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Windlass Mechanism

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Asher, Jan 18, 2009.

  1. Asher

    Asher Well-Known Member

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    Hi there

    Can someone explain to me the paradox of the windlass mechanism where we don't want the 1st MPJ to bear so much force as to dorsiflex the first ray (ie: arch flattening) but we need it to bear enough in order to get the hallux to dorsiflex.

    I suspect its an issue of timing of forces but would someone mind explaining it for me. Many thanks.

  2. Rebecca:

    Beautiful question! This type of question demonstrates your excellent grasp of the concepts which are important to understand first ray function.

    The answer is not necessarily an issue of timing. The answer is an issue of the foot's internal resistance to first ray dorsiflexion, or first ray dorsiflexion stiffness. If the first ray has good internal resistance to dorsiflexion motion due to the plantar ligaments being more stiff, due to the peroneus longus and posterior tibial muscle being stronger and also due to a good medial longitudinal arch height, then ground reaction force (GRF) plantar to the first metatarsal will not dorsiflex the first ray excessively, the first metatarsal head will bear it's share of GRF and the hallux will dorsiflex easily during propulsion.

    However, if the first ray does not have good internal resistance to dorsiflexion motion due to the plantar ligaments being more compliant, due to the peroneus longus and posterior tibial muscle being weaker and also due to a low medial longitudinal arch height, then force plantar to the first metatarsal will dorsiflex the first ray excessively, the first metatarsal head won't bear it's share of GRF, the hallux will plantarflex due to an excessive increase in plantar fascial tension, causing a functional hallux limitus and too much GRF plantar to the hallux, and not enough GRF plantar to the first metatarsal head.

    Therefore, the component of first ray dorsiflexion stiffness that is caused by all the tensile load-bearing structures that don't insert into the hallux are the key to determining how much GRF the plantar 1st metatarsal head will bear in late midstance and propulsion.
    Last edited: Jan 19, 2009
  3. efuller

    efuller MVP

    Hi Rebecca,

    The first met does not need to bear weight in order for the hallux to dorsiflex. You do have to decrease force on the entire 1st ray to allow the hallux to dorsiflex in the presence of functional hallux limitus.

    Ground reaction force on the met and hallux cause a dorsiflexion moment of the first ray. (When the MPJ is rigid.) This dorsiflexion moment has to be resisted by something. One possible item is tension in the plantar fascia. Pull on the fascia pulls the base of the hallux backward. The phalanx can't go backward because force from the first metatarsal head. The force from the metatarsal head acting on the proximal phalanx and the pull of the plantar plantar fascia on the phalanx create a plantar flexion moment acting on the phalanx which is what causes a functional hallux limitus.

    There is an equal and opposite reaction to the force from the met head acting on the phalanx. That is a force from the phalanx acting on the first metatarsal head. The metaarsal's proximal movement is resisted by a force from the cuneiform acting on the base of the metatarsal. These two forces acting on the metatarsal create a force couple that creates a plantar flexion moment on the metatarsal. This is the moment that resists the dorsiflexion moment from the ground. So, the higher the dorsiflexion moment from the ground the higher the plantar flexion moment there will be needed to resist the dorsiflexion.

    The above is easier to understand with diagrams. I could write more, repeating what I wrote, but I think there is enough written above to explain my point. If not let me know.


  4. Admin2

    Admin2 Administrator Staff Member

  5. Romeu Araujo

    Romeu Araujo Active Member

    Hi all,

    In a few days I'll give some classes and I'd like some visual aid to better explain the windlass mechanism - a video, animation...

    Any help will be appreciated.

    Best regards,
  6. Craig Payne

    Craig Payne Moderator

    Here is Hick's original:

    Attached Files:

  7. Craig Payne

    Craig Payne Moderator

    I also use this image to explain what "windlass" actually means:

    Attached Files:

  8. Romeu Araujo

    Romeu Araujo Active Member

    Thank you Craig! :drinks
  9. David Smith

    David Smith Well-Known Member

    Rebecca, There are several points in you question that require clarification.

    1)"we don't want the 1st MPJ to bear so much force as to dorsiflex the first ray"

    Any vertical component of GRF applied to the 1st MPJ will tend to dorsiflex the1st ray. For arguments sake lets say that GRF applied to the 1st ray dorsiflexes the 1st ray.

    2) "but we need it to bear enough in order to get the hallux to dorsiflex."

    No we don't, the hallux can dorsiflex when there is no GRF on the 1st MPJ e.g. you can dorsiflex your hallux and activate the windlass mechanism even when non weight bearing.

    As you say GRF acting on the 1st MPJ will lower the medial arch and extend it. This extension causes tension in the plantar fascia that unwinds the windlass and plantarflexes the hallux. When the hallux comes into contact with the ground then GRF acts plantarflex to the hallux, the windlass can now unwind no more and any further arch extension increases the PF tension. Increased tension in the plantar fascia and aponeurosis will tend to stabilise the arch. At this point the heel lifts and soon after the contralateral heel contact is made. This double stance phase reduces total force plantar to the foot of interest and allows the tension in the plantar fascia plus the windlass mechanism to raise the medial arch..

    It is between heel lift and contralateral heel strike, i.e. propulsive stage when GRF plantar to the forefoot is greatest, that the hallux must be allowed to dorsiflex in order for forward progression to progress unimpeded. The hallux can only dorsiflex by the application of GRF. At this point if the medial arch is extended by too much, the force required to shorten and raise the medial arch, via the windlass action, is very high. This is a function of its geometry and forces can be calculated and shown using trigonometry. Put simply the height of the virtual triangle of the medial arch represents the lever available to the hallux windlass system to raise or increase the arch height. The lower the triangle height the shorter the lever arm available and so the more ground reaction force required sub hallux. The problem is compounded by the fact that as the hallux windlass requires more force to dorsiflex it then it also becomes stiffer to GRF. This in turn increases the functional length of the 1st ray, which as you might realise increases the lever arm length available to dorsiflex the 1st ray. Therefore moments about a joint of interest, lets say the Navicular-cuneiform joint, tend to increase when ideally they should be reducing and the joint should be tending to plantarflex. (Tendency is a relative term i.e. as the dorsiflexion moments about a joint reduce there is a tendency to plantarflex). This results in what we see clinically as FncHL.

    As an issue of the timing:

    From experiments that I have done, I would say that a clinically significant test using a pressure mat might show that, after heel lift when FncHL is present, the pressure sub 1st MPJ is decreasing as the pressure sub hallux is increasing.

    In the non FncHL foot after heel lift the pressure sub hallux and sub 1st MPJ reach a peak and decrease simultaneously to toe off.

    Therefore you might see that it is important for the arch not to extend or lower or dorsiflex too far and the tendency for it to do this would be related to the relative load applied to it. So the load should only be as much as that which allows the operation of the windlass.

    I don't know if this is very clear since it's difficult to describe without diagrams and maths.

    Cheers Dave
  10. Even though I agree with most of what Dave says in his last note, I do take exception to the notion that "the hallux must be allowed to dorsiflex in order for forward progression to progress". It is my belief that hallux dorsiflexion is not required for forward progression. The most common cases of hallux dorsiflexion not occurring, but forward progression still occurring are the many patients with hallux rigidus who propel over the more lateral metatarsophalangeal joints (MPJs) and the many patients who have had surgical arthrodesis of 1st MPJ who actual propel very well over the hallux and first MPJ after surgery. These patients have little to no hallux dorsiflexion but may still manage to walk in a fairly normal manner despite a complete lack of first MPJ dorsiflexion ability.

    Even patients with functional hallux limitus that have excellent hallux dorsiflexion off-weightbearing but little hallux dorsiflexion during walking can still undergo forward progression, but don't have normal propulsion mechanics. If the ability to dorsiflex the hallux were a precursor to normal foot function, then 1st MPJ arthrodesis surgeries would not work as well as they do.

    I believe that patients that have a rigid 1st MPJ, and that also have a more normal to laterally deviated subtalar joint (STJ) axis location, will be able to walk quite nicely over their lateral MPJs with normal forward progression since they can easily supinate their foot the few degrees that is required to reduce the ground reaction force (GRF) plantar to the first ray/1st MPJ to allow the lesser digits to receive the majority of the propulsive forces of gait. However, those patients that have a rigid 1st MPJ and also possess a medially deviated STJ axis will have a difficult time walking normally since they can not easily supinate their foot off the first ray/1st MPJ to allow the lesser digits to receive the majority of the propulsive force during gait.

    I thought it may be worth making this set of observations known relative to the internal and external STJ moments occurring during late midstance and propulsion since I am currently looking for a good discussion to sink my teeth into.
  11. David Smith

    David Smith Well-Known Member

    Yes Kevin, I entirely agree with this statement in terms that the subject will be able to adequately progress forward. This however, as you have pointed out, will require some trick movement to facillitate this. As we have discussed before, but not entirely agreed upon, the saggital plane progression perturbation requires some compensatory action to facillitate adequate forward progression. This compensation reduces the magnitude of the perturbation.

    The point I was really trying to get across was that it is the mechanical state of the arch and windlass system relative to the load that allows it to work rather than causes it to work. This is analogous to a car differential that allows, but does not cause, one wheel to rotate faster than another on the same axle. A locked differential does not stop the car moving forward but the compensation required is that it should have low friction at the tyre - ground interface to stop the diff winding up or breaking a drive shaft.

    Cheers Dave
  12. In discussing windlass mechanics, please remember that the plantar fascia does not only insert into the hallux.

    I've just invented a new pathology: "functional lesser digit limitus".
  13. My point is, Dave, that forward progression will occur whether hallux limitus is present, whether hallux rigidus is present, whether the 1st MPJ is arthrodesed or whether the hallux is amputated. The hallux doesn't allow forward progression...forward progression will occur regardless of hallux mechanics.

    Allow: 1. Let something happen. transitive verb to permit something to happen or somebody to do something.

    Does hallux dorsiflexion let forward progression occur? Not in my opinion.
    Last edited: May 29, 2009
  14. Indeed. Moreover, isn't the windlass mechanism just a brake?
  15. Simon:

    The medial band of the central component of the plantar aponeurosis may act as a "brake" in those patients with low medial longitudinal arch contour and medially deviated STJ axes but may also increase the dorsiflexion stiffness of the medial longitudinal arch and increase the propulsive power of the first ray/1st MPJ complex in those patients with more normal medial longitudinal arch height and more normal STJ axis location. In addition, during running the windlass mechanism probably adds considerable energy back into the body during the support phase. Therefore, I think it is incorrect to say that the windlass mechanism only functions as a brake, since it can function as an accelerator also!:drinks
  16. If the lesser metatarsals had huge sesamoids (and not a thin plantar plate) and had greater dorsal to plantar diameters (and not small dorsal to plantar diameters) and were not located lateral to the medial border of the foot (instead of in the central to lateral border of the foot), then this may be a mechanical possibility. Until I see one however, Dr. Spooner, I will consider "functional lesser digit limitus" to be an imaginary beast, right up there with the Rothbart foot, functional foot typing, and frontal plane heel position during relaxed calcaneal stance position predicting foot function and pathology.:rolleyes:

  17. How does it increase propulsive power?
  18. You're just jealous because I invented it first, Kevin ;). All it needs is for plantarflexion moment to be >/= to dorsiflexion moment, momentarily during propulsion for functional lesser digit limitus (FNLLDL) to exist. BTW, individual digits affected are to be identified using the "this little piggy system" for example, FNLLDL of the 3rd toe would be called: functional lesser digit limitus roast beef; the second would be FNLLDL none etc.

    If the pantar fascial insertions into the lesser digits are insignificant, why are they there? Why should the force exerted by the slip of the fascia that anchors into the hallux be greater than the slip to the second toe? (not talking moments, talking force). Now if we are talking about the smaller digits, (not the 2nd) in general the plantarflexion lever arm about these MTPJ's is possibly smaller due to the relative decrease in the radius of the metatarsal head. But, the dorsiflexion moment lever is also reduced due to the shorter toe length. Indeed, your theory regarding metatarsal head size assumes that the axis of rotation is at the centre of the metatarsal head- we know that the centre of rotation of the 1st MTPJ is variable and can exist outside of the metatarsal head from the work of Sherreff. Do we have any clue where the lesser MTPJ axes are? Theoretically, the plantar fascial slip to the 2nd toe might have a greater lever arm than the slip to the hallux in the same foot, simply because the centre of rotation about the 2nd MTPJ is external and dorsal to the metatarsal head, while the centre of rotation at the 1st MTPJ is somewhat more plantar within the first metatarsal head.

    Finally, functional lesser digit limitus may be the result of increased force and or altered phasic activity of those muscles capable of generating plantarflexion moment about the lesser MTPJ's .

    While I accept your scepticism Kevin, I'm sticking with it :wacko: because this is the first original thought I've ever had ;)
    Last edited: May 29, 2009
  19. ps, I haven't forgotten about lesser digit dorsiflexion stiffness (LDDS) either. Or Spooner's test which is basically the same as a Jack's test but for the lesser toes.
    Last edited: May 29, 2009
  20. David Smith

    David Smith Well-Known Member

    Yes I agree, but we are talking at cross purposes, I am saying that the windlass is allowed to work and not caused to work and not that the operation of the windlass is necessary for forward progression per se.

    I wrote
    this statement could have been phrased better and I can see where the confusion arrived. DoH!

    Obviously this would be a great disadvantage if the failure of the mechanism of such a small appendage would stop forward progression. Imagine all those people just stuck halfway to work or in the middle of a field, unable to progress any further. Each waiting for a rescue skate board to be fitted to the FncHL foot to enable them to be on their way. It would be like a Monty Python sketch eh?

    Regards Dave
  21. In all seriousness: We perform a jacks test = normal, i.e. STJ supination, STJ axis = lateral to 1st met head, continue across the digits performing Spooner test's on the lesser digits when you reach the digit where the STJ pronates, you know you are now lateral to the STJ axis, so the axis must lie between the digit where Spooner's test = STJ supination and where Spooner's test = pronation. You often find one digit where you get neither pronation nor supination = the axis is beneath this MPJ. If when you perform your Jacks test you don't get STJ supination then the STJ axis is either beneath or medial to 1st MPJ = medially deviated STJ axis. Try it.;)

    We can call it the Spooner assessment of weightbearing STJ axial position, or just the Spooner test. Or perhaps we'll call it the Kirby test after my friend and mentor for all these years.
    Last edited: May 29, 2009
  22. Dave:

    Maybe changing your statement to "dorsiflexion of the hallux during the propulsive phase optimizes the forward progression of walking", would be more a more accurate way of stating what you probably meant to say.:drinks
  23. Simon:

    If the origin of the plantar aponeurosis didn't place a horizontally directed tensile force on the medial calcaneal tubercle, then this test might work to locate the STJ axis. However, since it does attach to the calcaneus, I would imagine your results might not be predictive of STJ axis location in all cases.
  24. So quick to burst my bubble Prof. Kirby, can't we at least try it and see...:dizzy: Moreover, what might it be predictive of? Why is it that when I dorsiflex certain toes I get STJ pronation, while when I dorsiflex other toes I get STJ pronation?
  25. Introducing a new treatment for lateral ankle instability and peroneal tendonitis the "Spooner Wedge": A wedge placed beneath the fourth and fifth toes such that the broad end of the wedge is distal and beneath the toe pulps and the thin end of the wedge is proximal, ending just distal to the fourth and fifth MPJ's. When it flows, it just flows. Test it, let me know your results :morning:
  26. Simon:

    I am assuming you meant to say "Why is it that when I dorsiflex certain toes I get STJ pronation, while when I dorsiflex other toes I get STJ supination? Must have been a late Friday PM when you wrote this one.;)

    You may have indeed created, Dr. Spooner, a new measure of the effects of increases in tension within the plantar fascia to each digit on the subtalar joint (STJ) moments. However, I would tend to doubt that the STJ axis will always be passing directly over the "transition digit" (i.e. that digit where neither pronation or supination occurs with digital dorsiflexion).

    Instead, the Spooner Digital Dorsiflexion Test (SDDT), as you might call your new test, would instead probably be indicative of which slips of the central component of the plantar aponeurosis, when placed under greater tensile force, produce a net increase in STJ pronation moment, a net increase in STJ supination moment or no net change in STJ moment. The results of the SDDT may or may not correlate directly to where the STJ axis passes over the metatarsal heads since the SDDT may also be significantly affected by spatial location of the STJ axis as it passes the medial calcaneal tubercle (where the plantar fascia originates).

    Good to see your creative juices are flowing, Simon. Keep up the good work!:drinks
  27. Perhaps it goes some way to explaining the variation in STJ motion observed with various propulsive strategies among individuals.
  28. David Smith

    David Smith Well-Known Member

    Osss! shihan, do Itashimashta, oregato gozaimashta.

  29. NewsBot

    NewsBot The Admin that posts the news.

    A dynamic model of the windlass mechanism of the foot: evidence for early stance phase preloading of the plantar aponeurosis.
    Caravaggi P, Pataky T, Goulermas JY, Savage R, Crompton R.
    J Exp Biol. 2009 Aug;212(Pt 15):2491-9.

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