starting strength gym
Page 1 of 3 123 LastLast
Results 1 to 10 of 24

Thread: Incorrect moment diagram in squat article?

  1. #1
    Join Date
    Mar 2019
    Posts
    9

    Default Incorrect moment diagram in squat article?

    • starting strength seminar april 2024
    • starting strength seminar jume 2024
    • starting strength seminar august 2024
    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.

  2. #2
    Join Date
    Jul 2007
    Location
    North Texas
    Posts
    53,562

    Default

    It's hard to understand, I know.

  3. #3
    Join Date
    Mar 2019
    Posts
    9

    Default

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

  4. #4
    Join Date
    Jan 2014
    Location
    RS WY
    Posts
    980

    Default

    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?

  5. #5
    Join Date
    Jul 2014
    Posts
    139

    Default

    Start here:Engineering Statics — Open Learning Initiative

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

  6. #6
    Join Date
    Jun 2015
    Location
    Garage of GainzZz
    Posts
    3,297

    Default

    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
    Join Date
    Mar 2015
    Location
    Indianapolis, IN
    Posts
    2,266

    Default

    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.
    Starting Strength Indianapolis is up and running. Sign up for a free 30-minute coaching session.
    I answer all my emails: ALewis@StartingStrengthGyms.com

  8. #8
    Join Date
    Jan 2017
    Location
    Germany
    Posts
    275

    Default

    He 80 IQ.

  9. #9
    Join Date
    Nov 2018
    Posts
    5

    Default

    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
    Join Date
    Jan 2018
    Location
    Houston, TX
    Posts
    9

    Default

    starting strength coach development program
    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.

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •