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Help with Physics Speak

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Asher, Feb 25, 2011.

  1. Asher

    Asher Well-Known Member

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    Hi all,

    I'm going through my notes from Biomechanics Bootcamp and I'm having trouble with some physics terminology, in particular, inversion moments.

    I understand the premise that orthoses work by altering forces, not motion.

    And I thought I understood what a moment is ... its a force that doesn't necessarily cause a movement.

    Now it seems that foot orthoses work by not controlling foot pronation, but by reducing the rearfoot inversion moment. This would make sense to me if it said 'by increasing the rearfoot inversion moment'.

    For example, Williams, Davis and Baitch's paper 'Effect of Inverted Orthoses on Lower Extremity Mechanics in Runners' which compared standard (Root) orthoses to inverted orthoses reports that although peak rearfoot eversion didn't change between the two orthotic conditions, the inverted orthoses significantly reduced the peak rearfoot inversion moment.

    I would have thought that an orthosis works by increasing rearfoot inversion force.
    Somebody please help me, I'm sinking :sinking:

  2. Hi Rebecca

    orthotics are an reaction force. So they can't increase they can only reduce.

    So we can think of them as an extension of ground reaction force or even as an orthotic reaction force (ORF).

    By that the device can only react to the force placed on it.

    So muscle can create force but liagments,ground and orthotic can only reduce.

    Hope that helps.

    Check your email in an hour I will send you some reading when I get to work
  3. Rebecca:

    The difference is internal inversion moments (e.g. posterior tibial tendon tension) is what the Williams et al paper was talking about whereas, when we, as podiatrists, normally talk about inversion and eversion moments, we are talking about external moments (e.g. ground reaction force). I think most podiatrists don't know the difference between the two...good for you to see the difference.

    Try my Thought Experiment #10....I think it will help.
  4. Asher

    Asher Well-Known Member

    Hi Mike,

    Really! That makes no sense to me. But thanks for explaining.

    I can sort of understand this bit, muscles can pull but ligaments can't, at least not actively like a muscle can.

    I eagerly await your 'further reading' for me. Thanks!

  5. Rebecca if I said create instead of increase does that help. Thought about the terminology and I think create is better.

    I tried a few ways to give a good example using space and helium balloons but doesn´t make sense.

    So I will ask this.

    Do you agree that Ground reaction forces can not create moments ?

    and if so ...

    In-Shoe Pressure Measurement and Foot Orthosis Research A Giant Leap Forward or a Step Too Far? Simon K. Spooner, David K. Smith, Kevin A. Kirby J Am Podiatr Med Assoc 100(6): 518-529, 2010

    ps You have mail .
  6. Johnpod

    Johnpod Active Member

    Hi Rebecca,

    Perhaps I can help?

    If a moment is thought of as a 'tendency' to move and the orthotic has the ability to 'resist' that tendency, you will see that a greater resistance ability requires a greater tendancy to move for movement to actually occur.

    The orthotic is passive and generates no energy of its own, and so, yes, can only resist forces applied to it.

    Clearer now?
  7. Asher

    Asher Well-Known Member

    Hi Johnpod

    Yes, this makes sense.

    Kevin's internal vs external moments seems to make more sense to me. I'll be doing the thought experiment over the weekend.

    Thanks for your help Johnpod.

  8. Ooo ooo let me try. We can play "who can make the complex thing simplest!!"

    You're right. This makes no sense. Which suggests to me that you've missed the point of the paper.

    Kevin hit the nail on the head, laser guided hammer that he is.

    So actually you are right. The insole works by increasing the inversion moment from the insole. So what was Williams talking about.

    Imagine a seesaw with a fat kid on one end and a thin kid on the other. The fat kid is sitting on the ground, the thin kid in the air. The see saw is the foot. The fat kid on the ground is the foot maximally pronated.

    Now, you want to invert / supinate the foot so you grab the fat kid under the armpits and try to lift him to make the see saw tip (invert the foot). But you can't cos he is properly huge. See yourself straining? Mind your back!

    You are the tibialis group trying to supinate the foot. You might just hurt yourself trying that hard! Careful. Put the kid down. let the foot back to maximally pronated.

    Now I come along and Lie on my back under the seesaw on the fat kids side. I push UP under the fat kids ample behind just as hard as I can.

    I am the orthotic, Increaseing (as you said) the inversion moment.

    Now with me doing that, you (remember you're the tibialis group) catch hold of him under the armpits again and try to lift him (supinate the foot). With me pushing up as well as you pulling up you should find it a lot easier now. So your job is easier. You have reduced the inversion moment YOU have to exert by sharing it with me.

    So as kevin, far more succinctly put it. The orthotic INCREASES the inversion moment from outside the body, the insole pushing on the foot. By doing this it DECREASES the inversion moment the tibialis group has to exert. Cos the insole has done some of the work for the foot, d'ya see?

    This is the difference between internal and external moments. An external moment is a force exerted from outside the foot (insoles, shoes, ground reaction force etc) whereas in internal moment is a force on a joint exerted from INSIDE the body (muscles, ligaments, bony end ranges on joints etc)

    So to come back to the initial point, in a nutshell. Much foot pain occurs in structures which exert internal supination moments (tibialis, deltoid ligaments, plantar fascia etc). So the job of insole is to reduce the workload in these structures, to REDUCE the INTERNAL inversion moments. How do they do this? By INCREASING EXTERNAL inversion moments.

    Hope this makes sense. I feel a Column on internal and external moments coming on for pod now. This is an important point.
  9. But you can´t push being the orthotic - you can only resist the fat kid.

    So decide the angle of your knees because they can´t extend or push up the fat kid.

    I think this is a very important concept that people miss.

    using your idea the the effort you provide is only equal to the push from the fat kid minus the effort from Rebecca.

    Which is why I´m slowly getting Simon geometry ideas of how a device works.

    the more extended your knees the less effort Rebecca requires to stop the fat kid moving towards the floor - but you as the device are only a reaction force and if the kid is very fat your knee will flex (deformation of the plastic/EVA) - But you the device cannot extend (push) only resist.
  10. Not sure I agree with you there Mike. If the orthoses is shaped a certain way, there will be more GRF under that part of the foot. "push" is bad terminology to be fair (for the sake of clarity for Asher) but there can be an increase in GRF in a certain area from a reaction force.
  11. Good discussion. I'm going to sit back for now and see how this one develops.
  12. Thats why the geometry concept is making sense, but it is still a reaction force,

    Using your example, but adding 1 thing a time a which the fat kid does not push down with any force - the swing phase if you will.

    Now your walking through the park as come across Rebecca try to resist the downward force of the fat kid, you say I´ll help and get under the fat kid and help.

    But then some Bloke called Murphy says on one condition your must decide the angle of your knees, they can flex but cannot extend beyond that angle - say 130 degrees.

    So you lie on the ground with your knees at 130 degrees, fat kid comes down your knee flex a little, but you resist the fat kids downward motion.

    Rebecca will have less work to stop the downward motion of the fat kid as well. But you cannot push up the fat kid past 130 degree.

    say we have fat kind 300 pounds Rebecca was providing 200 pounds of work to stop the fat kids downward motion the Ground was doing the rest 100 pounds.

    Now with you there is 300 pounds Rebecca 100 pounds you 200 pounds of resistance against the downwards motion - Rebecca less work.

    Now I come along with a fat suction machine such out 150 pounds of fat. The work required by you and Rebecca has reduced. The reaction force required by you is less because the opposite force is less.

    The above example works better if Rebecca says thanks for your help and gets and ice cream.

    So the 300 pounds of of "work" is all on you, reduce the size of the fat kid the "work" required by you is less. due to the downward force being less .

    ie an orthotic is a reaction force it can not push or create a moment.

    But then the fat kid sees Rebecca buying ice cream and hopes off the see-saw . Skinny kid hits the ground and your knees are still at 130 degrees.
  13. Johnpod

    Johnpod Active Member

    What flavour does the ice cream have to be?
  14. Chocolate is probably more calorific and so is best for this analogy.

    I see where you're coming from Mike. Pushing implies a movement as well as a force and if we set aside the flexion component of orthoses there is no movement in reaction force.

    I think this hinges on whether we are seeing the function as a series of instantaneous snapshots or as a 4 dimensional construct. Things on paper loose not only depth but also time.

    Granted that the seesaw analogy does not hold up to substantial extrapolation. So I conceed that point. It was just to try to illustrate the concept of insoles reducing internal inversion moments in a simple way.

    So lets take instead, a foot and a medial heel skive.

    Does a MHS increase GRF under the medial side of the heel?

    I'd say so. As the heel loads the medial heel loads first (because the foot contacts it first). So at that instant the GRF (now ORF) is increased under the medial side of the heel. As the heel loads the effect depends on the vector of the force and that will depend on the friction co-efficient of the material of the orthoses. If it is high then the vector will be more vertical and the Force vector will pass more medial. If it is low then there will be more load under the lateral heel. On a hypothetical frictionless surface the force will all be in the dell on the heel cup.

    So taking an average friction surface on a standard insole. I would say that the ORF medial to the STJA increases until it overcomes the reactive pronation moment from the stiffness in the AJC and takes up all the "slop" in the fibrofatty padding, at which point kinematic change must take place.

    In another hypothetical situation of a fused STJ and a solid heel, the medial wedge will mean ALL of the GRF will be exerted on the medial part of the heel. So yes, the reaction force HAS increased.

    And to you...
  15. True - but some of this comes back to discussion re an any given time force must be equal and opposite

    ok. :drinks
    I´d say yes as well, as for the friction vectors a bit of a black hole for me I am afraid , But would agree the more vertical the vector the more medial the orthotic reaction force.

    again I´m not sure we can say the load would be more lateral heel with a less vertical vector, it may mean greater internal loads say on the Tibialis anterior, I think it depends on the initial strength of the Pronation moment. (I can be barking up the wrong tree here)

    I assume AJC means Ankle Joint C? Depends on the body we may get a peroneal spasm and an increased pronation moment and no change.

    But, In theory yes if the Medial heel Skive is affective we should get a kinematic change

    I´m not saying an ORF can not increase ( I did say that, but as a way of explaining what I mean keep reading ) in reality it will change depending on what the person is doing, with the greater the pronation moment from the person.

    ie walking v´s running with 10 kgs in a backpack the greater the Supination moment from the Orthotic if we ignore deformation of the plastic.

    So the greater the pronation force the ORF must increase as well, unless internal supination forces work harder - but it can only increase in reaction to increased pronation force in this example
  16. Asher

    Asher Well-Known Member

    I am following what you are all saying, I'm quite sure. What I don't understand is why would you want to word it as above. It's totally ambiguous. Why not say either:

    1. Significantly increased external peak rearfoot inversion moment.
    2. Significantly reduced the internal peak rearfoot inversion moment required.

    It's like an intentional ploy to confuse.

  17. Johnpod

    Johnpod Active Member

    Just a couple of paraphrases:

    "The more I strive to be clear, the more obscure it becomes" Horace

    "The art of genius is knowing what to leave out" Sweitzer

    And just a thought.... you can carry words in your head....carrying a seesaw and a fat kid around to demonstrate a principle is bloody hard work!:wacko:
  18. Rebecca:

    The problem you are having with the terminology "rearfoot inversion moments" is understandable. The problem arises from the fact that most biomechanics research papers that use inverse dynamics to determine foot and lower extremity joint moments has, as its convention, a description of these moments as internal moments and not external moments. Therefore, what most foot and lower extremity biomechanics research papers describe as rearfoot inversion moments does not mean the external rearfoot inversion moments that come from, for example, a varus heel wedge of an orthosis, but rather mean the internal rearfoot inversion moments that come from, for example, the contractile activity of the posterior tibial muscle or the tensile force in the deltoid ligament.

    As a result, if, for example, a varus heel-wedged orthosis was added to a runner's shoes, the inverse dynamics software program would tell the researcher that the rearfoot inversion moments had decreased. For those of us reading the paper, in reality, this means that the internal rearfoot inversion moments had decreased as a result of the varus heel-wedged orthosis. This creates great confusion for many clinicians since, for the past 25 years, myself, and many other podiatrists who lecture on subtalar joint axis location theory, have used the convention of speaking about external rearfoot inversion/eversion moments which, basically, will yield the exact opposite answers to what would occur from much of the rest of the international biomechanics research community that use the convention of their inverse dynamics software programs. So when the external rearfoot inversion moments are increased from a movement of the center of pressure more medial on the plantar foot, the internal rearfoot inversion moments will generally be decreased also. If you simply are aware of this difference of convention for external-internal moments when papers on foot and lower extremity kinetics are being read, then the confusion level should be greatly diminished for the reader.

    I have had this very same discussion with researchers such as Irene Davis and Chris MacLean over the past five years and they are now, when they are lecturing, trying to make sure that their audiencse understand that they are reporting internal moments, and not external moments, so that they will avoid confusing the audience about their research results. It would be nice if the journals also made this internal-external moment distinction crystal clear when these papers are reviewed for publication since, I believe, it is currently one of the biggest obstacles that is preventing clinicians from understanding the clinical significance of these research studies that use force plates, 3D motion analysis and inverse dynamics to analyze foot and lower extremity kinetics.

    Hope this explanation helps.

    This is a great topic and you are, Rebecca, very smart to recognize this and bring it up for discussion on Podiatry Arena.
  19. Robert, Mike, John:
    Restoring force of the orthosis?
  20. In reading the other responses on this thread, I am confused by the responses, Rebecca, so you are not alone.

    A moment is a rotational force. Moment is defined as M = F x d, where M = moment, F = force component which is perpendicular to moment arm and d= length of moment arm.

    Both a force and moment are governed by Newtons Laws of Motion. A moment may either 1) accelerate rotational motion, 2) decelerate rotational motion, or 3) stabilize rotational motion against counter-opposing moments.

    Hope this helps.
  21. Quasar

    Quasar Member

    Kevin is spot on.

    For example, by resisting inversion of the rearfoot, an orthosis applies a torque counter to the pronatory torque in the first part of stance phase.

    Now, there always is a reactive force to any force by Newton's Third Law. This is due to the inertia (resistance to the applied force) inherent in all matter. By resisting the applied force, the forced object turns the applied force against the source, hence the reaction force.

    The pronating heel encounters the orthosis and applies a force to it. The orthosis applies an equal reactive force to the heel at some remove from the axis of rotation thus creating the torque countering the pronatory motion.

    This also allows the PT to be more effective by coming into play with an increased lever arm about the pronatory axis.

    Were there no orthotic help the weight forces would have an overwhelming torque, whereby the PT would not be able to create an adequate countering torque due to a decreasing lever arm.

    I hope this helps. Condensed explanations involving many elements can be trying!

    In forty years of on and off Physics teaching as an adjunct, Newton's Third Law never failed to be a mystery to students. In short, all objects resist being pushed, and in their resistance turn the force back on agent initiating the force: action...reaction.

    Hope this helps.
  22. Quasar

    Quasar Member

    P.S.: A moment of force is a torque.:deadhorse:
  23. Here are some nice articles on inverse dynamics, external and internal moments for those who want to become more educated on these subjects:


    http://books.google.com/books?id=oW...s, internal moments, external moments&f=false



    Great author, this last one.;)

    Have a nice weekend, everyone!
  24. Craig Payne

    Craig Payne Moderator

    That took me years to get the hang of in the context of the higher adduction moments being a risk factor for medial kee OA. I could not work out why some studies claimed a high abduction moment was a risk factor and then other studies were showing a high adduction moment was a risk factor .... I missed the bit about external and internal moments and Newtons 3rd....
    ...then I went back to being confused when some studies showed a high varus torque was a risk factor ... :drinks .... I finally got it.
  25. Quasar

    Quasar Member

    Glad it finally made sense, Craig. Never ceases to amaze me how simple things seem once we get them. Getting there could be a bear.

    BTW, I had an aurora expert from Australia speak to my Astronomy class. He actually designed and built an IMAX camera for this purpose. I greeted him with, "How does it feel walking upright??":dizzy:

    Greetings to all Down Under...
  26. Quasar:

    Where do you teach at? Is your background more in physics than biomechanics?

    By the way, within the international biomechanics community, the term "torque" or "moment of force" is not used much anymore, being replaced over the last 20 years by the synonymous term, "moment".

    Good to have you contributing here on Podiatry Arena.
  27. Quasar

    Quasar Member

    Hi Kevin,

    I currently am semi-retired. I closed my practice last year due to declining revenues due to insurance and other matters. My patience in dealing with insurance, especially Medicare had run out.

    My main area of interest in practice was in applied biomechanics; viz., prevention and improvement of function. Part of my practice was seeing patients of a Russian sports medicine specialist.

    My background was in Physics. I have taught Physics since working on my D.P.M. at the Ohio College of Podiatric Medicine. At that time it was at Cuyahoga Community College. At this time I am teaching Physics and Astronomy at Lasell College in Newton, MA and Quincy College.

    I enjoy teaching, but I must admit that the quality, both skills and attitudes, of students has dropped by several orders of magnitude since I started...an experience acknowledged by every teacher I've spoken to.

    I'll quit here, as one quality of age is the accumulation of soap boxes!!

    Best regards.
  28. Arthur:

    You should stick around here and contribute more on Podiatry Arena. Your practice experience and physics knowledge could be very helpful to many here on this international forum.:drinks
  29. Hi Simon, there will be a component of elastic energy return, But as Nigg discusses, this energy return must be at the right time.

    As the energy return will occur micro seconds after the impact we don´t know what effect it will have.

    So as COP moves anteriorly and the STJ axis is at new equlibrium points we may not be able to know the effect of the orthotic returning to it´s orginal shape.

    If there is no heel contact when the energy return occurs then then will be no effect.

    So the further anterior COP the less of an effect the elastic energy return of the plastic may have.

    This will be material dependent as wll some materials returning to shape faster.

    But granted the returning energy may create a moment.
  30. efuller

    efuller MVP

    The other "definition" of moment that was left out was

    net Moment = moment of inertia x angular acceleration.

    The term net moment is important because you can have several different moments acting on the body you are interested in.

    One thing that helps with internal and external moments is carefully defining the moment you are talking about. For example The moment from ground reaction force acting on the subtalar joint. Or the supination moment from the posterior tibial tendon. If ground reaction force and posterior tibial tendon tension were the only sources of moment acting on the STJ then you would add them together to get the net moment. When the moment from ground reaction force is greater than the moment from tension in the post tib tendon then you will get a net pronation moment and you will see STJ pronation.

    To evaluate the moment from ground reaction force you use (moment = force x distance) the distance from the line of action of the force relative to the STJ axis. The bigger the distance the greater the moment. This is how orthotics change moments at the STJ. They change the distance of ground reaction force to the axis. The location of ground reaction force can be averaged to a point called the center of pressure.

    An orthosis with a varus wedge effect will shift the center of pressure more medially. A medial shift in the center of pressure will increase the supination moment from the ground. However, the center of pressure may not have been moved to a position that is more medial than the location of the STJ axis. So, even though it was moved medially, it may still be lateral to the STJ axis. So this would still cause a pronation moment, but a smaller pronation moment than was present before the orthotic was used.

    Hope this helps.
  31. efuller

    efuller MVP

    Orthotics don't reduce GRF either. They change the location of ground reaction force which changes the moment from ground reaction force.

    I think you meant react, rather than reduce.

  32. Should have read react - sorry folks
  33. Asher

    Asher Well-Known Member

    Here's another question, this time about dynamic loading at the knee joint.

    In normal walking, GRF passes media to the joint centre. This produces a compressive force on the media aspect of the knee joint surface. This is termed an adduction moment.

    I can easily imagine the compression on the media aspect of the joint. But how do you call it an adduction moment?

    Adduction occurs in the transverse plane. Knee joint compression has to happen in the frontal plane, doesn't it?

  34. Asher

    Asher Well-Known Member

    That's a dumb question I think. Because hip adduction and abduction happens in the frontal plane too. There's obviously a different way of describing motion for the foot compared to the rest of the body.
  35. efuller

    efuller MVP

    The definition of adduction that I learned was that the distal part of the distal bone of the joint moved (or has a moment trying to move) toward the midline of the body. So, there is a lot of similarity between inversion and adduction. So, adduction can occur in the frontal plane.


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