Incorrect moment diagram in squat article? Incorrect moment diagram in squat article?

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Thread: Incorrect moment diagram in squat article?

  1. #1
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    Default Incorrect moment diagram in squat article?

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    Hi Mark.

    In your Squat Mechanics: A Clarification article you include the following diagram: squat_diagram.jpg

    A dotted line indicating a moment arm a perpendicular distance from the barbell to the knee joint has been drawn. I think that is incorrect.

    The knee joint does not 'see' the barbell. The knee simply joins the femur and shank, and since there are no external loads on either of these segments, the knee experiences no external moments.

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    It's hard to understand, I know.

  3. #3
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    Quote Originally Posted by Mark Rippetoe View Post
    It's hard to understand, I know.
    Sure -- maybe you should fix the diagram.

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    since there are no external loads on either of these segments
    How do you figure? Are you suggesting the forces across the patella are independent of the load on the barbell? The quads exert the same force getting off the privy as in a #405 lbbs?

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    Start here:Engineering Statics — Open Learning Initiative

    You don't need an account to review the material.

  6. #6
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    If the lifter was sitting and leaned over with a barbell on his back, yes; he is standing, however.

    Newton always governs; the moments sum to zero at this snapshot in time.

    The hip, knee, and ankle joints are not hinges in this case. Because the lifter is trying to move the barbell in space in a controlled manner, there is moment across each joint down to ground.

    Remove the back segment and load the femur directly, which is what the drawing above does. The load from the barbell is transmitted down the back, through the hip, and into the femur, producing the moment at the knee as denoted.

    Do the free body and moment diagrams and you’ll see how it all works out.

  7. #7
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    Quote Originally Posted by me80iq View Post
    Hi Mark.

    In your Squat Mechanics: A Clarification article you include the following diagram: squat_diagram.jpg

    A dotted line indicating a moment arm a perpendicular distance from the barbell to the knee joint has been drawn. I think that is incorrect.

    The knee joint does not 'see' the barbell. The knee simply joins the femur and shank, and since there are no external loads on either of these segments, the knee experiences no external moments.
    If you draw out the individual free body diagrams, decomposing it as you suggest, you'll find the drawing is accurate.

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    He 80 IQ.

  9. #9
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    In the knee system, you can imagine that the entirety of everything after the knee is condensed into a single point with the mass of everything above the knee. The knee doesn't "see" the barbell, per se, but it "sees" that on the upper end, there is a weight (encompassing the weight of the barbell and everything above the knee) with a center of mass above the middle of the foot. It doesn't really matter what weight is on the bar versus on the body, a moment arm is about the total mass and the distance between the joint and the center of mass of thing being moved (on the plane normal to the opposing force) -- it just so happens that the barbell is responsible for most of the mass (and because your body as a whole will already be balanced over midfoot even without the barbell), so as a simplification diagrams just point out the barbell. Because the knee is working to straighten the leg, it is moving that center of mass (barbell plus everything above the knee) vertically while reducing the distance between the joint and the center of mass (again, on the plane perpendicular to the force), ie reducing the moment arm, which is why the squat gets easier as you get closer to the top.

    Now that you hopefully believe there's a moment arm between knee and the barbell, the point is that it's more efficient to have a larger moment arm between the hips and barbell than between the knees and barbell because A) larger moment arm means less efficient transfer of energy from the joint to the barbell, B) the hips are capable of generating more force than the knees, so they should be the ones carrying the greater weight if the goal is to move as much weight as possible (which it is, if you're doing SS). Compare to the front squat, with a more vertical back angle, causing a larger moment arm between knees and barbell, and smaller between hips and barbell. Notice that much less weight can be lifted in this way.

  10. #10
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    The diagram is a simplified view of a complex system with many competing forces. In short, the knee is affected directly by the load on the end of the femur at the hip joint and the reaction force of the ground acting on the shank at the ankle joint. The net effect is the moment shown in the diagram. If the knee were directly below the center of mass, it would experience no net moment but would be under two opposing moments that cancel out.

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