Welcome to the Podiatry Arena forums

You are currently viewing our podiatry forum as a guest which gives you limited access to view all podiatry discussions and access our other features. By joining our free global community of Podiatrists and other interested foot health care professionals you will have access to post podiatry topics (answer and ask questions), communicate privately with other members, upload content, view attachments, receive a weekly email update of new discussions, access other special features. Registered users do not get displayed the advertisements in posted messages. Registration is fast, simple and absolutely free so please, join our global Podiatry community today!

  1. Everything that you are ever going to want to know about running shoes: Running Shoes Boot Camp Online, for taking it to the next level? See here for more.
    Dismiss Notice
  2. Have you considered the Critical Thinking and Skeptical Boot Camp, for taking it to the next level? See here for more.
    Dismiss Notice
  3. Have you considered the Clinical Biomechanics Boot Camp Online, for taking it to the next level? See here for more.
    Dismiss Notice
Dismiss Notice
Have you considered the Clinical Biomechanics Boot Camp Online, for taking it to the next level? See here for more.
Dismiss Notice
Have you liked us on Facebook to get our updates? Please do. Click here for our Facebook page.
Dismiss Notice
Do you get the weekly newsletter that Podiatry Arena sends out to update everybody? If not, click here to organise this.

Bipedal spring mass walking sagittal plane theory

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Oct 7, 2010.

  1. Members do not see these Ads. Sign Up.
    All, I have been interested for a while now in the concept of modelling walking gait as a bipedal spring mass system. This was first proposed by Geyer in his PhD thesis attached. It strikes me that Geyer's work may offer an alternative explanation to the observations of Howard Dananberg in describing the compensation patterns observed in association with a sagittal plane "blockade". I have asked Howard to contribute to this discussion and he has kindly agreed to read the work of Geyer; this is a great starting point.

    I have a number of questions to ask of Howard regarding the duration of double limb support, leg stiffness, force time curves and centre of mass displacements. Give me a heads up when you've read the work and are in a position to continue, Howard.

    Many thanks in advance for the time and consideration you are giving to this.

    Attached Files:

  2. David Smith

    David Smith Well-Known Member


    "Give me a heads up when you've read the work and are in a position to continue, Howard".

    That's quite a task you've set right there 140 pages of technical argument to read, digest and comment on Phew:eek:

    Regards Dave
  3. David Smith

    David Smith Well-Known Member


    This first paper Geyer SMW seems to have a problem with the premise of his argument in that it is no argument at all. Don't we already accept that the knee bends during walking and is not a stiff legged vaulting action as per the models he uses. Running is then not a different model just the same model but with one spring damping phase instead of the out of phase parallel spring damping of walking, I.E. double leg stance. This is what he is saying and yet we knew that already.

    I'll read on

  4. Dave as far as I now walking is normally modelled on an inverted pendulum, this is why this model related to leg stiffness is so intersting. Walking would be seen as a stiffer spring or the body has increased (kleg)

    For anyone if they want to have a look Ive attached a paper with discusses the inverted pendulum model in relation to energy usage, but gives a nice outline and good pictures to look at when reading Geyers model ie spring mass when walking.

    Which Ive just noticed is 1 of the same people (Kuo) that Simon posted up in the muscle strength and arch height thread........small world and all

    Attached Files:

  5. Dave, as Mike has already said, modelling walking gait in this way is pretty new. I would have you (yes that's you David Smith) think about how independently manipulating the left and right leg spring characteristics might influence the duration of double limb support and moreover, how this might result in the force/ time curve characteristics that have been suggested in association with sagittal plane "blockages"- kind of what we were talking about earlier this year. I'm sure you understand the physics of springs and harmonic motion.

    I've attached a few other papers which may prove helpful. I appreciate that its a lot of reading, but I've been reading and thinking about this for a while now... Anyway I think you'll find this interesting and maybe, fit the pieces into the puzzles we were missing when we last looked at this, Dave. Glad to see you contributing again, Dave! Who knows, we might even get that paper written?

    Attached Files:

  6. David Smith

    David Smith Well-Known Member


    Time has different priorities these days but I'll try and read thru your assignment. ;)

    From a practical point of view, and I hope I'm not going off topic here, Consider the Vertical force time curves characterised by a plantar force/ pressure measuring device.
    I usually see that where the braking peak is low on the ipsilateral leg then the same curve on the contralateral leg is high, when the push off peak is low the opposite peak is high, where the push off peak is high on the ipsilateral the contralateral braking peak is high etc, etc. This allows for a constant or mean velocity of saggital plane progression even where there are significant variations in CoM accelerations within each stride or gait cycle. There is a lot of leeway in that proposition but it seems to me that this would be difficult to achieve with a stiff inverted pendulum model and I have always considered this model a convenient reductionist modelling theory. Perhaps I should read some more eh?:cool:

    Regards Dave
  7. My contention is that we modulate left and right leg stiffness to maintain constant or mean velocity in sagittal plane progression in the face of obstructions such as functional hallux limitus and that this explains the postural compensations reported by Howard. Keep reading- you are key to this discussion and this thread was as much a baited hook for you as it was for anyone.

    Yet, take the time for the important things in your life too, Dave. This thread will still be here tomorrow.
  8. If it counts for anything , Ive mentioned this before I´ve had this idea that increased dorsiflexion stiffness of the 1st or Functional hallux limitus is required at certain times during the gait cycle to change the lever arm length of the foot, which will help load the spring of the gastroc/soleus complex , which will then lead to greater energy return in propulsion. The problem is if the stiffness it too high at the wrong stages of the gait cycle. This is only what I´ve been thinking about since the earlier discussions on Leg stiffness and have no research to back up this claim, but thought I would add it. See if anyone wants to agree or to tell me to take my bat and ball and be off.
  9. No, you were on my list of fish to catch too, since you have an excellent understanding of leg stiffness. Bruce and Graham aren't biting yet, but they will. Asher, you on board?
  10. Why thankyou sir I´ll be reading Pohl in the mean time
  11. Something for the weekend, Sir? Read that, then think about this: if I wanted to increase the duration of double support, how could I achieve this by manipulating only the stiffness of the left and/ or right leg springs? Lets assume the left leg is the leading leg (i.e. at heel strike) and the right leg is trailing. Hint: look at the joint motions at this time to determine which spring is compressing and which spring is decompressing- as you know, Mike: this will predominantly occur at the knee, followed by the hip and then the ankle.
  12. after a quite evening with Pohl and a couple of different malt friends from Ireland (wife and little man are visiting the grandparents) - This what I came up with.

    In gait modelling the importance of the position of centre of mass (COM) and how it travels through the gait cycle is clearly shown in the articles posted by Geyer and Kuo et al. So I focused on how I would slow the speed at which the COM moves forward in Double support. ( Ive drawn a picture to try and show what I mean ). In spring mass modelling the osscilation of the COM on the leg ( the spring) is important , the spring compress store energy and decompress to return energy, which will help us move the COM forward.

    The left leg (the front leg) should be decompressing or increasing leg stiffness (kleg) - it will increase kleg by internal extension moments at the knee, hip and ankle ( they maybe still flexed but extending - when thinking about the hip)

    The right leg (the trailing leg) should be compressing or decreasing kleg - it will attempt to achieve this by internal flexion moments at the knee, hip and ankle.

    If we think in terms of acceration of the centre of mass in forward progression. The right leg being the leg which accerates the COM forward due to the energy return I discussd ealier in normal gait - ie it should be decompressing( a thread and discussion already done ) to slow the forward progression of the COM we want the right leg compressing this will slow the forward progression of the COM, if the left leg is decompression ie increasing leg siffness at this time the resistance to the forward progression of the COM will also increase as the COM must also move upwards, which should also mean there is a greater gravity effects, there should also be greater resistance to the forward progression of the COM due to the longer , stiffer level arm of the left leg, due to increasing leg stiffness. thats my thinking.

    Sorry for the long post - though it might help for those reading to see why there has been 2000 posts re leg stiffness in the last 6 months - If I was just reply to you Simon, left leg increasing Kleg, right leg decreasing kleg will increase the time spent in dbl support phase due to great resistance to forward progression of the COM.

    Attached Files:

  13. Asher

    Asher Well-Known Member

    In the background.
  14. David Smith

    David Smith Well-Known Member


    Nice one Mike, Now this is a nice interesting uncomplicated premise to think about.

    The relative K of each leg and its influence or effect on forward progression during walking. Why would the K be relatively different or varying and what conditions would cause this.

    Hmmm! Tick tock tick tock --- I haven't finished Simon's reading assignment yet tho. (newly married you know:D)

    Regards Dave
  15. Well done Dave, nice one. Great news.

    Why would each leg have different Kleg ? heres what Ive got

    weakness in muscle - injury or enviroment
    Reduced ROM of the hip, knee and ankle on one side - OA or joint trauma
    Neuro changes which effect one side only
    change in surface stiffness - in one paper they discuss that the body can modify kleg after surface stiffness change after 1 foot flat, maybe in walking it happens faster due to the slower speed of walking ?
  16. Well done gentlemen, to this list I would add: step length; functional / structural hallux limitus, limb length discrepancy??

    If we assume the body modifies Kleg to maintain a stable progression of the CoM, how else might these variations in left and right leg stiffness manifest in the force / time curve.

    Basically during walking gait with a have two situations: double and single limb support. ~During double limb support effectively we have two springs in parallel similar to this,

    During single limb support we have only one spring or the other.

    Lets say the stiffness of the left and right legs are the same, what happens at the transition from double to single limb support. Lets say our left foot has just struck the ground and our right is preparing to leave it.. How does does the body maintain a smooth CoM, when the stiffness of the support structures effectively halves when we transition from double to single support... it has to minimise this shift by stiffening one leg while reducing the stiffness of the other- right?

    Now lets say the stiffness of the right leg was half that of the left and examine the transition periods.
  17. With leg stiffness of the right being half of that of the left and remained so with no leg stiffness modification the single leg support be longer on the right side than the left.

    In my 1st answer I wrote twice as long but I´m not sure on the twice, but would guess so.(?)

    As for the transition periods from double to single I would also think that going from lower leg stiffness to increased leg stiffness should take longer than higher leg stiffness to lower - again maybe twice as long lower to higher but notsure on that.
  18. Dananberg

    Dananberg Active Member

    The idea that each extremity operates as a spring-mass model seems over simplified to me. I have read through Geyer’s paper and found many issues which simply don’t make much sense in actual human ambulation. In order for his math to work, he has made each limb a “mass-less linear spring” and writes “on the contrary, in swing, the respective spring has no physical meaning”. Clearly, if you have ever sat on a child’s swing and pumped your legs to go higher and higher, there must be some mass of the lower extremity contributing to movement. Ignoring this may work to smooth out the RGF curves and make his theory plausible, but in clinical practice, I am not sure that they help.

    That said, the idea of a spring storing and returning energy does makes sense. My feeling is that this spring involves the entire body, and not just the extremities. If you read Grecovetsky’s “Spinal Engine” text, there is considerable mathematics to explain energy storage and return via pelvic and shoulder girdle counter-rotation. In other words, all body parts contribute to propel the body during the gait process.

    My final comment involves the foot end of his model. The human foot is a 3 part pivotal system, transferring rapidly during the 500ms single support phase. Using a solitary point to model a 3 point actual structure has to lose something in the translation. My sense is that foot movements, particularly at the MTP joints, cause considerable motion via the windlass. As the windlass winds through toeoff, it not only 1) matches external rotation from the pelvis down, but also 2) moves so as to prepare for impact attenuation when the next impact occurs at heel strike. It is this movement which permits the contact energy to be stored at heel strike. Without it, the foot would strike at end range pronation, with impact injuries resulting.

    Thank you for posting this paper.

  19. Spring-mass modelling of human running is widely accepted as a useful tool within biomechanics research.
    Agreed, yet this approach brings us further forward than the inverted pendulum model since it actually reproduces the GRF patterns and the energetics observed during human walking
    Yeah, read the spinal engine theory many years ago. We a talking about a spring mass model, the springs don't have stop at the hip. The key is in looking at the whole body system as a spring-mass system.
    So the foot stiffness adds to the leg stiffness, we already know that. When researchers have previously measured leg stiffness they have included the foot stiffness in with the leg stiffness. In my private discussions with Hartmut Geyer he has suggested the addition of a foot spring within the system, it's just adding another spring in series so pretty easy to model.

    Howard, in addition to changes in the duration of double versus single limb support, you have previously reported "flat spots" in the force/ time curve. How do you explain these changes and flat-spots?

    From Stiffness - review of research in running and jumping-2.pdf (linked above), vertical stiffness can be calculated using vertical force /time data

    "Because force is equal to mass multiplied by acceleration and because mass remains constant, vertical force can be graphed
    as vertical acceleration. Then, vertical acceleration can be integrated to produce vertical velocity (single integration), and then vertical velocity can be integrated to produce vertical displacement of the CM (double integration). "

    So what do "flat spots" mean in terms of centre of mass velocity and displacement?

    Moreover, if we have a bipedal spring mass walking system, how could we produce flat-spots in the force time curve it produces? Manipulating the leg stiffness should allow to modulate the duration of single and double limb support, I'm reasonably convinced it can reproduce these "flat-spot's" too. I appreciate it's a reductionist model, and not the real thing, but humour me. If we accept the model as being able to provide GRF and energetics data which matches in-vivo walking, then surely we can explain the in-vivo GRF in terms of this model.
  20. Dananberg

    Dananberg Active Member

    "So what do "flat spots" mean in terms of centre of mass displacement?

    Moreover, if we have a bipedal spring mass walking system, how could we produce flat-spots in the force time curve it produces?"


    The most direct answer is because the spring fails to function, and we advance via direct muscle action and impeded motion of the COM. In other words, a very apropulsive gait.

    Since F=M(a) and since M would be constant for any given step, the less acceleration....the more constant the force. Flat lines on F/T graphs would therefore indicate no changes in M acceleration. Since motion of the COM is what causes the force/time curve shape, then flat lines would indicate constant forece, ie, no motion.

    This is not to say that the subject is not moving, just that the COM is not moving as driven by max effieicent means. Flat lines = fatigue.

  21. Howard, I'm not sure it's because the spring fails to function, I think it is because of the effective spring stiffness which arises from the left and right leg springs acting in parallel at these given times and/ or due to the transition in the stiffness as one foot leaves or makes ground contact. Constant force = constant acceleration, this isn't always the same as no motion- is it? Leg stiffness gives us a global measure which includes muscle function. I honestly think there is mileage in this approach, Howard, or i wouldn't trouble you for your time.

    I just need to think through the calculation of vertical stiffness from this data...

    Thanks for taking the time to discuss this with us.:drinks
  22. David Smith

    David Smith Well-Known Member


    This is not quite true, The force cells in a pressure mat measure applied force not acceleration but acceleration can be extrapolated from the F=Ma equation. However the cells measure the change in force by the change in cell deflection therefore a curve or slope in the FTC is equal to a change in force and so a flat line is equal to constant force and so requires constant acceleration, which may also be no perceived acceleration relative to the ground reference and gravity but not necessarily. Therefore CoM velocity can be increasing and have a lot of movement and still produce a flat line on the force characterisation graph.This is easily proved since gravity causes a constant acceleration and yet when standing still on a pressure mat the FTC is flat.

    If you turn the example on its side and imagine a car accelerating with a passenger in the front seat then the force applied to the passenger by the seat is constant if the acceleration is constant. The car will be increasing in velocity or speed but the acceleration is constant and the force applied is constant.

    Therefore an active dynamic spring system could produce the conditions required to observe flat line FTC and accelerating CoM (remember acceleration is in both direction positive and minus)

    Regards Dave

    Regards Dave Smith
    Last edited: Oct 11, 2010
  23. With you so far Dave, so... If a flat spot = constant acceleration, how can I create constant acceleration, using a bipedal spring-mass model through manipulation of the spring stiffnesses of the left and right springs at the times when "flat-spots are reported to occur in the force/ time curves?- Mike?

    When are these "flat-spots" supposed to occur, Rebecca?
  24. David Smith

    David Smith Well-Known Member

    Ah! now that is the hard bit, As you may remember I did try reproducing this mathematically using MS excel to model the differential equations and fourier transformations but I hit a wall trying to reproduce a flat spot. Haven't got back to it and the maths gets really hard for me, we need a maths boff :confused:.

    In conceptual terms its just two springs accelerating a single mass but each spring has infinite variation in stiffness. Just to complicate matters also there is a change in force and acceleration due to the angular displacement of the CoM pathway in the walking example i.e. the sinusoidal displacement curve plus consideration of the inertial forces of swinging limbs.

    Got to go now

    Regards Dave
  25. Taken from the leg stiffness thread..... Which still reads pretty well to me.

  26. Rather than looking at the infinite variations, is it possible to find gross solutions in which equilbirium in terms of the acceleration of the CoM might be produced?

    So, for example: if we have two springs attached to a point mass, one spring which itself has mass X, is pulling downward on the mass with a force Y, the other spring which is identical to the first and also has a mass of X is pushing in the opposite direction and acting on the point mass with a force of -Y (equal and opposite), will the point mass be in equilibrium?

    If my assumption here is correct the leg springs need only have the same mass, resting length and spring constant, and be exactly 1/2 cycle out of phase with each other in their oscillations in order to flat-line the vertical CoM (assuming linear springs) during double limb support.

    In which case, flat-lines might appear in the force/ time curve when the deforming force of one leg and the restoring force in the other leg are equal and opposite in terms of their action on the centre of mass. This applies to single and double limb support. If flat spots = bad, then equilibrium in the forces exerted on the CoM by the limbs = bad in terms of forward walking. This makes sense to me. Controlled falling is OK, hitting a cliff-ledge half-way down is as bad as hitting the floor at the bottom without adequate control.

    Which then leads to the question : can equilibrium in the CoM occur in any situation other than when the restoring force of one limb being exerted upon the centre of mass is equal and opposite to the deforming force that the opposite limb is exerting upon the centre of mass? Which leads to the hypothesis that flat spot's in the force/ time curve occur when the restoring force in one limb is equal and opposite to the deforming force of the other limb. It's just a hypothesis, please prove it wrong.
  27. Dananberg

    Dananberg Active Member

    Simon, Dave and everyone else,

    Sorry….but I missed the point entirely. Try this.

    Why the flat lines? For the F/T graphs to be developed, the interface through which the force must travel is the foot. Its ability to provide adequate spring tension (stiffness) is dependent upon its ability to stabilize against support surface, ie windlass. Since the windlass is a direct function of MTP joint motion, there is a correlation between spring stiffness (ie, support) and sagittal plane motion.

    Force sensors detect load based on their relationship to the support surface (which Dave has described). As the two essential load bearing components (heel and met heads) approach a vertical alignment with the sensor, their ability to transfer loads increases with peak transfer occurring at absolute vertical for each segment. These correspond to the peak points on the double hump curve. Therefore, there is a correlation between sagittal plane motion and force load expression as measured by in-shoe pressure sensing.

    When functional hallux limitus exists, for instance, there is a functional inability to express this load transfer thru the sensor to the floor as progressive metatarsal plantarflexion (bases pivoting about the heads) fails to occur for brief but significant periods of time. This results in a flat line expression on the F/T graph. It also represents the loss of stiffness at the transfer point to the ground, as stiffness and windlass relate to sagittal plane motion.

    With the knowledge that CoM is in motion above the foot, coupled with the foot being incapable of changing the metatarsal attitude with the floor, (ie, functional hallux limitus), then a disparity must exist. It has been my contention for quite some time that this is how late phase pronation can be measured, and more importantly, an ability to measure once the correct orthotic Rx has been achieved.

  28. David Smith

    David Smith Well-Known Member

    Hi all

    Just got in but one thing that I thought worth mentioning in terms of GRF FTC flat spots. To make an assumption or conclusion about the effect of the CoM motion on the FTC or vice versa we must view the characterisation of the sum total forces acting on the CoM i.e. the GRF of both feet not each foot separately. A flat spot on the GRF FTC of one foot cannot tell us anything about the motion of the CoM. You will notice from the idealised model below, that I synthesised from force plate data using MS Excel, that the two peak VGRF at double Stance is around 1750N (subject body weight around 95kg) the first and last peak at about 1200N are single leg heel strike right foot and the last peak is single leg push off right foot. So you see total force acting on the CoM from both feet is about 40% -50%higher than single leg peak force. The conclusions drawn about CoM acceleration would be much different perhaps. Total time 200ms.


    To make this clear and take this to its logical conclusion: if I place a pressure mat on the floor, skip over it, and land on the other side without ever touching the plate what can the output data characterised on the PC screen tell me about the motion of my body in terms of the CoM trajectory?

    We tend to make assumptions about the shape of the FTC based on our assumptions about the normal progression of human gait but this is intuitive logic and when gait is far outside normal we can no longer use those assumptions and to make reasonable conclusions we must rely on reliable data as above.

    Below are the FTC of a young man with brain damage and subsequent loss of motor control in some muscle of the left side. (Extract from gait analysis report to physio) I have a video but cannot post it publicly because I can't hide his ID. However the FTC right has many peaks and troughs and the FTC left is fairly normal. This is because he compensates for having no propulsive or swing thru power left side by modifying his right side action. He uses his pelvic rotators in the saggital plane, using posterior pelvic rotation, to swing the left leg thru then propulses with the right leg. Propulsion with the left leg is compromised but he anteriorly rotates the pelvis to load the left hip extensor muscles which gives his some propulsion left. This end with the pelvis fully anteriorly rotated, which is ideal for activating the next left swing thru action. Subject weight 85kg


    Regards Dave
    Last edited: Oct 12, 2010
  29. Dananberg

    Dananberg Active Member


    My comments regarding F/T graphs were not intended to be viewed in a vacuum. They were designed to describe how force measurements DURING GAIT can be viewed. I would agree that if you were to skip over a force plate, it would not show what is transpiring at the CoM. As I have said many times, in shoe pressure analysis MUST be accompanied by some type of visual (video) analysis. So, my comments are assuming that someone is walking over the force plate or with an in-shoe pressure sensor in place....

    The other assumption that can be made is that a subject is essentially neurologically sound.
    Swing phase in neurologically normal subjects is very consistent. Once neurologic impairment
    exists, the parameters of gait change dramatically. The terminal phase of swing phase in a
    neurologically normal subject will exert a pull on the CoM in the final stages of single support.
    With this knowledge, interpretating these curves becomes far more valid.

    Looking at Ricky's data that you posted, it is rather obvious that he has very prolonged contact phase of the heel curves. The basic leveling of the heel curves and FF curves would tend to indicate a very apropulsive style of walking. The R graph would indicate that he appears to speed, slow (ie, lean backwards), and then speed again. The L does not demonstrate this same finding, although there is a prolonged heel contact period on both, and heel lift fails to occur during the basic parameters of single support phase (peak to peak of both force curves). Without seeing a video, I would only venture a guess that the R limb is longer...and therefore a more difficult vault, particularly in a neurologically impaired subject.

  30. Howard, I think Dave's point regarding the fact that in order to draw inference regarding the COM from force plate data it is necessary to combine left and right foot data is sound. This is similarly to one of your points regarding the Geyer paper assuming a mass-less limb during the swing phase. If we combine contact data for left and right limbs, do we still see a" flat-spot"?

    Which leg is stiffer? Unfortunately, we don't have all the data required...

    P.S. A more difficult "vault" for what?
  31. Long shot, but does anyone here have the programming skills to build a computer simulation of the bipedal spring mass walking model that would allow us to manipulate the leg stiffness, resting leg length etc? I can CAD model it and maybe run some basic simulation's, but I probably won't be able to set the springs out of phase.

    The closest off the shelf I can find is this:
  32. Dananberg

    Dananberg Active Member


    Of course Dave's comment on L and R feet is correct.....didn't I allude to that in the comment that you have to look at the person walking, and take the entire gait analysis picture into perspective....and not just look at individual foot pressure maps?

    I used the term "vault", to describe how one moves over the limb in single support phase. Longer limbs form a more difficult "vault" than shorter limbs. The term has been used in gait related papers many times in the past.

  33. So the COM "vaulting" over the planted foot? Does vaulting still occur when we have compliant legs? Indeed, does the swing leg pull the COM up and over the contact limb- is this still "vaulting"?

    Indeed, in single limb support isn't the observed vertical loading the resultant of the swing limbs vertical acceleration + the contact limb, anyway?
  34. Dananberg

    Dananberg Active Member

    Wikipedia defines "vaulting" as "Jumping, the act of propelling oneself upwards"

    Since the CoM moves from its lowest point in the gait cycle during double support, and reaches the highest point in the middle of single support phase, then there is some amount of "propelling oneself upwards" which must occur. How the power is supplied to create this is what I believe we are discussing. Since we do rise and fall during the step sequences, it would seem that vaulting could the appropriate term. We are not, however, jumping....so if you have a better term, I am all for it.

  35. Howard, no problems, I'm just trying to fully explore the concepts.:drinks
  36. Let's try and move forward (excuse the pun): Howard, the observation of increased knee flexion; reduced hip extension and delayed heel lift in association with a functional hallux limitus would suggest a decrease in leg stiffness. Do you ever see this change in knee flexion occurring unilaterally or is it bilateral regardless of whether the functional hallux limitus is unilateral or bilateral?

    P.S. Reducing leg stiffness should increase the duration (period) of oscillation for each leg, therefore the speed of forward progression during gait should be reduced. Viz, gait is slower- does this fit with your observation's, Howard?
  37. Sorry for the multiple posts, this is coming to me in a stream and I want to get it down in writing before I forget it.
    Lets say we have one limb with a longer resting length than the other; for example the right is longer than the left, but the stiffness of the two limbs may be different too. If the right limb was longer but less stiff than the left, would it still be "a more difficult vault"? As the COM is elevated by the right leg spring during single support for the right leg, it's resting length might be greater, but under loading it's length might be the same as the left leg at the same point during the gait cycle because it is more compliant (has lower spring stiffness). My point? = this is what the body attempts to achieve = compensation for leg length difference occurs via modulation of leg stiffness and attempts to make the longer leg more complaint than the shorter leg to make the periods of oscillation the same for both limbs so that the COM doesn't shoot up higher during the midstance of the longer limb than at the midstance of the shorter limb. It (the body) may fail to achieve this if it cannot make the longer limb compliant enough, or if it cannot make the shorter limb stiff enough during the previous period of single limb support, ie. it forces the legs to function outside of their zones of optimal leg stiffness (ZOOLS) :cool:, cool, double cool- thanks Howard, for starting me down this train of thought...

    Now, tell me about that quadriceps muscle mass and EMG relationship with limb length discrepancy again? Yeah, baby, I'm on it now. Here you go: http://www.ejbjs.org/cgi/content/abstract/83/6/907 increased quads activity in the longer limb. My conjecture: as they attempt to control the required increase in knee and hip flexion in order to make the longer leg more compliant than the shorter one, the quads activity is increased...QED?
  38. David Smith

    David Smith Well-Known Member

    Simon, Howard

    Just reading your last cogitations I had this idea, what if you combine the idea of spring compression and vaulting.??

    If you consider that the momentum of the CoM is turned into inertial force as the CoM is braked at heel strike, then is that inertial force alone enough to allow vaulting over a stiff leg i.e. a compass gait? I should think not (no maths ), If then as Howard contends the short leg required less force to vault over, which seems reasonable since the vertical displacement is less, then since the braking leg is at an angle to the ground and so is the direction of inertial force from the CoM, then the spring can be compressed by the force of gravity and the inertial force of the CoM. The CNS might then modify the lef stiffness or KLEG to allow the optimal leg length to allow vaulting using the inertial force and the minimum or optimum amount of added force from the posterior or 'propulsive/push off' leg.

    Haven't really thought this thru but as were brain storming I thought I would get it down and out there for consideration.

    Howard, I realise this as I have read all your papers you write in conjunction with Dr Norman Murphy in the Tekscan range and probably all your papers period. But others are reading and following so I was just getting / ensuring everyone is on the same page so to speak.

    Regards Dave
    Last edited: Oct 12, 2010
  39. Dave, I think we were cross-posting and coming to the same conclusions at the same time.... See my post above.
  40. David Smith

    David Smith Well-Known Member

    Great minds or drowning men, one or the other eh!! :D

    LoL Dave

Share This Page