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Muscles and longitudinal arch

Discussion in 'Biomechanics, Sports and Foot orthoses' started by gendel99, May 17, 2013.

  1. gendel99

    gendel99 Active Member

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    Dear collegues!
    Help to understand. If muscles do not play crucial role in longitudinal arch http://jbjs.org/article.aspx?articleid=13975
    why the disfunction of the posterior tibial muscle is the major factor in developing adult flatfoot? What is primary factor spring ligament tear or PTTD? What biomechanicaly major factor in flexible flatfoot children?
  2. The article you cited does not report that muscles do not play a crucial role in support of the medial longitudinal arch.
  3. gendel99

    gendel99 Active Member

    "reveals that only heavy loading elicits muscle activity". As I remember physiology, if absent electrical activity (depolarisation) = absent muscle contraction. Or not? And if absent contraction=the muscles don't play in support os longitudinal arch.
  4. Rob Kidd

    Rob Kidd Well-Known Member

    When the going gets tough, you go back to basics.

    Which muscles are you referring to? Intrinsics, or extrinsics?

    Refer back to Mann and Inman 1964 re: intrinsics; if any one here knows different, tell me, and I will update.

    As for Tib Post, it is a frequently abused muscle in flatfoot - but is not next weeks news. Its severance from the navicular is catastrophic in terms of foot collapse, but again, there is nothing new about that. Did you mention the spring ligament? If this breaches there is the Mother and Father of all collapses. I feel that I have said nothing new - you should have known all of this.
  5. But the muscles do kick in in high loading. This is not the same as saying they do not play a role in supporting the arch. Moreover, the study as I recall did not look at phasic activity of the muscles during dynamic function.
  6. gendel99

    gendel99 Active Member

    As I understand muscles support the arch only when we walk. In stance phase of gait cycle musles "don't work" i.e. PTT is dynamical supporter. As for PTTD or spring ligament I mean "which came first, the chicken or the egg" i.e. spring ligament tear firstly or PTTD and than spring tear. And I am interest why absent articles about PTTD in children? What is major ethiological factor in flatfoot in children?
  7. efuller

    efuller MVP

    Muscles support the arch when the CNS tells them to. In static stance the load of 1/2 body weight is applied to the arch of the foot. If the static structures can comfortably support that load the lazy CNS won't add any muscular support. As load is increased the CNS probably senses stress and increases muscle activity to reduce stress in the passive structures. When the CNS doesn't know there is stress (Diabetic Neuropathy) then there may not be a signal to support the arch.

    As an aside, the posterior tibial muscle shifts weight to the lateral side of the foot from the medial side. It's effective attachment is at the medial side of the navicular and hence it doesn't really support the arch. Yes there are fibers that go plantarly, but they don't slide with the tendon so they are more of a ligament than a "muscular support" of the arch. When the STJ has range of motion to pronate, and does pronate so that there is more load on the medial forefoot, this overload causes medial arch collapse. So, the posterior tibial muscle "supports" the arch in preventing overload of the medial part of the arch.

    So, when the PT muscle is not working, the ground will cause STJ pronation that not slowed by the PT muscle. The passive structures that stop pronation will recieve high loads at the end of the range of motion. One of the things that resist further pronation is the medial forefoot. With high loads on the medial forefoot, there will increased strain on the spring ligament. I believe that is a logical explanation of why the muscle dysfunction comes first.

  8. Gendel:

    I have spent a lot of time thinking, writing and lecturing on this subject over the past quarter century. Maybe I can help explain things better.

    The medial longitudinal arch (MLA) is best understood, I believe, by analyzing the MLA relative to the external forces and the internal forces which not only cause it to flatten, but also to raise in height. In other words, understanding MLA function is helped considerably by using the concept of rotational equilibrium to determine what rotational forces (i.e. moments) tend to flatten the MLA (i.e. medial forefoot plantarflexion moments) and tend to raise the MLA (i.e. medial forefoot dorsiflexion moments).

    For example, during relaxed bipedal stance, ground reaction force acting on the plantar rearfoot and plantar forefoot along with Achilles tendon tension force pulling superiorly on the posterior calcaneus causes a medial forefoot (MF) dorsiflexion moment which tends to cause MLA flattening. However, passive tension within the plantar ligaments of the MLA, central component of the plantar aponeurosis, along with muscular contractile forces within the plantar intrinsics, posterior tibial, flexor hallucis longus, flexor digitorum longus and peroneus longus muscles will create a MF plantar moment which tends to cause the MLA to raise in height. These structural components of the MLA form a load-sharing system for the MLA where the passive and active tensile forces generated by one set of structures reduces the loading forces on the other set of components of the load-sharing system (see illustration below). My fourth Precision Intricast Newsletter book, scheduled to be published in early 2014, will have a section devoted to this subject.

    When the summation of these MF dorsiflexion moments exactly equal the summation of these MF plantarflexion moments, then we will observe that the MLA is resting at a certain arch height, neither flattening or raising or, in other words, the MLA will be in rotational equilibrium.

    If, for example, the posterior tibial muscle contracts more vigorously to cause more posterior tibial tendon tension force, not only does an increase in MF plantarflexion moments occur, but also a decrease in MF dorsiflexion moments occur (as Eric noted with transfer of GRF to the lateral forefoot). This will cause an imbalance of MF plantarflexion moments which are greater than MF dorsiflexion moments which will, in turn, cause an increase in MLA height.

    However, if one of the structural components of the foot that creates a MF plantarflexion moment is weakened, partially ruptured or completely ruptured, then the relative decrease in MF plantarflexion moments compared to the MF dorsiflexion moments will cause an acceleration of MF dorsiflexion motion and MLA flattening to occur, until a new MLA height of rotational equilibrium is again achieved. For example, if the plantar aponeurosis is ruptured then this will decrease the MF plantarflexion moment which will cause an increase in MLA flattening since, now with plantar aponeurosis rupture, the MF dorsiflexion moments "win out" over the MF plantar moments to allow some additional MLA flattening to occur (see illustration below).

    Trying to fully comprehend this process without first being aware of and understanding the external and internal forces that cause and resist MLA flattening, the internal and external moments acting on and within the joints of the foot and lower extremity, and the rotational equilibrium equations of these joints is, in my opinion, a futile process since the human medial longitudinal arch is an exceedingly complex load-sharing system which relies on coordinated mechanical interactions of multiple structural components with afferent input into and coordinated efferent output from the central nervous system to ensure its normal functioning.

    Hope this helps.:drinks
  9. The spring ligament complex passively supports the MLA while the posterior tibial muscle actively supports the MLA. Put in more precise biomechanics terminology, the spring ligament complex causes a passive increase in medial forefoot plantarflexion moment during medial forefoot loading while the posterior tibial muscle causes an active increase in medial forefoot plantarflexion moment. In other words, since these two anatomical structures share functions, when one is compromised (i.e. torn or stretched) then the load on the other structure is increased.

    Posterior tibial tendon dysfunction doesn't occur in children likely because children's tendons are not subjected to large magnitudes of tensile loads and their tendons stretch, instead of tearing, when subjected to large magnitudes of tensile loads. As childhood obesity increases, so too will the age that we start seeing posterior tibial tendon dysfunction.

    The major etiologicial factor for children's flatfoot deformity is likely the reduced stiffness of their supporting plantar ligaments, combined with other genetic structural factors.
  10. gendel99

    gendel99 Active Member

    Thanks a lot Kevin for explanation! But what will be rupture firstly in load-sharing system plantar fascia or plantar ligaments?
  11. Gendel:

    We can't know what structure of the plantar arch load-sharing system will first fail unless we first know whether there are any defects or abnormalities (e.g. thin areas, low elastic modulus, weak attachment points to osseous structures) within each of the plantar tension load-bearing structures of the foot. In addition, we would also need to know what forces the foot will be subjected to predict load failure sequences of the plantar arch structures.

    Can engineers always predict which structures within a bridge will fail first? No. And engineers are at a great advantage compared to clinicians since engineers know the exact material characteristics of each of the structural components of the bridge before they go into the bridge structure. We, as clinicians, don't know the elastic modulus or exact structure of each of the plantar structures so we can only make an educated guess as to what will fail first. Then when a structure partially or completely fails, we then treat it accordingly with appropriate mechanical and medical measures. This is called the Tissue Stress Theory of Treatment of Mechanically-Based of Foot and Lower Extremity Pathologies. It is the way of the future for podiatric and other foot-health clinicians.
  12. ptulaya

    ptulaya Member

    Dear Kevin kirby,

    Your explanation is excellent. It helps me to understand more about flatfoot mechanics. May I ask a question? Why you focus on the forefoot moment? Why not at the talo-navicular joint which is the location of static and dynamic stabilizer structures attachment.

  13. Tulaya:

    Good question. By the way, welcome to Podiatry Arena.:welcome:

    When I say "medial forefoot plantarflexion moment", I mean all the joints from the midtarsal joint distally are tending to plantarflex relative to the rearfoot (i.e. calcaneus and talus). This would include the talo-navicular, naviculo-cuneiform, and cuneo-metatarsal joints.

    In other words, a medial forefoot moment would either tend to raise the medial longitudinal arch (i.e. medial forefoot plantarflexion moment) or would tend to lower the medial longitudinal arch (i.e. medial forefoot dorsiflexion moment). I use the term medial forefoot moments since the talo-navicular, naviculo-cuneiform, and cuneo-metatarsal joints all will allow sagittal plane rotational motion creating changes in medial longitudinal arch morphology. However, I do agree with you that the talo-navicular joint probably produces the largest ranges of sagittal plane motion in most feet.

    Good luck with your PhD and if you need any more help, please feel free to ask. Podiatry Arena is a great resource.:drinks
  14. efuller

    efuller MVP

    To say what Kevin said a little differently. The forefoot dorsiflexion moment from the ground is what applies stress to the structures of the talo navicular joint. When the forefoot is dorsiflexed the navicular will slide relative to talus until the plantar structures of the joint resist further dorsiflexion. In that position, the forefoot dorsiflexion moment will probably determine whether the plantar structures break. Biomechanically, it's hard to focus on the anatomy without looking at the load.


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