the way of explanation with their high level of academic style ( I am not rich in words as a non-native speaker of English Language). That is what keeping me in this biggest learning class ever on the earth.

    I have read an abstract it is being exposed in i-FAB in Sydney these days by Chris Nester ( I have been working with him in a different project for a short period of time) from University of Salford, UK and his colleagues. http://www.jfootankleres.com/content/5/S1/K5. They have used intracortical pins for anchoring reflective markers to provide a direct representation of the kinematics of the individual segments. during walking and running. This may helpful for future studies.


    Last edited: Apr 12, 2012
  30. Salih:

    Say hi to Chris Nester for me, I will be visiting with him in a few months at Salford. We have corresponded on midtarsal joint mechanics for many years and he is one of the world's leading researchers in midtarsal and midfoot kinematics during walking and running.

    As far as this discussion is concerned, many of us here on Podiatry Arena enjoy the intellectual stimulation that these discussions provide for us. I am happy that you have been able to participate.:drinks
  31. salihangin

    salihangin Member

    I have came on an interesing post in the thread of Strengthening intrinsic and extrinsic foot muscles
    Below image extracted from a research paper The In Vivo Elastic Properties of the Plantar Fascia During the Contact Phase of Walking
    It is old, however it made me think how the plantar intrinsics collaborates with the plantar fascia during the arch supporting.


    Attached Files:

    Last edited: Apr 13, 2012

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