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Bojsen-Mollers high gear/low gear

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Craig Payne, Apr 29, 2006.

  1. kevin miller

    kevin miller Active Member

    As hypermobile as both feet looked, the trigger for the most pronated foot shouldhave been found behind the knee, where his fibular head has been driven. I have ween watching this one for a while...Bojen Moller is one of my favorites. He has it almost right and,in contradiction to what Kevin K. says, is nowhere near snake oil. Kevin K. made a statement that there is no reason for the COM to change side during gait. Did I misunderstand that? If not, why does an F-scan make a sin curve during normal gait. More to ths topic, why do you thing the normal foot has a parabolicmet head shape? To create angular momentum. Then all of this momentum hits this huge 1st met, and it is turned into vertical lift. The Com has to be raised in the center of gravity to "feed" the spine. The COP, with the COM following - or maybe hte other way around - DOES shift from one foot to the other to get this loft. If you do not see this in your patient, then the foot is already broken down. As for High gear/low gear, I just described low gear. In high gear, there is no need for angular momentum because the indvidual is in full run moded and energy storage and return is via fascia and other soft tissue. When will we stop using pathologial feet as our "norms" and simply consider how the foot is put together and functions in synergy with the rest of the body. The feet are the gateway to energy transfer to the spine, they have alot of moving parts with a lot of joints that WILL fail under either extreme force or long term-low load. You all know this. Why is is so hard to grasp that the fett, then are most likely to fail? They take ALL of the force generated by sagital plane COM lift and filter it to make the rest of the kinetic chain work. Of course the are the first to go. The body can adapt and keep you walking....maybe not as efficiently...and high gear may well be just as efficient for a long time after the foot begins to fail simply because the foot is not the primary mode of force transfer in high gear, as stated above. Bojen-Moller was no idiot, he was ahead of his time....and ours, apparently.

    Kevin M
     
  2. I don't think f-scan measures centre of mass position. If you are referring to the COM analyis software which goes with the system, I thought the sign wave is supposed to be the vertical displacement of the COM, not the lateral displacement?

    It rarely is parabolic: see the independant work of Demp and Robbins.
     
  3. kevin miller

    kevin miller Active Member

    Hey Simon,

    It may be. If Bruce Williams were lurking out there he could tell us, but i thought it followed COP from one side to the other and a break in the "normal" sin pattern had some meaning. It should now be painfully obvious that I know nothing about how the machine really works. :)

    Thanks, Simon,
    Kevin M
     
  4. kevin miller

    kevin miller Active Member

    Sorry to not get this in on the first one, Simon,

    You wrote: It rarely is parabolic: see the independant work of Demp and Robbins.

    While I appreciate that, I hope to soon have some work finished that demonstrates the opposite. Evne in cadavers, If I cut the tendons so that I can get every joint congruent, the met heads fall into a parabola. If the lat cun disengages from the NAV and Cuboid, leaving the medial and lat columns to opperate independently, the 1st met head, with the second and third following, increase their distance froma given point on the plantar surface of the calcaneus. In other words, of the foot pronates - at all - the parabola is lost. On a naturally high-arched person, they could appear to have normal arches, but demonstrate what i describe. I haven't gathered enough data to run my mouth just yet, but it sure is looking suspicious.

    Cheers,
    Kevin M
     
  5. drdebrule

    drdebrule Active Member

    Re: Bojsen-Mollor high gear/low gear

    :dizzy:

    Craig,

    I have enjoyed all the comments on this post.

    Now that it is 2008, do you have more to say about pressure patterns and the gearing mechanism of the foot in propulsion?

    My experience with F-scan pressure studies suggests that the low gear pattern may include the hallux. Whay say you?

    Michael DeBrule DPM
    Marshall, MN
     
  6. Re: Bojsen-Mollor high gear/low gear

    Michael,

    Now, nearly 2 years later, I still think high gear-low gear propulsion is a very weak and misleading concept that has little mechanical significance.
     
  7. ellabellawish

    ellabellawish Welcome New Poster

    Re: Bojsen-Mollor high gear/low gear

    thank so much craig for this well explained concept of high gear and low gear.
    Now, i understood how it works.

    cheers
     
  8. David Smith

    David Smith Well-Known Member

    Re: Bojsen-Mollor high gear/low gear

    Craig and all

    This thread resurfaced and I had not read it before (or could not remember reading it before) so I read it this time. It is a long discussion so I hope I don't go over too much that already has been said.

    For a start I had not previously read BM high low gear theory although I thought I knew the concept. In fact I didn't. Previously I thought it was to do with the high gear of the hallux windlass versus the low gear of the lesser digit windlass action.
    But having read Craigs summary I see it is not. From Craigs description I see it is to do with lever / moment arm lengths of the 1st 2nd MPJ axis versus the moment arm of the 3rd-5th MPJ axis relative to the achilles tendon insertion. IE the 3rd - 5th is shorter. Therefore since power = moments * angular velocity the shorter moment arm potentially will produce less power for propulsion. Therefore the theory is that the CoP moves across the foot lateral to medial so that potential propulsive power is at its optimum. Also the secondary purpose of directing the forward progression the correct direction onto the next step ie toward the medial.


    This is a convenient theory but may I propose some other alternatives.

    1) The CoP or CoF (centre of force) that is commonly spoken of is that of vertical force as characterised by a pressure mat. The true CoF also has a horizontal component. Therefore CoP can be anywhere on the foot and still direct the CoM medially during propulsion.

    2) The body doesn’t need to do anything, it is allowed or tends to to follow the line of least resistance perhaps. The switch is just a natural progression. Due to the mechanics of the foot and the nature of foot placement in 'normal' (I prefer the term 'standard') walking the CoP must go from lateral to medial. If it does not some other force must be applied to stop its natural standard course. IE the foot remains supinated. Therefore the low gear propulsion is an effect of non-standard mechanics and not the cause.
    (Which is basically what Eric said and Simon showed, I think, IE the foot/stj will remain pronated and in the high gear position until there is some other supination moment acting on it)

    3) In the reference frame of the foot the CoP may be deviating to the medial but in the global reference frame there could be zero or far less medial deviation if the foot is abducted ie toe out, which it usually is. Therefore the CoP does not deviate because it needs to rather the CoP naturally progresses to the medial foot and the higher propulsive forces are bourne by the more massive structure of the 1st ray and MLA. In this case even though the CoP deviates to the medial, in the foot reference frame, the CoM is not necessarily being directed medially.

    4) As Kevin said the anatomical position and the compliance of the rays will determine the position and relative transverse displacement over time of the CoP.

    5) Assessment of 2D single camera photography of 3D events can have confounding elements that make evaluation of the action of interest unreliable.

    6) Do any studies compare the GRF difference and power output between lateral foot push off and medial foot push off? Is there any difference in CoM velocity or is power loss at the ankle compensated for by increased power at the contralateral hip as in unilateral Transmetatarsal amputees. TMA subjects do have decreased ankle power output but it is not clear if this is due to a short foot or antalgic response to chages in forces. Only those amputations that are above the MPJ's have low power, if the MPJ head is intact then power output is very similar to normal. Wouldn't this be the same for low gear push off?

    7) If you have a short lever you can increase the force and achieve the same power output. Antalgic response to higher forces may cause gait compensations and therefore low gear push off becomes less propulsive to avoid tissue damage.

    Bruce wrote
    This seems to make sense at first but I would say that if the foot is propulsive in the low gear position then it is because of compensation for some other pathology and the supinated position itself will not necessarily lead to excessive internal tissue stress.
    If the patient just walks slowly then they may not increase tissue stress to a pathological level. If the compensation was for say, a hallux rigidus and the clinician, only viewing the low gear push off, added a valgus forefoot wedge and restored the high gear push of. How would this affect foot pathology? Wouldn’t the patient be worse off.


    I would take the view that the lateral or low gear push off is an alternative to the natural progression of the gait cycle where push off is on the medial or high gear side. It is a compensation for pathology or anatomical variation and not a pathological condition itself. Propulsive power may be less but not necessarily and the CoM direction onto the next step is not necessarily sub optimal. The clinical presentation of a low gear / lateral push off may be a good clue to diagnosis but is not a direct indicator of pathology itself.

    Just my view for what its worth,

    Cheers Dave
     
  9. podpup

    podpup Member

    "Propulsion is via met heads one & two (ie BM's high gear or transverse axis) ... a good thing.

    Propulsion is via met heads 2 to 5 (ie BM's low gear or oblique axis) ... a bad thing.

    Here is the picture above marked with SiliconCoach drawing tool to illustrate this":

    More on this one!

    Has there been any research conducted on using video analysis to look at propulsion in this way? By this I mean High gear vs Low gear and the application of video analysis to this theory. Practitioners have used silicon coach to better view medial or lateral toe off. This technique seems mainly anecdotal....I would like to use anything in the literature on this in some research of my own. Very interesting thread!

    Regards podpup
     
  10. markjohconley

    markjohconley Well-Known Member

    Craig, for the 'oblique axis' to be the 'path of least resistance' then the counter rotatory force to the force applied by the achilles tendon (ATF) must be greater in the 'transverse axis' scenario, assuming the ATF is equivalent for both 'axes'. Why would this be so? If respective ATF's = counter ATF's then difference in lever arm lengths wouldn't account for it. Thanks, mark c
     
  11. Moreover, this assumes that there are two rotational axes, in reality there are five, one for each MTPJ. The dorsiflexion stiffness of each digit will be important- see Spooner's Test ;). I know Eric Fuller has spoken about this previously and the assumption of constant velocities etc. Perhaps he'll chime in here...
     
  12. efuller

    efuller MVP

    from post #16 of this thread

    Just because you have a longer lever arm does not mean a faster velocity of push off. An erroneus assumption of constant angular velocity has been made. With high gear push off ground reaction force has a longer lever arm to resist ankle joint plantar flexion. So much greater force would have to be present in the tendon, with long gear push off, to have the same angular velocity as a short gear push off. We do not know Achilles tendon tension, so we cannot conclude that high gear is automatically faster. A real world example. Get on your bicycle pointed up a slight incline and put the bike in the lowest gear and in the highest gear and start pedaling. In the high gear you might not be able to move the pedals because the resistance is so high. This may be the case for the foot. In the lowest bike gear you will certainly, be able to move the pedals, but you may not be able to go very fast. As you try different gears, you may find one that is optimal for speed. We don't know where the "gearing" of the foot is in relation to optimal speed production.

    Cheers,

    Eric Fuller
     
  13. Petcu Daniel

    Petcu Daniel Active Member

    Looking at the COP trajectory I've seen, generally, 3 patterns:
    -1- COP is oriented directly to MT1 joint, with an increase in the pressure under MT1,
    -2- a normal COP, as described in some papers - COP directed laterally and then towards MT1-MT2 space and finally towards hallux
    -3 - a laterally deviated COP especially in the forefoot area with an unloading of MT1
    I'm thinking that low gear concept could be related with the timing for a proper loading of the MT1 and windlass mechanism. Practically I think that low gear is producing a 'delay' through a initial lateral deviation of COP [a lengthening of the COP trajectory] assuring a proper timing of the MT1 loading. Without this delay/ lateral deviation the MT1 could be too soon loaded [a shorter COP trajectory] while a bigger delay could lead to a hallux unloading.
    What do you think?
    Thanks,
     
  14. efuller

    efuller MVP

    I'm not liking the concept of "timing for a proper loading." Different tasks require different loading. What is proper loading?

    The center of pressure path in gait is the result of different muscular output. The Hicks paper, from the 1950's, is still valuable for understanding this. Hicks put a cadaver foot, with its plantar surface, on a board and then put a pin under the board and applied a downward force to the top of the tibia. He then moved the pin under the board until he found a balance point. That balance point is the center of pressure. Then he put tension in various muscles and then moved the pin to the new balance point. He showed how tension in the tendons shift the location of center of pressure under the foot. The posterior tibial tendon shifted the balance point laterally. The peroneal muscles shifted the balance point medially. With walking, heel lift occurs because of tension in the Achilles tendon is shifting the balance point anteriorly (and a little laterally). Whether you see a medial shift, or lateral shift of the center of pressure after heel lift is in large part dependent on the relative tension of the posterior tibial and peroneal muscles. Some other factors are available, among them; range of motion available of the joints, and STJ axis position. STJ axis position will vary the resistance to motion caused by the posterior tibial and peroneal muscles.

    We should be getting away from what we think is supposed to happen and look at what does happen. When there is relatively more tension in the peroneal muscles, there will be a tendency toward more load on the first ray. More load on the first ray will tend to put more load on the windlass (medial slip of the plantar fascia). Yes, tension in the plantar fascia will generally cause a supination moment at the STJ. But, this supination moment may not be high enough to overcome the pronation moment from the peroneal muscles. For this step, the STJ will stay pronated and there will be a tendency toward functional hallux limitus because of high tension in the plantar fascia. This is what happens. There are other events that could happen with different muscle activation.
     
  15. Petcu Daniel

    Petcu Daniel Active Member

    What means "more load on the first ray" ? "More" compared with what? How it is seen this 'more' in terms of plantar pressure in walking?
    Thanks,
     
  16. efuller

    efuller MVP

    I wrote:
    When there is relatively more tension in the peroneal muscles, there will be a tendency toward more load on the first ray.

    The peroneal muscles will tend to cause STJ pronation. If there is range of motion available there will be eversion and with eversion there will be less pressure on the lateral forefoot and more pressure on the medial forefoot. Is that what you were asking?

    You could look at the center of pressure paths you described a few posts back.

    "-1- COP is oriented directly to MT1 joint, with an increase in the pressure under MT1,
    -2- a normal COP, as described in some papers - COP directed laterally and then towards MT1-MT2 space and finally towards hallux "

    Example 1 could be someone with a medially deviated STJ axis. These feet in static stance will tend to have high loads on the first met head and low loads on the lateral forefoot. In gait the lateral forefoot contact will cause rapid eversion and high loads relatively quickly on the first met head. Relatively quickly would be before the heel lifts. This would cause the center of pressure path to move, what appears to be directly for the first met head. The person may have learned that it is not worth the effort to use the posterior tibial muscle to such an extent that the supination moment from the muscle overcomes the pronation moment from the ground.

    For example # 2 is fairly typical for a late stance phase pronator. You see the heel lift causing the center of pressure move towards the forefoot. Once the center of pressure reaches the forefoot, the peroneal muscles could be contracting causing STJ eversion and an increase in load on the medial forefoot. A possible explanation for this is that the person has a more laterally deviated STJ axis. The anterior shift of the center of pressure may not be far enough lateral to cause a pronation moment from ground reaction force to overcome the small supination moment from the Achilles tendon. After years of walking the person has learned that the safest thing to do is to use the peroneal muscles to pronate the STJ to prevent an increased likelihood of an ankle sprain.

    This is just arm chair theorizing on potential explanation for different center of pressure paths. It could be tested by comparing peroneal emg with STJ axis locations and center of pressure paths.
     
  17. Petcu Daniel

    Petcu Daniel Active Member

    I've attached a small movie from the same subject showing for the left foot a pressure path as the one described at point 1 [1- COP is oriented directly to MT1 joint, with an increase in the pressure under MT1,] and for the right foot a pressure path as the one described at point no.3 [3 - a laterally deviated COP especially in the forefoot area with an unloading of MT1].
    -What can be a possible explanation for this behavior?
    -The plantar fascia [windlass] will be loaded in a different manner for the left foot [earlier ?] compared with the right [delayed ?]. Which are the possible implications ?
    Thanks in advance!
     

    Attached Files:

  18. efuller

    efuller MVP

    Both feet are interesting in that there is a delay in midfoot loading. Usually you see the forefoot and midfoot load at about the same time. I've seen that delay with a flexible cavus foot. The foot on the right in the picture takes a really long time to load the first metatarsal. It appears as if the foot is staying inverted until after heel off. This could happen with a partially compensated varus where the foot does not have enough eversion to fully load the medial forefoot. Another possibioity is that the person is choosing to use their posterior tibial muscle to keep the foot supinated to delay loading on the first metatarsal head. You could use the maximum eversion height test to differentiate between these two possibilities. As for the implications for the delay in plantar fascial loading for the foot on the right side of the picture. It doesn't need to load the plantar fascia until late because most of the bending moment is on the lateral column. If this pressure pattern happened on most steps this foot is more likely to have lateral column pain and less likely to have symptoms related to the windlass. (This is what I was getting at in the previous post when I said we should worry about where the force is rather than on where we think the force is supposed to be.)
     
  19. Petcu Daniel

    Petcu Daniel Active Member

    Thanks Eric,
    And for the left foot? It doesn't seem to be a foot with medially deviated STJ axis!
     
  20. efuller

    efuller MVP

    The left foot could be a lot of different foot types. There is enough eversion range of motion for the medial forefoot load early in the step (compare to right where medial forefoot loads very late). As heel off occurs, with this much range of motion available, the location of average point of force on the forefoot will be determined by the relative activation of the posterior tibial and peroneal muscles.
     
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