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Thread: Scientific Bias

  1. #61
    Mike Duffey Guest
    Quote Originally Posted by Jeff Martin View Post
    The extension of the hips, the primary power component of the second fire, is driven by the contraction of the gluteus maximus. How is contraction of a muscle caused by "shear forces"? My understanding is that shear forces are created where the feet and ground interact, and would result from asymmetric extension of the legs and hips, through a force couple.

    Jeff
    Jeff,,
    This post is finally about the golf swing. I am willing, right now, to drop all of the pants-on-fire stuff and talk about the willing to talk about the golf swing. Are you willing to do this as well?

  2. #62
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    Sure.




    Jeff

  3. #63
    Mike Duffey Guest
    Quote Originally Posted by Jeff Martin View Post
    The extension of the hips, the primary power component of the second fire, is driven by the contraction of the gluteus maximus. How is contraction of a muscle caused by "shear forces"? My understanding is that shear forces are created where the feet and ground interact,
    A shear force is typically defined as a force that acts parallel to a surface or perpendicular or perpendicular to the extension of the substance (note that this is not "knee extension", this is more of a materials testing definition, so extension in this case would be would be like pulling both ends ends of he femur away from each other).

    So yes, in general, all forces that are horizontal and act parallel to the ground woul dbe considered shear forces. But shear forces can also occur at joints, and in fact the very often do. For example, the quadriceps can act to actually pull the tibia forward, not just rotating it about the knee joint center. This may actually be a possible mechanism for ACL tear (The hamstrings should contract somewhat to prevent the forward shearing of the tibia relative to the femur.

    So, muscles can and do create shear, but it is almost always very small (the quadriceps are one of the few that can really do this and that is because the patella gives them a mechanical advantage). But this kind of shear does not help joint motion. As mentioned above, the antagonist muscle (the 'opposite acting' muscle) will almost always contract to help prevent the shearing motion and allow for rotation only.

    Quote Originally Posted by Jeff Martin View Post
    My understanding is that shear forces are created where the feet and ground interact, and would result from asymmetric extension of the legs and hips, through a force couple.
    Jeff
    Shear forces are caused by many things, but not really by the 'asymmetric extension' you describe. Simply twisting your spiked golf shoe on the ground will create shear forces. Pushing sideways with your shoe will create shear forces.

    A force couple is actually a very, very specific combination of forces that result in perfect rotation without any horizontal translation. That wouldn't apply in this context.

  4. #64
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    Quote Originally Posted by Mike Duffey View Post
    A shear force is typically defined as a force that acts parallel to a surface or perpendicular or perpendicular to the extension of the substance (note that this is not "knee extension", this is more of a materials testing definition, so extension in this case would be would be like pulling both ends ends of he femur away from each other).

    So yes, in general, all forces taht are horizontal and act parallel to the ground woul dbe considered shear forces. But shear forces can also occur at joints, and in fact the very often do. For example, the quadriceps can act to actually pull the tibia forward, not just rotating it about the knee joint center. This may actually be a possible mechanism for ACL tear (The hamstrings should contract somewhat to prevent the forward shearing of the tibia relative to the femur.

    So, muscles can and do create shear, but it is almost always very small (the quadriceps are one of the few that can really do this and that is because the patella gives them a mechanical advantage). But this kind of shear does not help joint motion. As mentioned above, the antagonist muscle (the 'opposite acting' muscle) will almost always contract to help prevent the shearing motion and allow for rotation only.


    Shear forces are caused bu many things, but not really by the 'asymmetric extension' you describe. Simply twisting your spiked golf shoe on the ground will create shear forces. Pushing sideways with your shoe will create shear forces.

    A force couple is actually a very, very specific combination of forces that result in perfect rotation without any horizontal translation. That wouldn't apply in this context.
    Guess I misunderstood Dr. Kwon. Or you two don't agree.




    Jeff

  5. #65
    Mike Duffey Guest
    It would probably help if you could post (or at least paraphrase) what he said, in context.

  6. #66
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    Quote Originally Posted by Mike Duffey View Post
    It would probably help if you could post (or at least paraphrase) what he said, in context.


    That might be helpful for you, but boring for me. I have a better idea. Why don't you put a post together using Jamie Sadlowski as a model and identify the major forces involved in his second fire (just the lower body)? Be sure to tell us which are shear forces and which are some other kind. You can also comment on why his hip rotation during the second fire isn't a result of a force couple. You can use these:










    Thanks!






    Jeff

  7. #67
    Mike Duffey Guest
    Jeff,
    What's the point of saying we don't agree if you are not willing to follow up with what we disagree about?

    Clearly you thought the topic was interesting and important enough for you to ask so your saying now that it is boring doesn't make any sense.

    Please go ahead and share what we disagree about rather than divert the topic; we can talk about Jamie soon enough.

  8. #68
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    Professor-

    Time to carry your weight around here. Just do as I asked, and that will clear everything up. Quit stalling.



    Jeff

  9. #69
    Mike Duffey Guest
    Jeff,
    When you originally posed the question, I thought you were kidding. I see that you were not and I am really surprised.
    I have stated many times that I think it is a bad idea to try to evaluate 3D motions based on 2D images. But that is not as bad an idea as to try to guess forces based on video.

    That is entirely the reason to have force plates (or at least pressure plates) - to take as much of the guessing and likely error out of it. To guess about forces based on video like you have posted would be foolish.

    So thanks for pointing out why that kind of analysis should be done with equipment meant for measuring forces. I agree that is a good idea.

  10. #70
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    Quote Originally Posted by Mike Duffey View Post
    Jeff,
    When you originally posed the question, I thought you were kidding. I see that you were not and I am really surprised.
    I have stated many times that I think it is a bad idea to try to evaluate 3D motions based on 2D images. But that is not as bad an idea as to try to guess forces based on video.

    That is entirely the reason to have force plates (or at least pressure plates) - to take as much of the guessing and likely error out of it. To guess about forces based on video like you have posted would be foolish.

    So thanks for pointing out why that kind of analysis should be done with equipment meant for measuring forces. I agree that is a good idea.


    hmmmm... Dr. Kwon had no reservations talking generally about the forces at play in Jamie's downswing based on videos from several angles. Maybe you should go take a course from him.

    Anyhow, until you publish something about golf, it's adios!




    Jeff

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