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Stiffness

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Jan 25, 2012.

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  1. markjohconley

    markjohconley Well-Known Member


    Members do not see these Ads. Sign Up.
    After copy & pasting 42 A4 pages from PA including sections of quoted articles, have I got it right,
    If my pt wants to "burn fat" I get her to run on jelly rather than eat it,
    but if a runner with stress #'s of the long bones of their leg wants advice I tell him to join the barefooters, preferably on concrete,
    mark
     
  2. Griff

    Griff Moderator

    Hey Mark,

    As Simon said we've been talking about this on the phone recently. Here's where I'm currently at with my understanding (courtesy of him):

    The CNS will modulate the stiffness of the leg 'spring' (kleg) in tune with the surface stiffness (surface encompassing the ground, footwear and orthoses).

    It does this to minimise any significant excursion of the Centre of Mass (CoM)

    It is fair to assume that there may be a zone of optimal leg stiffness (Spooners ZOOLS) for a given individual performing a given activity. (Where performance is maximised, and injury risk is minimised - although it should be said that these 2 variables may essentially have differing kleg values)

    Several factors can and will change the kleg (e.g. Foot type, running technique, running speed, change of surface stuffness etc)

    Generally speaking the injury profile seen with increased stiffness will be bone injury, and with decreased stiffness will be soft tissue injury

    As a rule - running on a stiffer surface, such as concrete, (or removing cushioned shoes) will result in the CNS modulating the kleg to become less stiff (more compliant). It seems intuitive therefore that this may be beneficial for those with bone injury (e.g. Tibial stress reactions).

    However,

    If a runner presents with exercise induced leg pain which turns out to be a stress fracture, our assumption is that there may have been an inability (or failure) of the CNS to effectively modify the leg stiffness, and therefore an approrpiate 'net stiffness' has not been achieved - in essence the leg is too stiff for the environment, and cannot change.

    Therefore,

    If we introduce cushioned footwear, or advise them to run on softer surfaces, we actually re-tune the net stiffness to be more appropriate. So we soften the surface to bring it in line with the kleg.

    I rang Spooner about this as it was bothering me also. The above is what we all do clinically on a daily basis (and we know it works) but taking the leg stiffness concepts to the letter it immediately makes you assume that to lower leg stiffness we need to increase surface stiffness. I think the key here is whether or not the leg stiffness can be modulated by the CNS.

    Hope that makes sense and reads ok - apologies if not - I'm typing it on my Blackberry whilst sitting in a conference.
     
  3. Btw what the kerdock paper showed was that if you want to burn more calories you should run on a hard surface.
     
  4. pod29

    pod29 Active Member

    Excellent summary mate! So how can we determine the ability (or inability) of the CNS to regulate leg stiffness?

    Cheers
     
  5. If there is lower limb overuse injury, this might be an indicator. Another method might be to look at metabolic cost. I think the key may lie in identifying left / right asymmetries in leg stiffness. Early days. If only I had a well equipped lab to play in........
     
  6. Just a couple of points
    Not necessarily minimise the excursion, more to maintain a constant displacement cycle. Also, its about minimising eye shake too.

    To reiterate, injury occurs because the leg has failed to successfully modulate stiffness to meet the environmental demands and in attempting to maintain a fairly constant net system stiffness (Knet= Kleg + Ksurface) the leg has been outside of it's Zone Of Optimal Leg Stiffness (ZOOLS). In this situation we can either: match the Kleg to the Ksurf by altering stride parameters, or we can match the Ksurf to the Kleg by altering the surface stiffness; or both.

    Something like that.:drinks

    BTW this is really just an extension of Niggs preferred movement pathway paradigm- I'm just saying it's the movement path of the CoM which is key and this is what the locomotor apparatus is trying to maintain in the face of variation in terrain.
     
  7. pod29

    pod29 Active Member

    Lower limb injury could be an indicator, but you might suggest that the horse has already bolted? left/right asymmetries seem like a good place to start in a clinical setting.

    In a lab setting... How about if we could manipuate the CNS / spinal reflexes in some way in order to determine if/where there is some "breakdown" in stiffness regulation? Maybe by using a nerve block? I've also seen 90Hz vibration used successfully to manipulate stretch reflex responses in the gastroc-soleus during running. Could we use techniques like this to manipulate the pathway?
     
  8. I think breakdown in stiffness regulation can be due to far simpler factors like joint ROM. The CNS can be functioning just fine, but if the range of motion isn't available...

    Are you familiar with the work on Soleus H-reflex stimulation and the phase of gait dependent motor responses which have been obtained? I talked about these studies some years ago on here and on the old Podiatry Mailbase- see if you can find them. If not I'll have a look later.
     
  9. The key is to match the leg stiffness to the surface stiffness so that they share the same resonant frequency. That way the transfer between kinetic and potential energy of the leg and the surface stay in phase. That way the surface isn't trying to transfer kinetic energy into the leg when the leg is trying to lose its kinetic energy to the surface. Too much energy in a tissue = tissue injury. This is what Tom Mcmahon realised.

    Energy is the new stiffness this season.
     
  10. Athol Thomson

    Athol Thomson Active Member

    This is a great post Simon!

    Energy storage seems to be important with respect to connective tissue regeneration and repair.

    A bit off topic so you might respond with a so?

    A book I have been reading about connective tissue regeneration says something along the lines of;

    The mechanism in which elastic energy is stored in connective tissue during locomotion brings about fibroblast stimulation in these tissues that directly effects gene expression and the regulation of cellular protein synthesis at the extracellular matrix.(via process called mechnochemical transduction).

    A link to the book is below with a nice summary on page 40:

    http://books.google.co.uk/books?id=...q=mechanical transduction in a tendon&f=false
     
  11. RobinP

    RobinP Well-Known Member

    Sorry to go back to basics but I, like Mark I think, have some difficulty in fully comprehending this topic. I read passages 2 or 3 times and cannot quite grasp the meaning so i am going to try and simplify it a bit if that is Ok

    The above quote interests me. If the ROM or stiffness of a given joint is insufficient to allow the CNS to effectively modulate the stiffness of the leg (kleg) then is it possible that the same could be said for the muscle strength being insufficient to regulate C of M. So, conditioning becomes a major factor, especially in cases where someone drastically increases activity over a short period of time

    Thanks in advance

    Robin
     
  12. David Wedemeyer

    David Wedemeyer Well-Known Member

    Ian and Simon thank you, I was completely lost on the previous stiffness threads and your posts really cleared it up for me. Epiphany moment!
     
  13. CraigT

    CraigT Well-Known Member

    Agreed- nice summary. Saves reading the whole other thread again...

    So...
    How does foot function have in influence?
    Does the stiffness of the foot help modulate total leg stiffness directly?
    Or does the stiffness of the foot have an influence on how effectively extrinsic muscles can regulate stiffness?
    Interesting stuff.
     

  14. After reading this stiffness thread, there seems to be a lot of theorizing going on, with little research evidence to back up the theory. As you all know, I enjoy theorizing also, but I'm not so certain I can agree with all of the statements that have been made so far.

    First of all, I'm not so sure that the key for the body is to maintain its CoM in a certain path as Simon says above. Rather I believe that the main driving force is for the body to minimize metabolic energy for running over a given surface stiffness by changing the kinetics and kinematics of the lower extremity, which will, in turn, cause the CoM to move in a certain prescribed path. In other words, is the driving force to change leg stiffness when encountering surfaces of different stiffnesses while running to make the CoM move in a certain path or is the driving force to change leg stiffness when encountering surfaces of different stiffnesses while running to optimize metabolic efficiency of running?

    I vote that the central nervous system chooses the most metabolically efficient leg stiffness for each surface of different running stiffness rather than choosing the leg stiffness that will make the CoM move a certain path.

    Secondly, in Ian's posting, he alludes to the theory that "Generally speaking the injury profile seen with increased stiffness will be bone injury, and with decreased stiffness will be soft tissue injury". Do we have any evidence for this generalization, or is this pure speculation? It seems, to me, to be a gross over-simplification, just as the barefoot runners like to focus on impact shock as the cause of all running injuries, to only focus on leg stiffness variations causing injury. What happened to such injury mechanisms such as increased magnitudes of subtalar pronation moments, increased tibial bending moments due to eccentric bending loads on the tibia, etc when you are discussing your hypothesis that increased stiffness causes bone injury and decreased stiffness causes soft tissue injury?

    Third, how do we measure or evaluate leg stiffness in our practices or whether our patients are properly altering their leg stiffness for the different surfaces they are running on? How do we use this theory that leg stiffness may be important in injury production to better treat our patients or to improve our patient's running performance? And finally, do we have any good research evidence that high leg stiffness in a runner causes any more or different injuries than in a runner that has low leg stiffness?

    Just some thoughts that popped out when I read some of the interesting statements made on this thread.:drinks
     
  15. Attached Files:

  16. Craig See some ideas listed above but it comes back to this discussion - Does the tibia drive the foot or does the foot drive the tibia?

    Yes, the foot will be a spring in the series of springs ( muscles) which make up the lower extremity, and if the effectiveness of one spring is reduced that `load`must be taken up by the other springs to maintain kleg , so we have individual muscle stiffness added together to give us net lower extremity stiffness or Kleg. Measuring muscle stiffness

    All of the theroy behind this is based around Hookes law - Hooks law

    Really good visual explanation Hookes Law Lecture 10: Hooke's Law - Springs - Simple Harmonic Motion - Pendulum - Small Angle Approximation
     

    Attached Files:

  17. Craig, I've attached this paper for you (I'm trying to avoid turning this into an attachment fest has occurred with the leg stiffness thread).

    Robbin, precisely. Which is why Janda's muscle imbalance theory may be significant.

    I'll come back to Kevin's post a little later when I find time.
     
  18. Phil Wells

    Phil Wells Active Member

    Simon (As the potential I-Phone Guru)

    Is there an app for the I-phone that would allow it to be used as a Accelerometer?
    If so, could we assess vertical and possibly horizontal displacement over, for example, the duration of a run?
    Could we then compare initial in-shoe pressure measurements e.g a force/time curve verses the same measures at the end of the run.
    Would an change in CoM correlate to increase foot pressures and consequently an decrease in the body to regulate stiffness?

    Just thinking about creating some basic evidence?

    Phil
     
  19. oops try again.
     
  20. Kevin, no disagreement from me. And as the CoM pathway and the metabolic cost are so obviously linked then I believe we are just saying the same thing in a different way. The specific CoM movement pathway I spoke about above which the body wants to maintain, is the CoM movement pathway which is most metabolically efficient for the given locomotor task. :drinks See also the work of Gordon: http://www-personal.umich.edu/~artkuo/Papers/ASB03GordonPoster.pdf and the .pdf attached; Ortega http://jap.physiology.org/content/99/6/2099.long etc

    This is the reason I corrected Ian: it's not about minimising the excursion of the centre of mass (as this will actually increase metabolic cost), it's about maintaining a steady, metabolically optimised, CoM displacement cycle for a given locomotor task. Viz. the preferred movement pathway for the CoM.

    However, things like pain avoidance may also trigger a change in the CoM pathway. But again, a steady state cycle would seem desirable.

    this comes from the Butler paper and the references therein. See section on leg stiffness and injury here: http://www.udel.edu/PT/davis/stiffness_update.pdf

    Clinically, Daryl Phillips F-scan module is probably the best way to determine leg stiffness without a force plate. I use a treadmill and camera and look at contact times and step frequency- it's crude, but gives a rough idea/ estimate. Failing that, a single leg hopping test. As I intimated previously, I tend to look for left to right asymmetries see http://www.ncbi.nlm.nih.gov/pubmed/22117105 and obviously the presence of pathology. In terms of treatment I use the information to modify stride parameters and orthoses stiffness characteristics. Early days as I said.


    See Butler paper: http://www.udel.edu/PT/davis/stiffness_update.pdf; this one too: http://www.ncbi.nlm.nih.gov/pubmed/22117105 And obviously this: http://www.udel.edu/PT/davis/myweb/Irene_Arch_structure.pdf when viewed in light of this: http://www.podiatry-arena.com/podiatry-forum/showpost.php?p=245340&postcount=25

    But then of course you also have this: http://w4.ub.uni-konstanz.de/cpa/article/viewFile/3337/3137
    Hopefully you will now find this thread a little more evidence based, Kevin.
     

    Attached Files:

  21. http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=56116
     
  22. So? Seriously, explain your thinking Mr Thomson...
     
  23. That sounds better now. I believe it is important to remember that the central nervous system (CNS) is driving the near instantaneous changes in leg stiffness that are occurring during running in different shoes and different surfaces. In fact, as far as we know currently, the key driving factors behind why the CNS chooses a certain kinematic pattern of running is to reduce the metabolic cost of running and to avoid pain and injury during running, not to make the center of mass move in a certain path during running.

    However, I am not so convinced by the paper provided that we can be so certain that lower stiffness lower extremities will always tend to develop soft tissue injuries and higher stiffness lower extremities will always tend to develop bony injuries. Again, I think those that truly believe this are placing way too much emphasis on lower extremity stiffness during running as the key factor in running injury production.

    For example, what if I have a high school female distance runner who is just beginning to run longer distances, has relatively low leg stiffness and then develops a tibial or metatarsal stress fracture more due to their low bone density and narrow diameter bones (i.e. decreased moment area of inertia) than due to whether their legs were more or less stiff.

    http://www.orthometrix.net/downloads/human12.pdf

    I have seen countless stress fractures in the metatarsals and tibias in young female runners much more so than in their male counterparts. And, typically, the male runners have much more stiff legs than do their female counterparts, but have far fewer stress fractures (stiffness being determined by how little time the runners feet spend on the ground for a given running speed).

    Certainly, lower extremity stiffness is one factor to consider and it is fun to talk about and theorize about. However, I am not convinced that it is the most important or even in one of the most important factors that tend to produce injuries in runners from what I have seen in the 27 years I have been treating injuries in runners.
     

  24. Lets take your example with two identical female athletes with the same cortical thickness in their tibias. They follow the same training plan. One runs with high leg stiffness, the other with more compliant leg stiffness. Which one is more likely to develop tibial stress fracture and why?

    How have you measured the leg stiffness of these countless young runners over the years in order to state that the male runners have much stiffer legs than the female runners? If you were just looking at contact times did you normalise the data for body mass? http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1624931/ There is an obvious comeback to this... Anyone? Think about y'all.

    More on gender differences in musculoskeletal stiffness and risk of injury here: http://www.jelectromyographykinesiology.com/article/S1050-6411(02)00002-0/abstract and here:
    http://www.jelectromyographykinesiology.com/article/S1050-6411(02)00003-2/abstract
    and here:
    http://www.jssm.org/vol8/n2/15/v8n2-15pdf.pdf
    etc. etc.
    With the greatest respect Kevin, you haven't been measuring leg stiffness in your patients over the last 27 years in the same manner in which the authors of the published papers who have suggested a link with leg stiffness and injury have done, nor have you collected and analysed data on leg stiffness and injury in the same way that they have. So why should we favour your statement over their data? Come on, you'd be the first to point this out to another, my friend.;) While your expert opinion is acknowledged, lets see where the hierarchy of evidence takes us on this...:drinks What we need are multivariate models of predictors for specific injury. That way we can see how strong a predictor leg stiffness is for specific pathologies, rather than speculating.

    Now, I'm not saying that leg stiffness is the be all and end all, it's far more complex than that. However, leg stiffness does seem to be being linked with injury and performance in a relatively large and ever growing number of peer reviewed publications in highly respected journals. This one I linked to earlier, being published just this month is the latest: http://www.ncbi.nlm.nih.gov/pubmed/22117105 Whether we like it or not, this kind of evidence trumps that of "expert opinion" in the hierarchy of evidence. As of today there is far better evidence in the peer reviewed literature suggesting a link between leg stiffness and injury/ performance, than there is to suggest a link between subtalar joint axial position and injury/ performance. So while the significance of subtalar joint axial position in injury "is fun to talk about and theorize about" there is little evidence to support it. Can you resist that contention, Kevin? :drinks To quote Craig: "you got to go where the evidence takes you". Only playing.

    In my "expert opinion" and with my 21 years of post-graduate experience treating running related injuries, leg stiffness is worthy of further serious study. Judging by the number of publications in this field it seems I'm not alone in this view. But each to their own.
     
  25. Athol Thomson

    Athol Thomson Active Member

    Again from the book.....I wish I could say it was my thoughts.


    Forces generated by muscle are stored as elastic strain energy during tendon deformation, then transferred to bone to allow for joint movement.

    Some of this energy goes into moving the joint and some of the energy is transduced into cellular changes via a process called Mechanochemical transduction.

    This dictates how cellular activity will respond to the forces and ultimately lead to changes in the mechanical properties and composition of the connective tissue.

    Stored energy drives mechanochemical transduction during locomotion so I was thinking that leg stiffness/surface stiffness will have a part to play. Especially when the practitioner is attempting to accelerate connective tissue repair after injury. Maybe as the patient returns to straight line running.


    More later,
    Athol
     
  26. I acknowledge that I am guessing, which I, again, am pretty good at doing. I'm not saying that stiffness isn't important, but is optimizing leg and surface stiffness the key to preventing the majority of running injuries?

    Do you actually think, Simon, that athletes with stiffer legs during running suffer more bone injuries and athletes with less stiff legs during running suffer more soft tissue injuries? What does the science so far tell us about this? Does bone density, and cortical diameter of long bones have less to do with the possibility of the runner developing stress fractures than does their leg stiffness?

    Can't wait to discuss this in Belgium over some good Belgian beer...yum....
     
  27. I think you need to answer the hypothetical Prof. Kirby: "Lets take your example with two identical female athletes with the same cortical thickness in their tibias. They follow the same training plan. One runs with high leg stiffness, the other with more compliant leg stiffness. Which one is more likely to develop tibial stress fracture and why?"

    Knowing your capacity for alcohol... knowing how strong Belgium beer is... It'll be a walk over.:drinks

    P.S. the parcel might be coming from an "Angel"- he assures me it is posted this week!!!

    P.P.S. Statements like this: "I acknowledge that I am guessing, which I, again, am pretty good at doing" make me smile, remind my why I love you and where I get it from. Will I ever be that confident?
     
  28. Re beer in Belgium

    make sure you 2 have a beer or 10 with Toni Arndt
     
  29. I don't think we have enough evidence to draw that conclusion. We need a good prospective trial.

    But that doesn't make you right.;)
     
  30. Even by my standards 10 may be unobtainable in Belgium; they brew it strong.
     
  31. well enjoy anyway it is the effort - he is the bone pin guy over here and the guy to help with the next stage of the patient study re .....

    so you can all talk behind my back :D
     
  32. Lets not loose this as it is potentially an important exercise to the understanding of leg stiffness and performance.

    Kevin wrote: ... "typically, the male runners have much more stiff legs than do their female counterparts (stiffness being determined by how little time the runners feet spend on the ground for a given running speed)."

    I replied:
    "If you were just looking at contact times did you normalise the data for body mass? http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1624931/ There is an obvious comeback to this... Anyone? Think about y'all."

    What is the "obvious comeback" given that Kevin suggested increased leg stiffness in males compared to females observed via contact times (not necessarily true, but lets run with that for this exercise)? You'll need to understand simple harmonic oscillators, the influence of spring (leg) stiffness and mass on resonant frequency and how this relates to contact time and how contact time relates to performance to answer this.

    If you're interested in leg stiffness, tell me why normalising for body mass becomes irrelevant if Prof. Kirby's contention is correct, men being generally heavier than women (it's not, but lets just help his ego here by pretending he's right; like he needs his ego massaging- "I acknowledge that I am guessing, which I, again, am pretty good at doing"- extraordinary :rolleyes:)
     
  33. Simon:

    For two runners, running at the same velocity, one with a support time of 200 msecs and the other one with a support time of 400 msec, why wouldn't the runner with the shorter ground contact time also have a stiffer leg? Wouldn't this be a good qualitative way of measuring relative leg stiffness between one runner and another, having them run at the same speed and measuring the duration of their support phase of running?

    In addition, one of the training effects I have noted in runners as they become more fit is that their support time decreases over time, which I assumed was related to them increasing their leg stiffness with increased conditioning. Do you know of any study that has measured leg stiffness in runners during a longitudinal conditioning study?
     
  34. Simon:

    I believe that any good clinician will have the ability to be "pretty good at guessing", since, in all reality, how many absolute truths do we know about the physiology and biomechanics of the human body? Therefore, the decisions we all make for our patients are based many times on educated guesses.
     
  35. The key is in regard to the differing mass- with males generally being heavier than females. As I said there is an obvious come back regarding the requirement to normalise for mass given your statement of the males having shorter contact time than the females. Assuming a heavier mass with a shorter contact time for males, why does normalising for mass for the lighter females become irrelevant? Let the others play, Dr Kirby... but think about spring mass oscillators. Just for you Kevin: what if a lighter individual had a shorter contact time than a heavier individual- would that mean they had a stiffer leg? The answer you are looking for is: not necessarily. So, looking at contact times in isolation might be a rather blunt instrument, I guess.
     
  36. Yeah, I guess it could also be argued that its better to look at the evidence than to guess. But lets not get hung up on this since it was honestly just a joke, Kevin. Goodnight.
     
  37. Simon:

    Let me ask the question again, but this time, more precisely:

    For two runners of the same body mass, running at the same velocity, one with a support time of 200 msecs and the other one with a support time of 400 msec, why wouldn't the runner with the shorter ground contact time also have a stiffer leg?
     
  38. The key is that you didn't previously specify the body mass, Kevin. Nor, I rather doubt, did you normalise the data for body mass when you looked at contact times in your patients over the last 27 years. So frankly without this information, you could only have been guessing about leg stiffness in looking at contact times. I was trying to be kind by suggesting that males are generally heavier than females therefore your contention could be right because in a spring mass system with a heavier mass and a shorter frequency the spring stiffness might well be higher than in a similar system with a lighter mass and a longer frequency and at the same time I was trying to allow others to explore the physics of simple harmonic oscillators, since this is a good model of the body in running locomotion.

    I think you know that I know the answer to your question, Kevin. But differences in foot length might be significant in altering contact times in the presence of identical leg stiffness and identical mass and velocity.

    Let me ask the same question again, third time lucky: "Lets take your example with two identical female athletes with the same cortical thickness in their tibias. They follow the same training plan. One runs with high leg stiffness, the other with more compliant leg stiffness. Which one is more likely to develop tibial stress fracture and why?"

    I really do need to go to bed now though.
     
  39. I believe that the injury risk of tibial stress fracture in runners depends more on the direction and point of application of the ground reaction force (GRF) vector relative to the long axis of the tibia than it depends on whether the lower extremity stiffness is different between these two hypothetical runners. Tibial stress fractures are probably most commonly due to abnormal bending moments in the tibia so that if the GRF vector is causing an eccentric load on the tibia, rather than an axial load, than a medial tibial stress fracture will be more likely to occur regardless of the inherent stiffness of the lower extremity.

    However, if the point of application of GRF, the 3D location of the GRF vector relative to the long axis of the tibia, the bone density in the tibia, the moment area of inertia along the tibia, the muscle strength in the lower leg muscles, the training habits, and racing frequency are all the same in these two hypothetical runners, then I guess that the runner with the stiffer leg would be more likely to develop a tibial stress fracture due to the greater vertical loading force occurring across the tibia over a shorter period of time.:drinks
     
  40. The correct answer is that if runners of identical mass, are running at the same velocity and one runner has a support period of 200 msecs and the other runner has a support period of 400 msecs, then in all likelihood, the runner with the shorter duration support phase will have the stiffer leg.

    I sure do see lots of short duration stance phases in the elite runners....wonder how many of these elite runners get bone vs soft tissue injuries...isn't the Achilles tendon a soft tissue? My guess, is that elite middle distance and long distance runners get just as many soft tissue injuries as bone injuries even though their leg stiffness is probably on the high side compared to the population of runners as a whole.

    Great discussion, Simon. Thanks for forcing me to learn more about this subject. Certainly seems that gaining a better understanding of leg stiffness has the potential to help clarify some of the mysteries of running biomechanics. Looking forward to speaking further in person with you on this subject over some Belgian waffles, Belgian beer and Belgian chocolate.:drinks
     
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