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orthotics how do they work ? - Kinematic v´s kinetic debate

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, Feb 12, 2011.

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    I don´t think this has it´s own thread yet.

    Its been discussed many times in threads but B Nigg Book - Biomechancis of Sports shoes has raised the question in the public domains. How orthotics work ?

    What is not under debate is that orthotics or as Benno is fond of calling them inserts work. The debate is how ?

    The following is taken from here as a start point.

  2. Griff

    Griff Moderator


    This is the paper that first opened my eyes to the kinetic effects of foot orthoses:

    Williams, D. S., McClay Davis, I., & Baitch, S. P. (2003). Effect of inverted orthoses on lower-extremity mechanics in runners. Medicine & Science in Sports & Exercise, 35, 2060-2068.
  3. Certainly the kinetic effects of foot orthoses are much more substantial than the kinematic effects. We know that from the research. The most important question to ask may be: why is this so?

    One of the biggest reasons we see mostly kinetic effects is that, for the most part, researchers are largely fixated on only measuring one joint of the foot, the subtalar joint (i.e. rearfoot complex). Unfortunately, the subtalar joint, in many people, is maximally pronated or is a few degrees from maximally pronated during the midstance phase of gait so of course the kinematic changes will be slight and often not more than the range of error of the skin-mounted markers that are used to measure that motion. Until researchers start measuring the kinematics of the joints of the foot that have much more range of motion, such as the talo-navicular joint, with and without orthoses, they will continue to see little kinematic effects from foot orthoses on foot joints.

    Another important factor that one must consider is that foot orthoses do not cross the subtalar joint, but they do cross the talo-navicular joint, midtarsal joint and midfoot joints. Therefore, foot orthoses have much greater mechanical potential to affect the kinematics of the joints of the midtarsal joint and midfoot than they do to affect the kinematics of the subtalar or ankle joints (i.e. joints which the foot orthosis doesn't cross).

    Orthotists have known for many decades that the best way to brace and limit motion of a joint is to design that brace to span the joint of interest to create counter force on both sides of the joint axis that limits its motion. Since foot orthoses do not cross or span the subtalar joint, but a high top boot or ankle-foot orthosis does span the subtalar joint, foot orthoses are mechanically limited in their ability to "brace" the subtalar joint. Therefore, high top boots and/or ankle foot orthoses are critical when increased "control" of subtalar joint motion is required such as in a patient with posterior tibial tendon dysfunction that has a highly medially deviated subtalar joint axis or in a patient with chronic lateral ankle instability.

    This is one reason that I am never too worried by those researchers that claim that foot orthoses "weaken" the foot since the foot orthosis is simply not powerful enough to limit the motions of the foot and ankle if the body (i.e. central nervous system) deems it necessary to use that joint motion to accomplish a weightbearing activity. However, what foot orthoses are very good at accomplishing is reducing the demand on certain structures within the foot and lower extremity so that the human locomotor can work more efficiently, with less stress on certain structures, with less fatigue of "overused" muscles and with less chance of injury to the structural components of the foot and lower extremity during the repetitive demands that are placed on these structures during daily activities.
  4. Kevin that one of the things from our Midtarsal joint discussion that really stuck with me- the fact that orthoses will have a greater effect on the MTJ rather than the Subtalar joint Kinematics.

    or as you say much more clearly -
  5. Foot orthoses can ultimately only ever directly influence kinetics, whether a kinematic change comes of these altered kinetics is down to the body. These days I'm more into deconstructing the foot orthosis, to better understand the way in which it influences kinetics. Here's where I am at: foot orthoses can ultimately only influence: geometry at the foots interface, friction at the foot's interface or stiffness at the foots interface. Ultimately all of these factors are geometry dependent.

    I'll be discussing this and more at biomechanics summer school 2011.
  6. Craig Payne

    Craig Payne Moderator

    Depends what you mean by "work"?

    If by "work" you mean the patients symptoms get better, then that happens by reducing the load in the tisues. That can only happen via kinetic changes.
  7. Or psychologically.
  8. N.Smith

    N.Smith Active Member

    If by "work" you mean the patients symptoms get better, then that happens by reducing the load in the tisues. That can only happen via kinetic changes.
    12th February 2011 10:16 AM

    At some point the Kinetics will have a Kinematic effect... if you keep applying force. Is this relivant? Has anyone seen a patient, saying the "orthotics aren't working anymore"?, add more correction, change a posting here or there or make a new job with more correction, which applies more force to the foot and the symptoms stop. Is there a point where the Kinetic force applied to the foot has a used by date, for symptom relief? and is it better to apply as much force as possible before TOP, to the foot so the surface geometry of the positive model mimics this? Just a thought!

  9. Kinetics directly affects kinematics. However, kinematics also directly affects kinetics. Kinematics can't be altered without a change in kinetics. However, since we can't directly observe the changes in kinetics that occur with foot orthoses and can only directly observe the change in kinematics that occur with foot orthoses in the gait examinations of our patients, then we use kinematic evaluation to infer how our foot orthoses are "working".

    Kinetics and kinematics are so closely mechanically intertwined, that a discussion of one, without a discussion of the other, results in only a partial mechanical analysis of how foot orthoses work.
  10. By Work I did mean patients get a reduction in symptoms - should have said so, rather than mechanical work.
  11. Which maybe another important role in the windlass mechanism - if we have a change in the shape of the MLA ie more curved the stiffness of the foot will have increased .

    ps hope your daughter (Grace?) is feeling better.
  12. Something I´m still working my way through - Which Eric helped with a while ago.

    if to every force we must have an equal and opposie force . When looking at a joint and axis, if we add an orthotic device which alters the location, magnitude, and temporal patterns of ground reaction forces acting on the plantar foot during weightbearing activities .

    So then we have X net force /axis/ X net force the forces must be equal an opposite at all times, the intersting thing is that what makes up the X can and will alter - is there anyway to predict what makes up the X force ?

    so we have ORF,GRF,muscle action, ligament resistance to force,bone compression force /axis/ GRF,muscle action, ligament resistance to force,bone compression.

    Is it possibleo predict where the ORF will make the opposite side work more ?

    Sorry if that does not make sense ,still working it all out
  13. I've had an itch for a long time. You've just scratched it for me! Thankyou kevin!!!

    My itch was this. Working on standard grf and vector concepts, and assuming a manter axis, I've been working on the basis that a higher friction co-efficient gives a more vertical force vector, passing more medial to the stj axis gives a more efficient supination effect.

    But against that there was an intuitive and demonstrable belief that a higher medial flange would give the most effective brake to navicular drift. This seemed at odds withe the above, because it would make the vector more horizontal, nearer to the axis.

    When you said about crossing the axis, it all clicked into place. Under the heel I do want my force vector as far from the axis as possible, to give the best lever arm. But in the arch area I want the force vector to go straight through the lever arm don't I!!! Because another pivot has been introduced.

    Was this just me being slow? Has everyone else known this forever? Or is this news to anyone else?
  14. I find my thoughts careening down a path which is at best heretical and possibly entirely wrong. For the first time in 5 years I'm questioning the applicability of SALRE in weight bearing function. I no longer believe the foot rotates around the sub talar axis in weight bearing!

    This is scaring me!!
  15. You are going to need to expand upon this, Robert. But I should say that I've never believed that the foot rotates about the subtalar joint axis in weightbearing function; I do believe that the calcaneus and talus rotate about the subtalar joint axes; the talus and navicular rotate/ translate about the talonavicular joint axes; the navicular and medial cuneiform rotate /translate about their joints axes etc.

    I've been trying to tell you for some time now that the navicular is not part of the subtalar joint, so why would the best way to control its motion be via the subtalar joint? Hence we had our conversation on the navicular shelf technique. The best way to control navicular drift should be through a three point pressure system: lateral heel, navicular and lateral forefoot.
  16. Neil this may have something to do with Davis law. With increased load which does not create plastic deformation soft tissue will get stronger. ie a training effect.

    So if we have add supination vector to the equation from an orthotic device around the subtalar joint axis , the pronation muscles and ligaments lateral to the Subtalar joint axis will have increased load/stress, if the force in not outside their Zones of optimal stress over time the pronation muscles and ligaments lateral to the subtalar joint axis will get stronger.

    By being stronger they will have a greater effect to increase the pronation movement (muscles) or resist the supination moment (muscles and ligaments), which may be the reason why the device has less effect and will also mean that the tip Over Point ( TOP ) will change over time.

    The above discussion excludes the fact that the device will also change due to loads over time.

    Edit I also used the transverse plane example to make it easier to visualise.
  17. Craig Payne

    Craig Payne Moderator

    Some people define "orthotics working" as being a realignment of the skeleton rather than a reduction of symptoms.
  18. N.Smith

    N.Smith Active Member

    True! But, in a perfect world, wouldn't that mean that the forces applied to the foot, weren't in the right spot, rather than a "realignment of the skeleton" with the forces not applied to the right "spot"? Just a question!

  19. It's not just the location of the forces, it's their magnitudes and lines of action. Realignment of the skeleton is interesting, I lectured on it out in Portugal last week. As Kevin noted kinematics and kinetics are inter-related. If we take a frontal plane view of the lower limb, a vertical column should minimise internal stresses when compared to the forces in a bent column under the influence of gravity. So an everted heel should have a bigger force couple attempting to further evert the foot than a heel and leg that are vertical. Viz an everted heel will require greater internal stresses to maintain equilibrium than a vertical heel. Root was right, but not necessarily for the right reasons.
  20. Cracked this off its own thread.
  21. Actually it is the position of the talus relative to the leg which is important, and/ or the calcaneus relative to the talus. Hence Mert's desire to perform measures in subtalar joint neutral- to take the angulation of the talus out of the equation. What we really need is to make the talus align either vertically with the leg, or vertically with the calcaneus, if we wish to model the force couple between the leg and the heel and thus, take the talus out of the equation (is this subtalar joint neutral?). I've taught myself something about Root theory today. Cool, :cool:, double :cool: As Jeff and some of the old hands will testify, I was pretty vocal about the failings of Rootian biomechanics back in the days of the old mailbase, but the more I look at it, the more I think there was plenty of sense in it. The problem is if we look from posterior to anterior in the frontal plane, how do we examine the angular relationship between the talus and the leg, or the talus and the calcaneus? This, I think, is what was missing...

    Remind me, what do the bone pin studies tell us about talar alignment to the leg and talar alignment to the calcaneus?
  22. I feel that Mert Root's main failing in his theory was that he was trying to determine subtalar joint function by only analyzing the calcaneal position/motion (i.e. posterior exit point of subtalar joint axis) without considering the position/motion of the head and neck of the talus (i.e. anterior exit point of subtalar joint axis). If he had spent more time thinking and analyzing this critical factor of talar head/neck position/motion, I believe he would have come up with a much better theory............thereby saving me a whole lot of time in my life trying to figure it out.
  23. I think the problem was that he had no way of isolating the talar position within the ankle mortice. If we (you ;)) were to draw a diagram looking from posterior to anterior in the frontal plane of the leg, talus and calcaneus segments, and then attempt to add in the forces, how would the frontal plane angulation of the talus within the ankle mortice influence the calculations? He was incorrectly assuming that the leg and the talus were a rigid body in the frontal plane.

    Here's my attempt to diagram it (top = leg, middle = talus, bottom = calcaneus), add in the forces...

    Attached Files:

  24. Romeu Araujo

    Romeu Araujo Active Member


    This is the right point. It's always around this, and "ever, ever, ever, ever... ;)"

    Great thread.


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