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Modelling Muscles?

  1. Aug 14, 2008 #1
    I don't understand how muscles create torque. More specifically, I am trying to model muscles inside a computer program. I have two line segments (bones) from A->B and B->C, so B is obviously the joint. I was thinking of modeling a muscle as a spring going from A to somewhere on the line BC (but very close to B). Is this what happens biologically? When I flex my biceps, does the muscle tissue "PULL" on my bone after the joint?
    Can someone please help me understand how this torque is generated?
    thank you
  2. jcsd
  3. Aug 14, 2008 #2


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    Yes. You have it right.

    Note how the muscle is joined just beyond the joint.


    P.S. If you are modeling anatomy, you might really consider a course or at least a book on the subject.
  4. Aug 15, 2008 #3
    ok. my biggest problem with that is that if ABC are on the same line, then I cannot ever possibly create this kind of torque! Note that in this case there simply is no vertical component of force applied (if you imagine them on a horizontal line). T=r x F and clearly r and F are now on the same axis! But on the other hand I can hold out my arm in front of me and just flex my biceps and I can still apply a lot of torque!

    But I think I know what you are going to say :s the source of force is not center of my bone, but my muscle, which is slightly above the bone. So for lines ABC, I should have one spring attached Slightly in perpendicular direction of AB away from A, and the other sprint attached slightly away from B on segment BC? Am I right again?
    Thank you for reply!
  5. Aug 15, 2008 #4


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    Yes. These factors will help too:

    - a limb is comprised of 3 dimensional components - i.e. the arm has a thickness, so the muscle is not attached to a "line" but a cylinder i.e. a short distance away from the longitudinal axis
    - the joint itself is a 3 dimensional component, so that even at full extension (180 degrees), the muscle has to bend around the joint.

    That's how nature handles it. when we model joints using stronger materials, we have a lot more options. Look at a piece of construction machinery and you'll see your idea for having short perpendicular components.


    (Actually, note also that the backhoe has only one "muscle" per joint. It can both pull AND push and so acts as both flexor AND extensor. In the body, muscles can only pull, they cannot push, so we need separate muscles to extend and flex.)
  6. Aug 15, 2008 #5
    ok I finally arrived at counterexample to the way you tell me nature handles things. At least I think. If you bend your hand to make right angle at elbow, and imagine you put it vertically against a wall, you can clearly push against that wall by applying torque around the elbow, trying to straighten your hand. I think the muscle in this case that is involved is the triceps. So scenario:

    *****C ---Force-->

    Where I try to show that you are applying force on C, pushing it to the right. (ignore the stars, had to put them in for indentation) How do you explain that? Muscles can only pull, so what is the triceps (located below line segment A,B) pulling on in this case? This doesn't make any sense to me :( And thanks for the suggestion on hydraulics, but i need to model a biological system...
  7. Aug 16, 2008 #6


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    See attached.

    Note how much shorter the distance between attachment points is in the flexed position.

    Attached Files:

  8. Aug 16, 2008 #7
    ooh i see. Thanks Dave. I find it so hard to believe that we can develop such a great force through our muscles, and that this is the mechanism behind it? Torque = rxF, and there is almost NO r at all. So strange.
    Thanks for the help!
  9. Aug 16, 2008 #8
    I think that would primarily because in order for the springlike force to take hold, you require Ca++ to be present in the cytoplasm of the sarcomeres via stimulation by Acetlycholine from a neuron in a myoneural junction. That's from what I recall, then again, i haven't looked at the material since about March.
  10. Aug 17, 2008 #9


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    Well, examine the real thing before making measurement conclusions.
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