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The stuff between the bony arch of the foot and the plantar fascia

Discussion in 'Biomechanics, Sports and Foot orthoses' started by scotfoot, Jul 3, 2020.

  1. scotfoot

    scotfoot Well-Known Member

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    Every now and again I find myself coming back to the same question .

    Superiorly we have the bony arch of the foot , and on the plantar aspect we have the plantar fascia ,a tough envelope of connective tissue that is continuous from medial to lateral aspects of the foot but which has specific areas of thickening which correspond with the metatarsals .

    Filling the volume of space between these two boundaries we have "stuff " .
    Leaving aside any specific functional roles that this stuff might have in the foot and viewing it just as a mass , does its presence change the mechanics of the foot during gait ?

  2. scotfoot

    scotfoot Well-Known Member

    Further to the above, when the foot accepts weight during gait the arch flattens.
    Now if this flattening produces a reduction in the volume of space between the bony arch of the foot and the envelope of the plantar fascia ,( and anatomically it is an envelope like structure not a series of separate bands ) then what happens to the "stuff " that occupies the area that has now reduced in volume and does the "stuff " passively oppose volume reduction .
  3. scotfoot

    scotfoot Well-Known Member

    So during gait we have a flattening of the arch , volume reduction between arch and fascia and a compression of the " stuff" in between .

    Is there any proof that this happens is the next question and the answer , in my opinion is, yes, there is clear evidence .

    Let's separate out from our mass of" stuff " the medial and lateral plantar veins . These are filled with blood and empty when the forefoot bears weight giving us the powerful plantar venous pump.

    The veins cannot empty by necking down as this is a nonsense , nor by muscular contraction since the intrinsic muscles lengthen during weight acceptance ,leaving pressure generated in the " stuff " between the flattening arch and fascial envelope as the only possible explaination .

    If you like, the pressure generated in plantar venous plexus tells us about the level of pressure generated in the " stuff that lies between " ( STLB) .

    So we know that the STLB is compressed during gait even when the heel and midfoot don't make contact with the ground, but how does this "core under compression" affect mechanics and why is it not acknowledged in the literature at all ?
  4. scotfoot

    scotfoot Well-Known Member

    Years ago Gardner and Fox produced seminal work on the plantar venous pump . In one experiment they studied blood flow out of the foot during weightbearing where only the heel and forefoot where in contact with the rig.
    They found that blood flowed out of the foot without the need for the midfoot to contact the ground .

    This led them to conclude that blood is propelled out of the foot by either
    1 muscular contraction
    2 stretching of the anchored plantar vessels as the foot lengthens .

    In another experiment they found that the pump functioned in a less efficient ,intermittent fashion in a paraplegic foot with little muscle ,but functioned never the less , leading them in the direction of a stretch and necking down pumping mechanism .

    In actual fact if you take a blood filled viscoelastic vein, hold it vertically, and stretch it you will not expel the blood at all .

    Also, with regard to muscular contraction as a possible pumping mechanism, it is true that NMES will move blood out of the foot as muscle bellies shorten and bunch in the proximity of the plantar veins . However ,during early stance the muscles contract eccentrically and the muscle bellies lengthen so this will fail to pressurised the blood filled plantar veins .

    The only possible way the pump can work is by the squashing of the stuff , STLB, around the plantar vessels as the foot flattens and there is a tendency towards volume reduction between the bony and the fascial envelope.

    The high pressures generated in the PVP reflect the pressures in the STLB .

    That the existance of an energy absorbing , stress dissipating system within the foot, that is all about compression of the STLB as the volume of space between the bony arch and the PF reduces , remains totally unacknowledged amongst anatomists, physiologist and foot health professionals is simply inexplicable.
    Last edited: Jul 5, 2020
  5. scotfoot

    scotfoot Well-Known Member

    Imagine a person walking " on tiptoe " , that is on their toes and balls of their feet only . In this situation we have ground reaction force and force from the achilles tendon acting in an upwards direction and a downward force from the tibia/ fibia.

    These forces act to compress the bony arch of the foot and compress and deform the" stuff " between the arch and the enveloping PF .

    Clearly then the characteristics of the STLB under compression will influence the mechanics of the foot and since this material is mainly skeletal muscle the characteristics can vary with contraticle activity within the muscle . The greater the contractile active within the muscle mass the more resistant it is to compression and deformation .

    Pressure within mass can be estimated from pressure within the PVP system .
  6. scotfoot

    scotfoot Well-Known Member

    A note on all of the above.
    It is perfectly true that passive dorsiflexion of the foot at the ankle joint will activate aspects of the calf muscle pump and move blood up the leg .

    However ,it should be noted that this pumping mechanism is most likely related to stretching of the some of the muscles of the calf and the evacuation of blood from venous sinuses within muscles and not from the action of compression of vessels that lie between muscles .

    The lengthening of muscle bellies adjacent to blood filled vessels is not at all likely to effectively empty them whilst shortening will.
  7. scotfoot

    scotfoot Well-Known Member

    The planar venous foot pump demonstrates the pressure which exists in the " stuff that lies between " ( STLB) during simple weight-bearing .

    During sprinting , where the heel might never touch the ground and where loads on the foot can exceed 8x or even 9x body weight , it can be appreciated that as the bony arch flattens and the volume between the arch and the fascial envelope reduces and alters shape ,the pressure and distorting forces acting on the STLB will be enormous.
    In return the forces in the STLB acting on the bony arch of the foot and the enveloping fascia will be correspondingly high .

    Thus the STLB has a critical role to play in foot biomechanics regardless of whether the muscle , of which it is mainly constituted, is in a contractile state or not .

    That said, if the muscle is an active ,contracting state as the STLB is squashed ,then the STLB will be able to better resist compression and deformation.

    I cannot for the life of me understand why the STLB and its compression and distortion during gait has never been mentioned in the literature before , other than by me on blog sites .
  8. scotfoot

    scotfoot Well-Known Member

    Most people are familiar with "silly putty " a viscoelastic material ( like the STLB) , which can bounce like a rubber ball or stretch and elongate when steady force is applied .
    The STLB ( stuff that lies between the bony arch and the plantar fascia) acts in a similar way responding differently to rapidly applied forces than it does to forces applied over a longer period . This may explain the results of a recent experiment by Kessler et al 2020 .
    She found that intrinsic foot muscle activity decreased as hopping frequency increased. Since the intrinsics make up the majority of the STLB mass , this reduced activity may be because short sharp forces acting on the STLB cause a more elastic response whereas forces with longer impulse times mean that the STLB starts to act more as a viscous fluid necessitating greater intrinsic activity to reduce and recover from deformation

    It should also be noted that the composition the STLB changes with disease or as we age . Typically skeletal muscle mass is lost to be replaced by fatty infiltrate . Poorly defined fatty infiltrate is less elastic than muscle and so foot function may be compromised
  9. scotfoot

    scotfoot Well-Known Member

    Still very much on the subject of the mass of tissue that lies between the bony arch of the foot and the plantar fascia is the paper referenced below from Taddei et al 2020 .

    These researchers increased the mass of the " literal foot core " or STLB with strengthening exercises and found although actual strength did not significantly improve in the feet of runners ,foot biomechanics did . Hence it may be inferred that" literal core "mass and composition matters in and of itself .

    ( note : some researchers have referred to a "foot core " but they do not mean "that which lies at centre of ".
    " Literal foot core " does mean this but to avoid confusion I have gone with STLB meaning the " stuff that lies between " .

  10. scotfoot

    scotfoot Well-Known Member

    Here are some photographs from a very recent paper which looks at the relationship between intrinsic muscle volume ,plantar fascia thickness and toe deformity in type 11 diabetics .
    In figures B and particularly D ,you can clearly see the large mass of muscle tissue that contributes towards the overall mass of the stuff that lies between .If that mass ,which in the healthy foot is mainly skeletal muscle , does not influence foot function by resisting compression and distortion , then it would be very ,very surprising indeed !
    How much longer can this elephant in the room be ignored ?


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