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Leg Stiffness

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, Mar 15, 2010.


  1. Members do not see these Ads. Sign Up.
    I thought ( which is getting more and more of a problem) we should have some discussion about leg stiffness.

    It seems to be playing more and more of a role within the biomechancis discussions. See the barefoot thread latest. http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=43282 see pages 8 and 9

    also we discussed it here in this thread last few pages http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=35726

    Questions that I have or maybe can make good discussion points

    What exactly is leg stiffness ?

    How is it defined ?

    When is it good to have increased/decreased leg stiffness?

    What effect does the ground surface have on leg stiffness?

    How does the body control the amount of leg stiffness - is the foot or the amount of knee flexion-extension or something else such as the CNS ?

    Is there a definative paper on leg stiffness?

    How does different striking positions affect leg stiffness?

    Clinically how do we define too much/ not enough leg stiffness and how do we make our clinical discissions ie hard device/soft device ?

    I´m sure there is much more but thats a start
  2. Attached Files:

  3. All up to speed?

    Lets say we had a patient with achilles tendinopathy and we increased the foot-interface stiffness (shoe + orthoses stiffness), what effect would this increase in surface stiffness have on ankle and knee kinematics?
  4. up to speed I´m not sure about, but this is what I´ve got.

    The increased foot stiffness should lead to a decreased leg stiffness.

    Now this is where I get a little confused, some of the articles I have read have said that the changing the foot interface stiffness. Ie running on hard then soft ground showed that there was no change in the kinematics of the knee or ankle. The body naturally adjusted the leg stiffness to this new GRF. This would lead to same joint kinematics but different muscle fuction. This is the bodies way of keeping the leg stiffness optimial, the zones of optimal leg stiffness that you wrote about in the barefoot thread.ZOOLS. So I guess the above is for when it all works perfectly.

    If we increase the foot interface stiffness too much we may decrease the leg stiffness to a point at which the body is not able to maintain ZOOLS and we then will get changes in the knee kinematics 1st and then ankle after, this seems come from muscle fatigue.

    What I can gather is that decreased leg stiffness outside of the ZOOLS may lead to muscle overuse and increased ZOOLS may lead to increased bone stress.

    Thats what Ive got so far
  5. Do we know what those changes in knee and ankle kinematics are?
  6. This is taken from 1 of the papers you posted.

    It appears that we get greater knee flexion and greater ankle plantarflexion.

    It also appears that the knee joint is the 1st affected and then the ankle, is also appears that it is about the balance between antagonist and agonist muscles of that area.

    ie one gets fatigued great dominance of the antagonist chane in kinematics.
  7. So in our achilles tendinopathy is this likely to be positive or negative in terms of tensile stress in the achilles?
  8. So Triceps surea main action- plantarflexion of Ankle and Gastroc helps in Flexion of the knee when contracting. Contacting = less tensile load unless the external moment increases, but the paper showed change in position, so

    Therefore in terms of tensile stress it would be positive. There would be less.
  9. Yeah, that's where I got to as well. Perhaps that's why I never found silicone heel lifts that efficacious in achilles problems- too soft?
  10. Another very intersting study in that one as well.

    As an aside everyone being different and all, say we have x % improve with harder heel lifts and x-y% improve with silicone. Due to the individual body types and different ZOOLS ( I´m a big fan of the ZOOS and ZOOLS ideas by the way make lots of sense) How would this help in our clinical set up.

    ie this is where I´m going with this.

    Patient comes in. Achilles pain. we test muscle stength and length of knee flexiors, extensors and muscle length and strength of ankle plantarflexior and dorsiflexiors

    from these tests maybe we could determine who get soft or hard heel lifts and maybe the optimal height ?
  11. It would be even better if we could measure leg stiffness in a standard clinical set-up. I'm guessing most folks don't have a Kistler force plate, but perhaps we could make similar calculation from a pressure mat system. I know Dave Smith is interested in inferences regarding centre of mass from pressure plates- perhaps he will comment on this?

    I think the joint stiffness at the ankle and knee will be significant. This is why I was asking Ron Bateman about predictive models for the lunge test and running. Unfortunately, he took my questions the wrong way.
  12. I guess it would help alot in the being able to predict outcome of some treatments plans a little more closely.

  13. I found this on Irene Davis uni web pages. It discusses loading rates and Plantar fascia problems. Ive just scanned it but it looked intersting.

  14. Nice paper thanks Simon for posting it up.

    These 2 sections from the Davis paper above jumped out at me.

    Simon and others, I´m not fimilar with inshoe force measurement devices, can they deterimine loading rates ?
  15. Slope of the force/ time curve = rate of loading.
  16. before I go off on off a side track. Do you think that Knowledge of Leg stiffness in relation to ZOOLS for specific activities maybe 1, if not one of the most important strings to the treatment bow in relation to the tissue stress models of treatment ?

    Edit I should have added something about injuries which occur due to changes in sagittal plane motion such as knee extension,flexion.
  17. Could be a significant part of it.
  18. So if we take a patient with PTTD.

    We decided on a medial skive device to laterally deviate the STJ axis, by changing the GRF or ORF vector. By doing this we have reduced the work on the Posterior tibialis (PT)muscle. In my head I see a change in the foot kinematics, but is it more that the force provided by the device has moved the imaginary line of the STJ axis, which has changed the eccentric and concentric load on the PT muscle?

    What has also occured in that the load rates have changed, what also will occur is a change in Leg stiffness depending on the stiffness of the foot orthotic interface. This will also mean that the potential for a great treatment response from patient is if the device has the leg working in ZOOLS
  19. Sort of. What we have done with the orthosis is to increase the external supination moment acting about the STJ. This may or may not have a kinematic effect at the subtalar joint. Remember, no change in kinematics probably equals no change in axial position. The increase in external supination moment, may result in a decrease in internal supination moment from the posterior tibial muscle and tendon unit.

    The loading rates may well be changed with the orthoses.

    The vertical leg stiffness may change in response to the change in surface stiffness. I think the limb will always be trying to work within its zone of optimal leg stiffness (ZOOLS) which is why we see kinematic changes within the joints, step length etc. when muscles are fatigued in order to try to maintain optimal leg stiffness. Its the extra muscular effort and kinematic changes that are required to maintain ZOOLS that are key. If the extra muscular effort and / or change in kinematics place the target tissue and/ or other tissues outside of their zones of optimal stress (ZOOS), the orthotic intervention will be less successful. Hope that makes sense.
  20. Makes sense, I need to take a bit of time to digest it.

    I guess the next step is to know when you have the leg working in ZOOLS and it seems inshoe force measuments will be important.
  21. In the Davis paper they discuss High arch and Low arch feet and loading rates.

    Which we maybe able to generalise that High arch- lateral Stj axis Low arch - Medially deviated axis.

    Maybe this will help determine clinically re leg stiffness.
  22. Would be nice to see a bone pin study tracking STJ axis and modelling the relationship to leg stiffness.
  23. Griff

    Griff Moderator


    Sorry to take this all a few steps back but I've only just had time to read the first article you attached by Ferris, Louie & Farley (1998). I'm darn keen to get a better understanding of the concept of stiffness, but don't mind admitting I'm struggling to crowbar it into my head.

    If I've interpreted it correctly it concludes that humans adjust their leg stiffness (CNS control) during running to accomodate changes in surface stiffness thereby facilitating very little change in some aspects of their running mechanics (peak GRF, CoM movement and ground contact time). If I've read that right then it seems to be in opposition to the research which suggests lower peak pressures/GRF on more compliant surfaces such as grass when compared to stiffer surfaces such as asphalt?
  24. efuller

    efuller MVP

    Hi Ian,

    One of the methods of adjusting leg stiffness is to increase knee flexion. So, on harder surfaces you would tend to run with a more flexed knee. This may keep the external landing forces the same. However, assuming the ground reaction force vectors are close to the same, a more flexed knee would lead to greater internal forces. Think standing with a slight bend in your knee as opposed to standing with a 90 degree bend in your knee. Ground reaction force, depending on the angle of the vector, may have a much longer lever arm and create a greater knee flexion moment that would require a greater internal knee extension moment.

    It is interesting to think about why we choose to keep inpact forces about the same by changing leg stiffness. Some of the earlier work was done by Nigg and Bobbert out of Calgary.


  25. Ian ( I´m sure Simon will explain it better) but on the train to work this morning I was going over a few things in my head, how would I explain this to others.

    So came up with this. If you consider a car with that fancy suspension that adjusts between what surface your driving on. That represents your leg, the surface which comes in contact with the road in the plantar surface of your foot, the car computer the brain.

    I´m sure you seen the advert on tv. I know that a car system works differently depending on what surface it drives on, but the vision of the suspension changing in the ad seems to help.

    So as we are running on road the GRF on the plantar surface would be higher, the brain feels this and would decrease the stiffness of the leg thru the CNS system , ie the computer in the car has made the suspension softer. The studies of running on different surface show that this change happens directly. So this says to me that the GRF must remain the same on each step with the CNS making adjustments very quickly. The higher GRF the lower the leg stiffness

    When we now run over the softer surface the GRF is lower so the CNS adjusts the leg stiffness by increasing it.

    So the foot interface will be higher or lower depending on the surface, ie grass lower GRF than the road, but the stiffness will adjust depending on the stiffness. So thats why cushion shoes lead to a higher leg stiffness.

    So the CNS system works to keep the leg in the Zones of Optimal Stiffness , ZOOLS that Simon mentioned in the barefoot thread.

    The problems occur with the system adjustment with fatigue, reduced joint motion etc.

    Hope that helps Ian, as I said Simon probably can explain it better.

    Thats how I´m looking at it at the min.
  26. Eric is on the money. The body wants to maintain a smooth CoM pathway and minimise head and eye displacement. It seems to do this by modifying the stiffness of the legs to accommodate changes in surface stiffness. As Eric points out by modifying hip and knee and to a lesser extent ankle kinematics, the stiffness of the leg is altered and the relation of the net GRF vector to the joint axes and therefore the external moments acting about these joints is altered. Alteration in the external moment requires a variation in internal moment from the muscular apparatus.
  27. You do yourself down Mike. I don't think I can put it any better than you have. :good:
  28. Interestingly, I found a paper yesterday- I'll try and find it again today!!! doh! in which they modelled leg stiffness in walking as well as running- traditionally the leg has been modelled as a compound pendulum for walking and a spring for running. This got me thinking about sagittal plane facilitation theory as it provides the opportunity to re-evaluate sagittal plane blockades in terms of leg stiffness. Think about the classical compensation patterns for functional hallux limitus- what influence should this have on leg stiffness?

    Found it! http://www.asbweb.org/conferences/2005/pdf/0035.pdf
  29. There should be reduced leg stiffness due the change in the greater amount of knee flexion and hip Kinematics changes.

    Go to know I´m thinking correctly thanks Simon
  30. So an increase in dorsiflexion stiffness of the hallux results in a decrease in knee and hip stiffness? Can we extrapolate similar observation for ankle equinus?

    I don't know whether any of us are thinking correctly.... just thinking.
  31. SO... If varying the dorsiflexion stiffness of the hallux = modification in leg stiffness, we should be able to modify leg stiffness by manipulating the shape of the orthosis beneath the 1st ray segment- right?
  32. Griff

    Griff Moderator

    So increasing 1st metatarsophalangeal joint (MTPJ) compliance with orthoses could potentially increase knee and hip stiffness?
  33. This might be a stretch- but when I read whta Irene Davis was saying re loading rates against FF - RF striking and the relationship with leg stiffness.

    The ankle equinus should lead a decrease in leg stiffness as well.

    But now I´m having 2nd thoughts due to the external knee extension moment from increased GRF. But then maybe that the problems

    stage 1 - Ankle equinius - reduced leg stiffness increase knee flexion from muscle function to maintain ZOOLS

    Stage 2- Fatigue of knee flexion muscles- greater effect from FF GRF vector, now a greater effect of external knee extension moment.

    now this is were it all hits the fan, from reading there will be greater plantarflexion moment and the ankle.

    I´m not sure where this is from but in some of my reading on leg stiffness there was discussion on joint couplings.
  34. 1 of the ways I guess.....

    I think so, but now the big question it that what we need to happen ?

    This might be a curve ball but why did that study found that most 100m sprinters in some olympic final had a limited range of motion of the 1st MTPJ , do you think the the reduced leg stiffness will mean greater use of muscle to provide forward motion ?
  35. Because sprinting isn't walking? ~I think I read that speed doesn't seem to influence leg stiffness

    Ian, hypothetically! If the increase hallux stiffness = decreased leg stiffness, then decreased hallux stiffness (increased compliance) could well = increased leg stiffness.
  36. Anyone got any ideas on what I wrote above.

    We know GRF will have an effect on leg stiffness, how do we work out which side wins and what treatment plans we implement..

    If we take this example

    an ankle equnius. think 1 mechanically 2 CNS maintainance of leg stiffness

    1 mechancially. Increase GRF Vector under the FF ---- Increased external knee extension moment---- increased leg stiffness

    2 Increased GRF under FF --reduced leg stiffness thru increased flexion moment from CNS-muscle contraction

    I feel a headache coming on.:D
  37. Griff

    Griff Moderator

    So is this a fair summation of what we have discussed so far:

    • GRF magnitude does not tend to change irrespective of the surface stiffness (although the GRF vector/lever arm may change)
    • A stiffer surface = reduced leg stiffness
    • A more compliant surface = increased leg stiffness
    • This CNS controlled stiffness modification is primarily to keep a smooth CoM pathway and minimise head/eye displacement
    • Leg stiffness alterations are predominantly made by a change in hip and knee kinematics (and these external joint moment changes result in a change in internal moments from muscular apparatus)
  38. I think so.
  39. Griff

    Griff Moderator

    Simon - where does orthoses stiffness fit into all this?

    Intuitively it seems to me that a stiffer shell material would decrease leg stiffness and vice versa - a less stiff (or more compliant) orthoses shell would potentially increase leg stiffness?

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