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Effects of Force on Object With No Object Motion

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Kevin Kirby, May 15, 2006.

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    Many experimental studies of foot and lower extremity biomechanics are kinematic studies, where the position or motion of the foot are noted to change or not to change when an intervention is made to the foot (e.g. application of foot orthosis).

    If an orthosis is applied to a shoe and the patient gets better with the orthosis, but the orthosis is also found to not alter either the position or motion of the foot, what does this mean?

    Does this mean that the orthosis could not have possibly have produced the change in symptoms because it has not produced a change in position or motion of the foot?

    Is it possible that a foot orthosis, which does not change the motion or position of the subtalar or ankle joint, actually causes some change in the foot by applying an external force on the foot without resultant foot motion?

    If it is possible for a foot orthosis to improve foot pain mechanically, without causing a change in motion or position of the foot, then how can the orthosis possibly do this??

    These are some questions to ponder for those of you who like to think about such things and also for those of you who probably should be thinking about such things.
  2. Donna

    Donna Active Member

    Hi Kevin,

    I have been thinking about these concepts a lot in recent times (since those awesome Thought Experiments :D )... it makes sense to think about how the supination/pronation moment arm and therefore torque of the STJ can change by using orthoses. I am still getting my head around the concepts properly, but those forces and moments are continually going through my head when assessing patient biomechanics. :eek:

    I've been reading your book (Foot and Lower Extremity Biomechanics II) with great interest, as the diagrams you have used are very effective in illustrating how the SALRE and Tissue Stress theories work. It's explained a lot of difficult concepts very well! Very clever!


    Donna :)
  3. Craig Payne

    Craig Payne Moderator

    The way I teach and explain this is that, imagine a foot that has pronated to end range of motion of the subtalar joint resulting in compressive forces across the STJ --> painful. There will be a certain force needed from the post tib muscle etc .... if that force is above a certain threshold, the tendon hurts.

    Now put that foot in an orthotic that applies a force medial to the STJ and watch the patient walk --- lets assume you can observe no changes in function (but you pretend you can) .... you send them away and they return in 2 weeks with the symptoms gone. How can that be if we did not make an observable change to motion?

    What happened was that the STJ has still pronated to end range of motion, but the joint surfaces are not only just touching and there are no compressive forces at end range of motion (ie the foot is still in the same position but the pathologic forces are reduced). Similarly, even though the STJ has not changed position, the post tib does not have to work as hard and force going through it are below a pathologic threshold.

    This way - forces have changed, but motion and position have not. As I keep saying, its the forces that damage the tissues, NOT the motions and positions. Why do we focus so much on motion for?
  4. davidh

    davidh Podiatry Arena Veteran

    I agree completely.
    I would only add that the orthosis allows the foot to work more around STJ-neutral.
    Unfortunately this brings us back to my contention that most feet are inverted when placed in STJ-neutral.....
  5. Ian Linane

    Ian Linane Well-Known Member

    I don't know if this is approprate to this thread so please delete if not.

    For a long time, as I do not use measured angular or degree measurements in assessment, I have tended to look a lot less at the foot to see if an orthosis is having an affect. Rather I have tried to observe a more efficient use of body movement through space, post fitting.

    e.g levels of vertical and lateral displacement in gait, position of neck and head in stance and gait, use of counter rotation of the shoulders, apparent improved firing of gluteal muscles.

    This has seemed to me a useful way of noting positive changes in a persons gait and posture and possibly links in, for me, that less can sometimes be more. So I need only try to introduce a miniscule amount of change (if it happens) to achieve a good affect.

    If orthoses reduce, slightly, the forces in the foot then is this reduction magnified in benefit because forces elswhere (those that come from more efficient movement in upper body action) have changed for the better? Don't know, Not even sure if I am asking the question right.

    I know this does not answer the questions Kevin but I can only come at these things from angles (no pun intended!) that work for me. I would agree that I often see little in the way of change in the position of the foot ( and if come from a view about the foot such as mine and Davids you may well not want to see much difference anyway) and so Craigs response makes sense.

    At our last training course I took some time out to walk barefoot on the unmown grass at the centre. My feet essentially remained pronated but most of the other joints were mininmally changed in position from that which I usually experience walking barefoot on a flat surface. I felt very stable and secure and my knees and legs seemd to feel easy in function. These minimal changes were different with each step. My upper body moved better as well.

    When it comes to orthoses and feet, post fitting, do we look to much at the foot?

  6. achilles

    achilles Active Member

    I remember having this discussion with you at Summer school some time ago ( aside from the pig!! ;) ).
    I feel that many of us, and certainly our students, as Craig's, are taught that no change in motion does not dictate that there has been no change in the forces being applied. The supination resistance test shows this very well, in that you can apply a medial wedge below the heel, without any significant change in joint position, but the force required to supinate the foot has reduced.
    As you have stated many times, this is probably due to the change in STjt axis position. I look forward to when you and Simon have the STjt axis locator available to demonstrate this very easily.

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