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Leg stiffness asymmetry: clinical implications

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Jun 22, 2014.


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    All, I've just returned from Lecturing at the BSS 2014. One of the things I said which seemed to raise some interest was regard to asymmetry in leg stiffness (I was talking about compensation for leg length difference).

    Lets take me as an example: due to traumatic injury to me right knee, I demonstrate asymmetrical leg stiffness when performing hopping tests: K = 40 x SF2 x mass

    K is the leg stiffness (N/m)
    40 is (2x∏)2
    SF is the step frequency (steps per second)
    Mass is measured in Kg

    my left K = 15,888 N/m;
    my right K = 11,610 N/m

    So, if we assume the body is trying to maintain a fairly consistant CoM displacement pathway during my walking gait cycle, how might my body compensate in an attempt to "equalize" the displacement under loading in light of variable left to right leg stiffness characteristics?

    Would my body benefit from functioning upon different surface stiffnesses under the left and right feet?

    BTW, these papers may help for background reading:
  2. Attached Files:

  3. toomoon

    toomoon Well-Known Member

  4. Except in this case the resting length of the legs is the same, only the stiffness is different... So the question becomes: given that the stiffness of a spring is partially dependent upon the resting length, should we induce an artificial limb length discrepency and/ or should we manipulate left/ right surface stiffnesses to resolve the issue and make the CoM trajectory more symmetrical?
  5. toomoon

    toomoon Well-Known Member

    I would go with option B.. I do not think inducing a LLD when it is absent is sound, but manipulating foot/shoe/surface stiffness makes some sense to me.. really interesting concept!
  6. Out of interest, when the authors from the same study that I took the graphic from above induced a leg length difference in their robot which has a leg length of 1m, they found stable gait solutions with up to 1.8% (1.8cm) asymmetry. But they made the leg stiffness the same despite of the difference in length- I don't think this happens in humans due to the neuromotor attempt to minimise the effects of leg length difference by modulating stiffness.
  7. Athol Thomson

    Athol Thomson Active Member

    Hi Simons,

    In theory you might be able to increase the stiffness of the injured (less stiff) limb by having it function on a more compliant surface and decrease the stiffness of the un-injured (more stiff) leg by having it function on a more stiff surface. Tough to achieve in daily living I imagine.

    Possibly by tuning the stiffness of the shoe midsoles (at the same height). Although Bishop et al (2006) did not find a change in leg stiffness with different midsole densities.

    If leg stiffness is adjusted within one step of loading on a new surface, as Farley et al suggests than I wonder how the CNS would go with independently modulating the stiffness of each leg walking on different surfaces? Would it get confused?

    I think the key would be to increase the passive stiffness of structures such as the tendons. There is some good papers that suggest certain strength and conditioning modalities can increase MTU stiffness and perhaps leg stiffness such as heavy slow resistance, isometric and plyometric training. The CNS does not have complete control of leg stiffness modulation.

    See http://www.ncbi.nlm.nih.gov/pubmed/22089697

    I can email a copy of this.

    To compensate the ankle and hip of the previously injured limb may be more stiff to make up for the more compliant knee when walking? Not sure if hopping stiffness tests translate to walking due to different GRF magnitude and changes to stretch shortening cycle.

    Real shame to miss the BSS this year. Sounds like it was fantastic. Well done to you boys.
  8. toomoon

    toomoon Well-Known Member

    Hi Athol.. how is Doha mate? Tanner looking after you? This is really interesting isn't it? A lot of unanswered questions, and unfortunately most of the literature involves modelling which I have to take with a grain of salt.. especially when it comes to leg stiffness. i have the Taube paper thanks. The Farley paper gives a good clue.. but we are talking a pretty major mechanistic difference with a LLD..
    I might be as simple as changing the texture of the surface (ie shoe) foot interface.. which would be a lot easier than trying to find L and R shoes of different densities. This would be a nice little study... anyone up for it?.. Simon??
    BBS was excellent.. I think probably the best of the 4 I have attended..next year mate.. book it now!
  9. Athol Thomson

    Athol Thomson Active Member

    Yeah great thanks Simon. Tanner doing a great job looking after me even if he is from Victoria....

    Still cycling to work in Doha but it was 49C the other day and I think I've hit my limit!

    I agree that models always assume something and that the reliability of some of the stiffness measures are not there yet. But....it is a fascinating area and great way to study the way the lower limb responds to loading during different tasks. I feel this is more "real" than looking at biodex-type measures with someone strapped into a chair. (I know that this has its place before anyone thinks I'm down on isokinetic measures)

    And since force is generated by the interaction with the surface (see Newton) I think we (podiatrists) should get involved with improving the the models or measurements etc. This can include rotational stiffness between a football cleat/stud and grass type, shoe-surface interaction of a handball player or whatever. Doesn't always have to be vertical or leg stiffness.

    Someone rich just needs to bankroll Spooner to research full-time and I reckon he would have most of it figured out pretty sharpish!

    Would love to get back to Townsville for your conference with KK. I grew up just up the road so can email a few spots to eat and drink.
  10. I don't think the injured limb's stiffness should be the target (at least in my case- unless my cruciates suddenly grow back), rather I'd target the uninjured limb, in terms of surface stiffness variation- make the surface stiffer to make the limb more compliant.

    Athol, think about walking with one foot on a compliant surface and the other on a stiff surface- you don't fall over- hence there is capability of the CNS to modulate the left and right limbs independently.

    You right though, rehab clearly has a role too.

    "Someone needs to bankroll me" Yep, I keep playing that Euro lottery. To be honest, I'd do the research for free if someone gave me access to the required kit.
  11. Athol Thomson

    Athol Thomson Active Member

    Good point - didn't mean you might fall over just wonder how it would work if indeed the CNS feedback modulates leg stiffness with info from previous step (by pre-activation of MTUs).

    As you know....on the ACL front. Depends on what you want to do. I think there is growing evidence that some people can return to sport without surgery (copers). Most likely different neuromuscular activation about the hip and knee compared with non-copers. Obviously depends on what you want to do. But it looks like you can increase at least the passive stiffness of say the hamstrings and quads that cross the knee. Probably not gonna let you be a lively loose forward again on the rugby field but ok for some sport.

    See http://link.springer.com/chapter/10.1007/978-3-642-15630-4_8


  12. The fact that you don't fall over suggests that the info from the previous step can be over-ridden.

    Smith and Watanatada (2002) did report differences in stiffness when running in shoes of varying stiffness. Here's what I'd like to see, anyone with access to the required kit can do this: a pair of shoes, one with a midsole made of wood at the heel (or some other relatively stiff material) and the other with a midsole within McMahon's "magic" range- lets see if there are any differences in left/right leg stiffness... If I get time, I'll make some shoes up like this in the next week or so (although it may not be in McMahon's magic range)
  13. Kenva

    Kenva Active Member

    Sorry guys, just got back from celebrating the Belgian win against Russia.
    This is really fascinating stuff! Athol, we should sit down and see if we can do something down this road together...
    Thanks Simon for the input - really sorry I couldn't make it this year to the BSS- much going on right now to fit it in...!
  14. Athol Thomson

    Athol Thomson Active Member

    Thanks I have never seen that paper and would like to read it. Found the poster he presented online there now. You got a PDF full paper?

    I think the wooden shoe should have the upper of a bowling alley "loan" shoe. Throw a patent on it now...it would look ace.
  15. Love that, **** you Spooner, it's your idea but I'll take that idea and get involved with Athol... Jeez. Only joking, or am I?:morning:
  16. Kenva

    Kenva Active Member

    Simon, don't get me wrong here - not wanting to steal any idea - sorry if it came out this way - I have way more respect for you than this... :drinks
  17. toomoon

    toomoon Well-Known Member

    yeah.. seriously.. I was the one who suggested the study!!:boxing:
  18. I'm joking, you two guys are far better placed to carry out the research than I am, might be nice to be asked along as a consultant though ;)
  19. Are you still here, Bartold? :D
  20. Kenva

    Kenva Active Member

    That would be one good way to finally sit together again :drinks
    Hope I can keep this offer in the back of my head ...?
  21. Anyway, back to the plot... When we create custom foot orthoses, if they are truly custom then their stiffness will be asymmetrical on the left and right sides (probably not so much under the heel (unless we stick an eva heel lift on one and not the other) but they will be asymmetrically stiff in the arch. What impact is this having on CoM displacement? I think the guy to talk to here might be Daryl Phillips who developed the CoMnalysis software for Tekscan, I'm sure he's observed changes in CoM symmetry using insoles via this software.
  22. Athol Thomson

    Athol Thomson Active Member

    Ken is moving to Doha to join us in the next few weeks. (Hope its ok to say this now Ken?) Hence the misunderstanding.

    Mind you..lets be honest.....I have never had an original idea in my life. Most things have spawned from things you nonchalantly throw out there at the end of a post! So thank you.

    I'm up for any of the studies suggested if you wanna go multi-centre Simon B, Simon S and Ken?

    Anyway, Back on track....
  23. I already knew Ken was moving to Qatar.

    Back to the plot, literally: here is another plot from the same robotics study in which the leg length is made asymmetrical, but the stiffness is maintained constant across the two legs. Note the timing changes, relative to the CoM displacements. The frequency of a spring is related to it's stiffness- stiffer springs will oscilate more rapidly than more compliant springs. So, if we wanted to bring the timing back to symmetry, how might you achieve it? Make the longer limb less stiff than the shorter limb, of vice versa or both. How might we achieve that? I think this is what the CNS does- it increases knee flexion of the longer limb (there are probably over methods since its a cat skinning situation), resulting in decreased leg stiffness on this side, slowing the oscillation. However there is a cost to that, and it has to maintain tissue stress levels within their ZOOS, sometimes it can, sometimes it can't and when it can't that's when we intervene, with heel lifts. Yet I believe we can intervene with surface stiffness modification too, in a fully functioning CNS. Lets help the shorter limb stay stiffer by providing a surface stiffness in which it can stay stiffer, i.e. a more compliant surface- hence the compliant heel lift approach.

    I know Craig reported on some work on asymmetrical desnity heel lifts in one of his boot camps- perhaps he can add to this discussion?

    Night boy's, good luck with the research. I'm off to win the lottery so I can retire and stop having to think about this ****.

    Attached Files:

  24. toomoon

    toomoon Well-Known Member

    yup..I am here!!
  25. Dr. Spooner:

    Interesting discussion, but I have a few points to make.

    First of all, we don't know that the CNS can even detect a small limb length discrepancy (LLD) to make "compensations" in stiffness during either walking or running. Therefore to speculate that the CNS will change the stiffness of the limb during walking or running for a LLD seems a stretch to me, at least for the magnitudes of LLD we most commonly see (less than 1.0 cm).

    Secondly, a "compensation" such as knee flexion for a longer limb, as you mention above, will not likely occur if the extra knee flexion demands a greater metabolic cost. Therefore, I wouldn't speculate that the driving factor by the CNS is to keep the limbs "equal in length", but rather think that the CNS is better able to detect changes in metabolic demand and optimize the gait of the individual based on metabolic energy demand, not on "equalizing leg length".

    Until you can present some research to me that the CNS can somehow detect that a LLD, I would rather hang my hat on the fact that the CNS probably will change the gait of the individual to optimize metabolic energy expended per distance the individual walks or runs, not to equalize the leg length of the individual.
  26. Thanks for your input Kevin,

    The magnitude of leg difference required for the CNS to make adjustments will vary from terrain to terrain, individual to individual. But lets assume we are talking about a limb length discrepancy of sufficient magnitude to warrant clinical intervention here. I believe the primary goal of the CNS here is to: a) avoid falling over; b) to avoid injury and c) to be as metabolically efficient as it can. I believe a) and b) will generally take precedence over c).

    I did not say the driving factor for the CNS is to keep the "limbs equal in length". Rather, I speculated that the driving factor should be to maintain a consistant displacement pathway of the CoM as support is transferred from limb to limb. Do you believe that the CoM displacement pathway is related to metabolic efficiency? Should left to right side symmetry in the CoM displacement pathway be more metabolically efficient than a CoM displacement pathway showing asymmetry? If this contention is correct, then how might the body attempt to bring symmetry to the CoM displacement pathway in order to optimise metabolic efficiency?

    Clearly, the modeling performed here: http://iopscience.iop.org/1748-3190/...el=sem&relno=1 shows that a stable gait can be achieved in a limb length descrepency with symmetrical leg stiffness being maintained. However, it also clear that changes in the timing of contact phases of the two limbs which impacts upon the relative position of the CoM at these events must be modified. So there are more than one way in which the CNS might resolve the problem of asymmetrical leg length.

    In terms of "compensation" I quote this from Craig's running research blog:"How does the body compensate for a structural leg length difference? According to the Bloedel & Hauger study, they flex the knee more on the long side"

    Bloedel PK, & Hauger B (1995). The effects of limb length discrepancy on subtalar joint kinematics during running. The Journal of orthopaedic and sports physical therapy, 22 (2), 60-4 PMID:

    Note also, the increase in quadriceps activity in the longer limb reported by Gurney in response to an artificial induced leg length difference. http://www.ncbi.nlm.nih.gov/pubmed/11407800

    Since leg stiffness is directly influenced by knee flexion, I think its a reasonable assumption that a change in leg stiffness may well occur in response to leg length discrepency, with the longer limb becoming more compliant at the knee at least; I think the shorter limb becomes stiffer at the ankle and believe the research backs my contention- since my contention is based upon my reading and interpretation of the research.

    Given what we know from the research on the interaction between surface stiffness and leg stiffness, I do not think it is too great a leap of faith to suggest that manipulation of surface stiffness MAY be useful in influencing leg stiffness characteristics.

  27. Most here will be aware of where I sit on the leg stiffness debate.

    back to Simon's original question, I can not see any mention of pain.

    I believe in a functional kr structural leg length discrepancy 1 of the bodies mechanical compensations is to adjust the stiffiness of the longer leg and make it more compliant, but I am not sure the body would do this if there was pain or if a joint such as Simons knee was unstable.
  28. Here you go, asymmetrical midsole stiffness: http://www.biomedcentral.com/content/pdf/1757-1146-7-S1-A8.pdf
  29. Athol Thomson

    Athol Thomson Active Member

    Thanks Simon.

    Really interesting. Keen to see full paper when it is published. They didn't seem to measure leg stiffness. The trunk accelerometer data suggests leg stiffness is modulated to maintain similar vertical impact peaks. This could be a nice simple way to postulate about the effect an intervention has on leg stiffness that can be taken out in the field (out of the LAB).

    Maybe a bit late to the party but been getting into some work byThorsten Sterzing recently along with Ewald Hennig. Big fan.
  30. No they didn't appear to measure leg stiffness, their premise appears to be that without modulation of leg stiffness then there should have been variation in the left right accelerometer readings due to difference in the left right midsole stiffness, since this wasn't observed they assume that leg stiffness modulation is occuring. It's a bit of extrapolation really, but interesting nonetheless.

    Sterzing appears to be suggesting in one paper that it is the heel portion of the midsole which is key, if I interpreted that correctly.
  31. Simon:

    Thanks for the reply.

    However, in this thread, it seems that everyone is lumping walking leg stiffness in together with running leg stiffness which, to me, makes no sense and invites errors due to the greatly different ground reaction forces, kinetics and kinematics between walking and running activities.

    Please everyone, when discussing leg stiffness in this and other threads, make sure you indicate whether you are talking about walking or running since we can't assume that what happens in the human body during running will also happen in walking.

    Running is not an accelerated form of walking!
  32. Agreed. However, the bipedal spring mass model of Geyer provides a single unified approach to studying both running and walking gaits.

    Lets go back a step.

    Do we agree that the centre of mass (CoM) displacement pathway and metabolic efficiency are related?

    I think most of us would agree that they probaby are. http://www-personal.umich.edu/~artkuo/Papers/APMR09.pdf

    Should a symmetrical (left and right limb support) CoM displacement pathway be more metabolically efficient than an asymmetrical pathway?

    If you'd have asked me this on Sunday, I'd have said yes... Now I've thought about it more, I'm beginning to think: "not necessarilly". But I should welcome others thoughts before I try to explain my reasoning since we're all trying to learn here, not just me- right?
  33. Arthur Kuo is one of my favorite authors on this subject. Minimizing CoM displacement does not seem to be the most metabolically efficient way of walking or running.

    Dynamic Principles of Gait and Their Clinical Implications
  34. Yep, flat-lining the CoM is not desirable since it increases energy cost, but I don't think we want excessively high vertical displacement amplitudes either.

    What about left/ right sided symmetry?
  35. I can not think why an asymmetric left/right would be a benefit, the only time I can come up with when the body might do this would be pain avoidance and thats about all I could think of.

    agree that there should not be too little or too excessive CoM vertical displacement, and that each activity and surface stiffiness, each individual will have a zone of vertical CoM displacement for a more metabolic efficiency
  36. Indeed, asymmetric motion of the CoM, along with vertical displacment appears to be a good predictor of metabolic efficiency in patients having undergone total hip replacement. http://www.ncbi.nlm.nih.gov/pubmed/23100147

  37. If we employ a bipedal spring mass model in which the left and right legs are modeled as springs of different stiffness, then the potential energy stored in each leg spring is given by:

    Potential Energy = 1/2 KX>2

    K is the leg stiffness
    X is the displacement

    So, what does this tell us?
  38. Funny I see a forumla and my brain goes to mush

    but ......

    Potential energy should be on an individual spring basis and added together to look at the body in total so 1 leg may have 60% the other 40 % ????

    but ..... to be honest just a random guess
  39. Let me put it another way: we have two springs of different stiffness, but we want to store the same amount of potential energy in both. How do we achieve it?

    Let K left be 16,000 Nm and X left be 0.04m
    If K right is 12,000 Nm what does X need to be so that the potential energy is the same in both springs?

    This site will solve the maths for you: http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html

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