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The foot as an inverted cable -stayed bridge

Discussion in 'Biomechanics, Sports and Foot orthoses' started by scotfoot, Jun 8, 2019.

  1. scotfoot

    scotfoot Well-Known Member


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    I am not sure if this has been suggested before , but can the foot be viewed as an inverted cable- stayed bridge ?

    Gerry
     
  2. scotfoot

    scotfoot Well-Known Member

    Of course the foot is a highly complex structure and a number of mechanisms are in play as gait progresses .

    However , I believe it is possible to tease out ,as a system within a system , an inverted cable- stayed supportive element ,where the tibialis posterior and peroneals (cable 1 ) support the more proximal part of the arch of the foot and the intrinsics (cable 2) support the more distal part .
     
  3. efuller

    efuller MVP

    Gerry, you should read the works of J.H. Hicks from the 1950's. I learned of the mechanical principles of beam, arch and tied arch from those articles. These are still really good articles. Gerry, since you are such a great autodidact, you should pick up a basic engineering text to see how mechanical engineering principles can be applied to the anatomy of the foot. An engineering text would help you see if there was a difference between a tied arch and an inverted cable stayed bridge.

    Eric
     
  4. scotfoot

    scotfoot Well-Known Member

    Eric ,
    The question was "I am not sure if this has been suggested before , but can the foot be viewed as an inverted cable- stayed bridge ?"

    Has it ?
     
  5. efuller

    efuller MVP

    I looked up cable stayed bridge. The foot can't be viewed as that. What is an inverted cable stayed bridge?

    The nice thing about the Hicks articles is that he explains which anatomical structures act as which part of a beam or tied arch.

    Eric
     
  6. scotfoot

    scotfoot Well-Known Member

    I am not attempting to challenge the existing concepts around a tied arch but to introduce an additional element ,also related to bridge construction , to explain the biomechanics of the foot more fully .

    A cable stayed bridge then . The bits we need are vertical supporting post/pillar ,a stretch of roadway that is attached to the pillar at one end and runs horizontally away from the pillar and two supporting cables attached to the top of the tower and then cable A attached to the road about half way along its length and cable B which is attached to the roadway at its end , furthest away from the pillar .

    In practice the cables support the span of the roadway and prevent it collapsing downwards when load is applied . ( cables attached to the top of the pillar and attached to anchorage would be necessary to balance forces out of course but we don't need that here )

    So if you draw this simple diagram out and then turn your piece of paper through 180 degrees you now have an " inverted cable stayed " structure . Now instead of resisting downward forces the structure best resists upward forces . We can call the distal aspect of the foot the road , the tib post and peroneals cable 1 , and the long toe flexors cable 2 .

    Going back now to the foot as a whole in effect you have a tied arch with two tie beams . Tie beam one is the plantar fascia ,tie beam 2 the extrinsic foot muscles . If you have two tie beams the load is shared .

    In post #2 I said "the intrinsics (cable 2) support the more distal part ." I have changed that as this is a theory under construction .

    Your thoughts please Eric .
     
  7. efuller

    efuller MVP

    Free body diagram analysis would quickly explain why it does not work this way.

    The load on a bridge is caused by gravity pulling toward the center of the Earth. The Earth resists that pull with ground reactive force. There are always both forces acting on one part of a bridge and a foot. A cable stay bridge won't work if you turn it upside down. The cable ties are under tension to hold the road bed up. When you turn it upside down, the cables are pulling the road bed toward collapse.

    Another reason that you can't consider the foot as a cable stay bridge is that there is no anatomical structure that corresponds to the central support.

    Technically the muscles and the plantar fascia are tension members and not beams. When something behaves as a beam part of the structure is under tension and part is under compression.
     
  8. scotfoot

    scotfoot Well-Known Member

    "Technically the muscles and the plantar fascia are tension members and not beams." Noted .

    Let's make the cable stayed structure more manageable by reducing it in size to a viewing platform extending out into the Grand Canyon . It's true that , with the platform construction right side up , the cables will resist gravitational forces , however they will not resist the vertical forces produced by a cable attached to the free end of the viewing platform and pulled vertically upwards by a Sikorsky helicopter (ground reaction forces in this analogy/ model ) . For the cables to resist this force the platform system must be inverted .

    "Another reason that you can't consider the foot as a cable stay bridge is that there is no anatomical structure that corresponds to the central support. "

    I am not saying that the foot should be viewed as an exact anatomical version of a cable stayed- bridge , but rather that ,with regard to the extrinsic foot muscles ,there are close similarities .

    Eric , could I ask you a direct question ? Would you agree that in closed chain , contraction of the tibialis posterior and peroneal muscles will act to cause dorsiflexion of the proximal aspect of the arch of the foot and plantarflexion of the distal aspect of the arch , which in turn will produce a shortening of the distance between the metatarsal heads and the heel ?
    In essence ,the basis of the short foot exercise as described by Janda .

    Gerry
     
  9. scotfoot

    scotfoot Well-Known Member

    Below is a link to a good article from Kevin Kirby . Here is a quote from the article -

    " The posterior tibial and peroneus longus tendons cross plantarly across the arch and cause a forefoot plantarflexion moment, which helps resist longitudinal arch flattening. "

    Along with the tib ant these are the muscles of Jandas short foot exercise .

    New Concepts In Longitudinal Arch Biomechanics | Podiatry Today


    https://www.podiatrytoday.com/new-concepts-longitudinal-arch-biomechanics
    24 May 2018 - Kevin A. Kirby, DPM ... The LALSS is a type of “load-sharing system,” which is a common design in both mechanical and electrical systems.
     
  10. efuller

    efuller MVP

    The line of pull of the tendon has to run plantar to the compression surfaces of the bone to create a plantar flexion moment. Or more precisely, the center of pressure of the articular surfaces.

    Peroneus longus does do this. I have done some modeling of peroneus longus and depending on STJ axis position, by itself, the peroneal muscle can do different things. The peroneus longus will simultaneously create a plantar flexion moment at the first ray and a pronation moment at the STJ. Pronation of the STJ will tend to increase ground reaction force on the first met head which will tend to dorsiflex the first ray. Peroneus longus is fighting against itself at the first ray in terms of arch raising. If it has a small lever arm to cause pronation, the first ray plantar flexory force wins. If it has a large lever arm to cause pronation, you can see dorsiflexion of the first ray from increased ground reaction force on the first met head. So, the peroneus longus, by itself, will not always cause arch raising.

    The posterior tibial muscle is in the perfect place to adduct the forefoot on the rearfoot. It's primary attachment is right in the middle of the navicular, so it's line of action of pull is not on the bottom. By itself, the posterior tibial muscle cannot plantar flex the forefoot on the rearfoot. However, in combination with the peroneus longus muscle, the increased supination moment from the posterior tibial muscle will counter act the direct pronation moment from the peroneus longus muscle and allow the plantar flexion moment from the peroneus longus muscle to create first ray plantar flexion.

    The perneus brevis muscle cannot raise the arch because its line of action is not plantar enough.
     
  11. efuller

    efuller MVP

    The anterior tibial muscle because of its line of pull will dorsiflex the forefoot on the rearfoot. However, in stance, the anterior tibial muscle will create a dorsiflexion moment at the ankle joint that will tend to shift the ground reaction force posterior and this will decrease the dorsiflexion moment from ground reaction force on the forefoot. (This would also help peroneus longus plantar flex the forefoot).
     
  12. scotfoot

    scotfoot Well-Known Member

    When the intrinsic toe flexors such as the flexor hallucis brevis and abductor hallucis contract , they will flex the toes . In closed chain (foot on the ground ) this will tend to dorsiflex the forefoot on the rearfoot potentially reducing the force with which the toes can be pressed into the ground . However , the tibialis posterior , peroneus longus and tibialis anterior can act to resist dorsiflexion of the forefoot (see above ) thus potentiating the effects of the toe flexors .

    In effect , in this instance but not all , we have the prime movers acting to maintain the "foot core" and the intrinsics acting as the "prime movers "
     
  13. efuller

    efuller MVP

    The abductor hallucis muscle plantar flexes the forefoot. Whether the attempt to plantar flex the toes does anything to the arch depends on what other muscles are acting.
     
  14. scotfoot

    scotfoot Well-Known Member



    "The abductor hallucis muscle plantar flexes the forefoot."


    I had always thought the same but in this video (below ) the toe flexors lift the ball of the foot , and hence the met heads , up clear of the ground . The angle of the foot as a whole has changed but has the forefoot plantar flexed on the rearfoot ?



    Janda Short-Foot Exercise - YouTube



    upload_2019-6-20_21-17-52.jpeg ▶ 0:24
    19 Apr 2017 - Uploaded by Testosterone NationAnkle position can affect gait and can sometimes be worsened in individuals who walk and/or run with a ...
     
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