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Lever arms???

Discussion in 'Biomechanics, Sports and Foot orthoses' started by bartypb, Sep 4, 2013.

  1. bartypb

    bartypb Active Member


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    I am tying myself up in knots trying to work out whether a longer lever arm ( ie route of any tendon) is weaker than a short lever arm, and if so does this predispose a structure to injury? This is a very broad question and I thought that I understood it but seem to be having a block??? Looked on PA for threads but can't find any!

    Any help, explanation would be great

    Thanks

    Marc
     
  2. Marc:

    A longer lever arm (i.e. moment arm) will produce a larger moment for a given applied force and a shorter moment arm will produce a smaller moment. However, for muscles, if the muscle moment arm is a shorter distance to the joint axis, the rotational velocity of the limb will be greater than if the muscle moment arm is longer.

    In the animal kingdom, muscle moment arms are relatively greater for animals that are diggers (e.g. moles) that need more force in their digging limbs whereas in running animals (e.g. cheetahs) the muscle moment arms are relatively smaller to optimize the velocity of the distal limb for faster running speeds.

    Hope this helps.
     
  3. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    eg shorter calcaneus --> achilles has short lever arm to ankle joint axis --> calf muscles have to work harder to provide plantarflexion moment --> hypothetically that makes running on forefoot less efficient and expose achilles to greater loads and injury risk

    eg medial STJ axis --> post tib tendon has shorter lever arm to supinate/invert rearfoot --> has to work harder --> inefficient and exposes tendon to increased injury risk
     
  4. Rob Kidd

    Rob Kidd Well-Known Member

    I think I read you right that you are asking the question not really about mechanical advantage etc, but about predisposition to injury.

    If that is the case, then are very many other factors that need to be considered. Examples are distribution of Sharpies fibres, the ration of this lever arm to its antagonists lever arm etc.

    With relation to foot function, you also need to look at the health the essential foot mechanics - pronated feet, by way of example have obliquely aligned insertions to their Sharpies fibres; this predisposes to premature failure.

    The place I would start is with a serious read of cancellous bone morphology. I recommend "Ghostly Muscles, Wrinkled Brains, Heresies and Hobbits" It is not all about cancellous bone, but is largely about biomechanics, in the broader sense of the word. Rob
     
  5. efuller

    efuller MVP

    Moment is equal to force times distance. So, if a tendon produces a certain amount of force it will create a greater moment with a larger lever arm. Now, when you are looking at potential injury, you also need to think about the resistance to motion created by a load. Ground reaction force is one resistance to a load. If you have a medially deviated STJ axis there will be a larger pronation moment from the ground that your muscles and/or other structures have to deal with when compared to a foot with an average STJ axis. A rocker bottom shoe (rocker point just proximal to met heads) will decrease the lever arm of ground reaction force that creates a dorsiflexion moment at the ankle. So, with a rocker bottom shoe, you will lessen the load the Achilles has to work against. So there is the force in the tendon and the resistance force. Keeping those separate may help you understand the levers better.

    Eric
     
  6. rdp1210

    rdp1210 Active Member

     
  7. I thought that is what I said, Daryl.

    Put another way, if the relative length of the muscle insertion to the joint axis (i.e. muscle moment arm) is small relative to the distance from the distal end of the extremity to the joint axis, then the joint moment will be minimized and the angular velocity at the joint will be maximized. This is how the limbs of the cheetah are arranged to maximize it's running velocity.
     
  8. rdp1210

    rdp1210 Active Member

    Thanks for clearing that up.

    If you don't have it, I'll have to share the chart that Dave Skliar made up many years ago, showing how each of the hip joint muscles change their directional moment on the hip joint based on the hip joint position. He did an excellent job, and it should be part of every biomechanists notebook.

    Best wishes,
    Daryl
     
  9. bartypb

    bartypb Active Member

    Thanks everyone a little clearer now still a bit hazy in parts but I'm trying to work my way through it- there may be more questions on this when I pull my brain into gear. Could I get a copy of the hip diagram thing you were talking about please Daryl?


    Regards


    Marc
     
  10. Marc:

    You may want to read this paper I wrote about 25 years ago on how positional deviations in the subtalar joint axis can change the moment arms for the posterior tibial and peroneus brevis muscles (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989). Here also is another paper I wrote from 13 years ago that may help better explain how shortening of a tendon moment arm, such as that seen in posterior tibial dysfunction due to medial deviation of the subtalar joint axis, may predispose this tendon to injury (Kirby KA: Conservative treatment of posterior tibial dysfunction. Podiatry Management, 19:73-82, 2000).

    Hope this helps.:drinks
     
  11. rdp1210

    rdp1210 Active Member


    I hope this uploaded OK. I have to give the full credit to Dave Skliar, who was a very close friend and taught with Dick Shuster at NYCPM and later taught at Barry for many years.

    The way to read the chart is this: look at the muscle, e.g. adductor longus. You'll notice that it sits at about 70 deg of flexion (on the vertical axis). This means that if the hip is less than 70 deg, the adductor longus is a flexor, however if the hip is more than 70 deg flexed, then it is an extender of the hip. Likewise all the other hip muscles can be either flexors or extenders of the hip, depending on the position that the hip is in. The abduction-adduction is on the horizontal axis, though the degrees are not marked, so it is only qualitative.

    Best wishes,
    Daryl
     

    Attached Files:

  12. rdp1210

    rdp1210 Active Member

    Thanks for the articles Kevin. You saved me a couple of bucks because right now I'm paying someone to scan all my hard copy articles, so that's a couple I don't have to have scanned.

    Some things that I've been thinking about recently:
    1) Several years ago you talked about a study that you had done on the distance from the supporting surface to the plantar surface of the medial and lateral plantar calcaneal tubercles, noting that the medial tubercle was closer to the weightbearing surface. I can't remember, though, did you deduce that the medial calcaneal tubercle bore more weight or had greater pressure?

    2) Have you repeated the study with patients standing on Kirby skive orthotics?

    Thanks,
    Daryl
     
  13. Daryl, I'm sure Kevin will answer for himself, but the anterior axial projection paper is probably the study you are thinking of and this more recent paper examined the influence of medial heel skives on plantar pressure beneath the heel. http://www.jfootankleres.com/content/5/1/20
     
  14. Daryl:

    My anterior axial study (Kirby KA, Loendorf AJ, Gregorio R: Anterior axial projection of the foot. JAPMA, 78: 159-170, 1988) is on its way to your mailbox. Our research did show that in all subjects, the medial calcaneal tubercle was more plantarly positioned than the lateral calcaneal tubercle, by about 21 degrees to the transverse plane, while the subjects were in relaxed bipedal stance position:drinks
     
  15. I'm interested in this too. If the fat pad beneath the heel is less thick under the medial tubercle of the calcaneous when compared to lateral, should we assume either of Daryl's scenarios as suggested above?

    In relaxed standing is pressure generally distributed relatively evenly across the medial and lateral aspects of the heel?
     
  16. To my knowledge, a research study on the correlation of plantar calcaneal morphology to plantar calcaneal pressure patterns has not yet been done. One must remember, however, that the heel fat pad is compartmentalized and can transmit pressures due to the pressure build-up within the plantar heel fat compartments. In addition, not only is the medial calcaneal tubercle more plantar than the lateral calcaneal tubercle, it is also 2-3 times the size of the lateral calcaneal tubercle.

    Therefore, it seems likely that the vast majority of the ground reaction force acting on the plantar calcaneus is going through the medial calcaneal tubercle versus the lateral calcaneal tubercle. My guess is that for most plantar calcaneus shapes, in relaxed bipedal stance, the medial calcaneal tubercle bears at least 75% of the weightbearing force acting on the plantar calcaneus.
     
  17. rdp1210

    rdp1210 Active Member

    We really need to know the distance of the tubercles from the skin surface in an unweighted condition, and then compare it with the weighted condition before we can answer the questions.

    Take care,
    Daryl
     
  18. Yep, seems like these guys have done some of this work, just a shame it is only an abstract for an oral presentation: http://www.biomedcentral.com/content/pdf/1757-1146-4-S1-I14.pdf

    Found this powerpoint which might contain some useful references: http://www.professionalevents.co.uk/_images/_products2downloads/77_143.pdf
     
  19. efuller

    efuller MVP

    Back when I had the EMED at CCPM I routinely noticed that the pressure distribution showed higher pressures under the medial side of the heel. I also started a study that was looking at pressure under the heel with a varus wedge. I didn't ever finish it, but it did look like it was going to show a medial shift in the center of pressure under the heel with the varus wedge.

    Eric
     
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