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High force, low force- who, what, why, when?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Dec 14, 2011.

  1. Members do not see these Ads. Sign Up.
    All, I was just reminded of something Bruce Williams and I touched upon when we were lecturing together out in Florida recently.

    One of the things Bruce mentioned in his workshops was that a shorter limb
    tends to have a higher peak pressure at strike as measured on F-scan. The rationale he gave was that the centre of mass (CM) is dropping down further on the short side. I said it could equally be explained by the shorter leg having a greater stiffness compared to the longer leg to maintain an even and constant CM displacement. Anyway, Ray Anthony then mentioned that despite the clinical use of pressure measurement for many years now, there is still no users guide to interpretation, for example: a high peak pressure under the heel may indicate....a shorter leg on that side.

    Here's the point of this thread (and it's really a rocket for Bruce so we can work some of this out) those of you who use pressure measurement regularly- I should like to know what kind of things correlate with high pressures and what kind of things correlate with low pressures? Perhaps we could begin the dummies guide to in-shoe pressure measurement here...
  2. David Smith

    David Smith Well-Known Member

    One problem with that proposal that immediately occurs to me is this; more often than not the short leg, which is usually the right leg, also has weak hip abductors and so the compensation is to rock the the trunk over the stance leg during contralateral swing thru and then rock back and drop onto the contralateral leg at heel strike. Observing this often gives the impression that the opposite leg is short.

  3. Orthican

    Orthican Active Member

    I only used it in school. :eek: But, to travel over a distance at an accellerated rate more on one side than the other side means a larger force at impact on the longer distance travelled side does it not? ...or at least until terminal velocity anyway..

    Just a thought.
    Last edited: Dec 15, 2011
  4. David Smith

    David Smith Well-Known Member


    First consider two objects of equal mass both accelerating at the same rate and one travels over a longer distance then that one will have a higher terminal velocity. Then consider the opposing force required to bring both objects to rest (or the impact, which is not instantaneous but has a time base i.e. a force / time integral or impulse). If they both come to rest over the same distance that they travelled while accelerating then the same force would be applied to both.

    The object we are considering could be considered as one object, i.e. the CoM of the whole body, with two force integrals. The force/time integral pattern for each would depend on Mass, initial /terminal velocity and magnitude, time of application and relative vector of the opposing force.

    In other words- mass, in this case, is constant (unless the subject eats a 1/2 pound double bacon cheese burger between steps) the initial velocity is variable, the magnitude of the applied or resisting force depends on the stiffness of the stance leg i.e. how bendy it is (moments about joints) which depends on muscle action and passive joint stiffness, also the angle of strike and the relative friction between ground and foot and then there might also be a consideration of footwear stiffness. All these things are variable and would alter the force integral pattern characterised by the GRF measured by a force plate or pressure mat.

    In other words it is not just a simple case of one foot travels a greater vertical distance than the other so therefore there will be a greater force on that foot. The intuitive proof of this is that a glass can break if it falls over on a marble work top but if it fall off the work top onto a cushioned vinyl floor it may not break even though the distance travelled was much greater and the terminal velocity was much higher the force applied during impact was less in terms of peak force, even though, interestingly perhaps, the force/time integral would be greater.
    Last edited: Dec 15, 2011
  5. Bruce Williams

    Bruce Williams Well-Known Member

    thanks for the invite. Craig P. sent me an email to alert me to this thread, so thanks to him as well.

    I agree with most of what you and David have written so far, except one thing. I see a short limb on the left side probably 60-70% of the time and I think David said he sees it on the right. I'm curious on that.

    The thing about stiffness is when is it necessary and usual for function and when it is compensatory? David did a wonderful job of outlining many of the parameters that must be understood when thinking about stiffnes and physics and all of those apply to lower limb function and comparisons right to left. They all will affect pressure distribution, CoF movement, timing etc as well.

    There has actually been quite a bit written about pressure data in the literature, but unfortunately little about interpretaiton in relation to what those on this list would consider important to assess. Things like Limb Length Difference, STJ position, DFion stiffness of the metatarsals, FnHL (functional hallux limitus) and AJ range of motion among others.

    All of the parameters I mentioned above are very important to me in my assessments of in-shoe pressure data and play a part in the how and why and what that Simon mentioned in the title of this thread. I cover quite a bit of this in my users meeting lectures for Tekscan a couple of times a year.

    I think that if people would like we could outline some common concepts and attempt to agree or at least to understand each others positions on the what, why and how of why pressure data presents itself as it does.

    This could be done a number of different ways. One way is to outline those things that users choose to primarily observe in the data, ie CoF progression, areas of high pressure, Force vs time curves, timing of the transitions and locality of potential delays. We could also just pick some common discrepancies and discuss what we would often "see" in function and data from the in-shoe data. I would lean towards the first suggestion, but whatever people want to know ask away and I'll chip in where I can.

    Please respect that my explanations and observations are usually kinematic based and not always physics and force explanation driven as Simon and David often do. I would think that the three of us, and hopefully others, could come to a meeting of the minds in a combination of explanations that would help to offer some help in furthering the understanding of in-shoe pressure data.

  6. Bruce Williams

    Bruce Williams Well-Known Member

    i see the short leg predominantly on the left side. I'm curious as to why you seem to think it is on the right side?

    I will see less force at impact often on the right and feel it is due to pronation compensations "softening" the impact so to speak. Less stiffness on the long limb side due to compensations from the foot to the knee and hip.

    my 2 cents.
  7. Gentlemen, can I suggest that we don't get too bogged down with the theoretical mechanisms as to why at this stage, as several theoretical solutions may exist to explain the observations. Rather it might be better to catalogue the observations and their structural correlates; start with say, elevated pressures, list where they occur and when and what structural correlates they might be associated with.

    So, for example:

    Higher pressure under one heel at strike = potential limb length discrepancy (shorter leg has higher pressure).

  8. Bruce Williams

    Bruce Williams Well-Known Member

    Ah, but just that one statement alone is already in contention from what David posted vs what you and I have stated!

    There are always caveats as well. What about a structurally short limb with significant gastro-soleal equinus that may only briefly touch down at the heel?

    Not messing with you Simon, it's just that there are many differring explanations for why things work the way they do in-shoe and sometimes you have to reason them out, even the zebras!

    I will agree with your initial statement that usually the short limb will have higher pressure at heel strike. That one bit of information alone is not enough for me to emphatically state a person has a short limb on that side. I would want physical exam correlation to go along with it.

    In-shoe pressure is a tool, as are those who regularly use it and attempt to share their knowledge at times! Present company excluded of course! :rolleyes: (self-effacing statement directed at myself only!!!!)

  9. And therein lies the reason why this technology has not become pervasive. As Ray said in Florida, if you want this to be taken seriously, somebody needs to put their neck on the line and record their opinions and observations. Personally, I'd just like to see a listed summary of your observations Bruce or Dave or anyone? I don't want detail just high pressure here at this time = this (maybe) so you might want to check to with other tests... etc. As far as I can see we don't even have starting points for any of this kind of stuff. i don't care how loose it is, somebody list it coherently, in one open-access place.
  10. Bruce Williams

    Bruce Williams Well-Known Member

    I understand your comments and the reasoning behind them, to a point.

    I think the biggest problem for why in-shoe pressure and other gait analysis technologies have "not become pervasive" has much more to do with the fact that everyone wants a simple yes or no response from the tests.

    I have had this conversation with many at Tekscan over the years and this is the primary reason why I have balked at writing such a manual. There is no cut and dried response to understanding the feedback from this type of instrumentation when dealing with a functional entitiy!

    That is why I continuously lecture on the basics of sagittal plane foot mechanics, right or wrong, when I lecture at users meetings. Right or wrong again, that is how I reason out the information that I get from the device. Right or wrong this is why I have a great desire to have more information from motion capture, emg and video capture. All of these devices can give you pieces of the puzzle, but you have to put them all together to get the full picture of what is actually occuring.

    In-shoe pressure, in my opinion, can take you farther than most of the others since not appreciating what is happening functionally at foot strike will leave you incredibly blinded when making a decision on whether or not to use an orthoses and what prescription to use if you do. I find all the other devices supremely lacking since the foot is effectively masked to functional observation to them all except to in-shoe pressure and maybe emg.

    This is probably why this thread is doomed to the same fate as others like it in which I have tried to participate. There are many "what-ifs" in this information. Not necessarily too many, but many nonetheless. I find it to be a wonderful experience in detective work with a tool that will give me immediate feedback as to whether what I did was right or wrong. At least I get objective feedback, instead of trying to get a definitive answer out of a patient!

    I'm sorry that I cannot give people what they want in this way of explanation. There are too many variables outside the foot that can and will come into play for why and how the information will be displayed and interpreted.

    I'm still willing to play along if someone else has questions about how something will look in-shoe. Just don't expect a cut and dried response very often.
  11. So, you can't list any of the factors that result in increased pressure under specific areas of the foot at specific points in time then, Bruce?
  12. Bruce Williams

    Bruce Williams Well-Known Member

    responded as asked...

    High pressures in these areas:
    neuroma, metatarsalgia under the metatarsal heads 2-4
    FnHL will often show high pressure sub hallux and higher pressures 2-5 mpj's
    High heel pressure often on short limb side.
    Early high pressures laterall forefoot may indicate AJ equinus or other such issues.
    High medial column pressures without an orthosis may indicate PTTD or charcot changes with extreme abduction of the foot and external rotaiton of the hips.

    Lack of or decreased comparative pressure in these areas:
    minimal pressure sub 1st mpj may indicate FnHL or structural HL.
    Decreased heel pressure may indicate long side limb in LLD.
    Decreased pressure medial arch w/ orthotic may indicate FnHL and medial arch that is too stiff.

    This work for starters Simon?
  13. Stanley

    Stanley Well-Known Member

    Hi Dave and Bruce,

    I was just wondering how each of you measure leg length differences?

  14. Perhaps we should all read: S Beekman, H Louis, JM Rosich, and N Coppola
    A preliminary study on asymmetrical forces at the foot to ground interphase
    J Am Podiatr Med Assoc 1985 75: 349-354. ? Maybe, if you could attach a copy?

    Or just this thread: http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=5768
  15. Stanley

    Stanley Well-Known Member

    Hi Simon,

    I wish I could, but JAPMA online doesn't keep PDFs of articles this far back, and in the last 26 years I have moved a few times and things seem to get lost. Maybe there is a student following this thread that has access to a podiatric library and could attach a copy for you.
    This is a primitive study, and was done 26 years ago, so I don't evaluate this way currently. However, this paper is a good place to start.

  16. David Smith

    David Smith Well-Known Member

    Stanley ( always good to hear from you)

    Q) Is apparent LLD due - Structural variation (e.g. bone length, pelvic innominate size) or anatomical variation (genu valgum, tibial bow, stj position) or tight muscles, fascia or ligaments.

    Quick method:

    Visualise levels of left and right iliac crest, same with greater trochanter, same with popliteal fossa in posterior knee. Then mobilise ankles and release ITB and hamstrings with that iliac/pelvic massage that you instructed me in some years ago and review iliac crest levels. View and consider with regard to STJ pronation and frontal plane knee position

    In depth:

    Same as above but also check iliac crest levels while patient is stretching hands towards the ceiling. Check iliac level while sitting . Measure from GT to lateral malleoulus. Check for torsions and versions of hip/femur and the relative knee and foot placement positions. Hip Abductor strengths and evaluating whether rocking gait style is due to short leg, weak abductors or both and what compensation the subject is making.

    With both above, and if differential heel lifts are fitted, then review after 8 weeks and remove if hips are level without the heel lifts. Also listen to what patient says feels best and visualise what looks the most symmetrical in gait. Adjust appropriately and/or as necessary.

    Regards Dave
  17. Stanley

    Stanley Well-Known Member

    Hi Dave,

    I always enjoy your insights. I am curious about your findings with the patient raising his hands towards the ceiling.

    Regarding your evaluation, you are using the iliac crest which can change with an iliosacral dysfunction (also known as pelvic torsion or a posterior/anterior innominate).

    I wonder how this compares to what Bruce is doing, as this can be why you are seeing something different than he is.

    Actually, this had to do with the fascial trains around the knee. (the superficial front line and the spiral line).


  18. Bruce Williams

    Bruce Williams Well-Known Member

    Stan and Dave;
    doing eval same as I"ve written up on the list several times before. Watch 'em walk, watch for shoulder tilt, arm swing comparison. PSIS, ASIS, pelvic level. Check internal and external rotation of the hips and compare knee and foot position on this as well. Watch for changes pre adn post AJ manipulation.
  19. David Smith

    David Smith Well-Known Member

    Anyway Bruce I've been looking at pressure scans of patients with apparent short limb length (although not many yet coz it takes a bit of time ) and so far there doesn't seem to be a correlation between short limb and high GRF but did you mean high peak force or high peak pressure?

    Regards Dave
  20. Stanley

    Stanley Well-Known Member

    Hi Dave and Bruce,

    You both are seeing the same thing.
    When the ilium rotates posteriorly, the crest rises when the axis is the sacroiliac joint.
    Dave, thanks for the explanation about the hands stretching overhead. I will check this in my evaluations. Also thanks for the kind words.


  21. Bruce Williams

    Bruce Williams Well-Known Member


    if you wish, please explain to those following the difference between peak force or peak pressure when using pressure mats or in-shoe sensors.

    My understanding, not always the best, was that essentially they are the same when derived from pressure mats or sensors. That is not to confuse the two when derived from force plates and compared to pressure sensors.

    Finally, regarding short limb and high impact force, I do see it often, but not always. That was the point I was trying to make to Simon. There are occurences, probably percentagely higher towards the short limb from what I see, that the heel pressure will be higher on peak pressure on the short sided lmb. Peak pressure is compared throughout the foot in one static image with the F-scan software right to left.

    It would be best to take all of our initial scans, without orthoses and mat and in-shoe, and compare them numerically to see if the physical exam from our notes correlates with the pressure numbers and to what percentage that it does.

    Anyone want to right a program for that? I can scan what I have if it is relatively easy and we can put this one to bed from my fairly large sample size and compare it to David's. Just a thought since discussing what we "see" will not necessarily correlate with the numbers. Also, as I've said for a long time, I think I see a trend for this, but it is in no way a hard and fast rule primarily due to stiffness issues of the foot and lower limb.

  22. David Smith

    David Smith Well-Known Member


    you wrote
    and then
    FORCE) - Peak Force is determined by the sum of all the forces applied to all the sensors acted upon at certain time of interest. (where time is defined as a temporal division of the stance phase, e.g. a 750ms stance phase and we are interested in the peak force at the 120ms point)
    Or it could be, but is not usually, determined as the largest force applied to a single cell at a certain time of interest.

    Pressure) Pressure is derived by dividing that force of interest, at that time of interest, by the number of cells acted upon by that force.
    It would be spurious to define pressure as defined by one cell amongst a number of cellssince this represents a force. Unless there happened to be only one cell acted upon by the force of interest at that time of interest,

    But let's consider a more practical scenario:

    So, if we consider only heel strike and also assume that the short leg is also heel striking because sometimes the short leg and equinus ankle lead to forefoot strike, which tends to attenuate total peak force. Then if we consider evaluating the force and pressure during a certain period of heel strike with respect to time, e.g. from initial contact to early midstance then we can define more precisely the peak force or pressure of interest.

    So the left foot has the long leg and the right foot has the short leg. The left STJ pronates a lot and is very compliant to GRF with a medial stj axis. The right foot is more cavus and the STJ is stiff to GRF with a lateral stj axis, a common picture in my book.

    Lets assume two scenario's

    1) The right foot has the highest peak force at heel strike
    2) The left has the highest peak force at heel strike
    and let's assume for this case that this has been verified using a force plate.

    Then the right foot and left, in both cases, have been sampled on a pressure mat system during normal walking.

    Ex 1) Right foot Highest peak force - The right foot strikes with a supinated posture and remains that way thru to early midstance. Therefore the area of heel/ground contact is relatively small in terms of the whole heel area available.
    The peak force was 800N but the contact area at the time of peak force was 4cm^2, therefore peak pressure was 800/4 = 200N/cm^2 or 200Gpa.

    Ex 2) Left foot highest peak force - The long left strikes with an everted stj and pronates to max very early with applied GRF. This time the peak force is still 800N but the area of contact is 20cm. Therefore the peak pressure at the time of peak force would be 800/20 = 40N/cm^2 or 40GPa.

    Further to example 2) the right foot peak force is only half that of the left at 400N but the same strike scenario as ex1). Therefore peak pressure = 400/4 = 100N/cm^2 or 100GPa. I.E. the peak force is much lower, at half that of the left foot but the right peak pressure is still more than twice that of the left.

    If we were to consider peak pressure as an indication of short leg then it would likely be confusing since any scenario that altered the ground/foot contact area would change the peak pressure output data completely independently of the fact of leg length difference.

    It may be however that if the short leg also, more often than not, has the equinus ankle and cavus foot posture and lateral STJ axis, then this probably would also result in the short leg having the highest pea pressure output. However that result, even tho repeatable, would be conveniently and arbitrarily coincidental.
    Having said that it may be a clinically useful indicator even so!!:dizzy:

    Further, if then one person evaluates short leg in terms of pressure peak and another person evaluates short leg in terms of force peak then the same clinical scenario could result in two different conclusions if the difference between pressure output data and force output data was not correctly understood and applied.

    Regards Dave Smith
  23. efuller

    efuller MVP

    I agree with dave. I would like to add a little more. Peak pressure is value achieved at one sensor. The location of that sensor may not be the same from one step to another, so it may make this value less precise than using peak force which the the sum of several censors. For example, if you have a metatarsal head that is directly over a sensor, that is about the same size as the metatarsal head on one step, and on another step the metatarsal head is in the middle of two sensors, then for the same force applied to the metatarsal head you would get very different pressure values. There may also be issues if the sensors are very small when compared to the anatomical structure of interest.

    Usually the software will allow you to choose an area of the foot over which the values from individual censors can be summed. If you are interested in peak force on the heel and the value you look at is peak force for the whole step, or whole foot, the peak force may be on the forefoot.

    A good question is how often.

    Given that there are other variables that may cause one foot to have a higher impact force than the other, can peak force under the heel be a useful indicator for limb length descrepency. Do, we have any evidence that impact force, in walking, is an indicator for anything.

    Of course we have to also show that side to side differences are bigger than step to step differences to be make any conclusion about one side consistently having higher forces than the other. Peak force over several steps needs to be averaged.

  24. Bruce Williams

    Bruce Williams Well-Known Member

    ok then, if I understand what you wrote, we should change the descriptor of what we are "seeing" at heel strike as peak force and dispense with pressure at this point, even though we are using pressure mats and / or pressure insoles for our evaluations. I have no problem with this.

    So, will the Force vs Time curves, if boxed out for the heel and forefoot areas, show mostly equal representation to the peak force under the heel during contact?

    I will admit that though I often witness more red color ( an indication of high pressures / force ) under the heel when comparing right to left heel strike, that the F vs Time curve graph peaks for boxing out the heel will often be the same. I suppose this would work well from the two examples you gave above.

    Does this make sense, or am I not explaining this correctly?

    BTW, I still have to make it through the slides you sent me, sorry will work on that this weekend.

    Good discussion!:drinks
  25. David Smith

    David Smith Well-Known Member

    Each cell measures force but the force measured is usually characterised as pressure because, unlike the force plate, it can be done. This is because force is divided over an area defined by the size and number of the force cells. However as you note if the area defined by the masking is about the same and the 'red area' within the mask is about the same then if the pressure is different between left and right then this will be equal to different forces. so if an equal area red area has more pressure then the total force will also be higher as shown on the graphic below


    you will note the right FTC braking peak force is highest and the total contact area is about the same and the heel 'red area' is about the same, therefore the mean pressure will be higher on the right also (NB this lady does have a short right leg)



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