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Thread: What is stress?

  1. #11
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    Quote Originally Posted by RayK View Post
    To an engineer, stress is force per cross sectional area. A 1/4 inch cable suspending a 200 lb load has twice the stress compared to a half inch cable approximately. (This is oversimplified since stress can vary a bit over tbe cross section) The same thing is true of human muscle, tendon, or bone. Do a google search of stress shielding in human tendons and you will see why Mark Rippetoe should be awarded a Nobel prize for his contributions to understanding human health. When a tendon is lightly loaded for a very short duration of time, such as by bouncing about with light weights, only the strongest and stiffest part of the tendon carries the load. During the last rep of a set of five squats the tendon is loaded for enough time, and with enough stress over the whole cross section, that even the weaker part of the tendon is loaded. Squats produce adaptation and rehabilitation. Much of what passes for exercise allows tendons to atrophy and raises the risk of injury.
    Every sentence is spot on here.

  2. #12
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    Actually I was wrong. Cross sectional area is proportional to the square of radius, so the 1/4 inch cable has 4 times the average stress. Embarrassing... Here is one of many links regarding stress shielding:
    ScienceDirect

  3. #13
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    Quote Originally Posted by RayK View Post
    Actually I was wrong. Cross sectional area is proportional to the square of radius, so the 1/4 inch cable has 4 times the average stress. Embarrassing... Here is one of many links regarding stress shielding:
    ScienceDirect
    I'm glad you caught that.

  4. #14
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    Quote Originally Posted by RayK View Post
    [..]During the last rep of a set of five squats the tendon is loaded for enough time, and with enough stress over the whole cross section, that even the weaker part of the tendon is loaded. [..]
    Would you mind elaborating on this?
    The articles you posted claim that 30 second long isometrics were effective in overriding the stress shielding response.
    Do you mean that a 5th heavy rep is slow/long enough to produce a similar effect, or did I miss something in the article?

  5. #15
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    Quote Originally Posted by sascha_herfort View Post
    Would you mind elaborating on this?
    The articles you posted claim that 30 second long isometrics were effective in overriding the stress shielding response.
    Do you mean that a 5th heavy rep is slow/long enough to produce a similar effect, or did I miss something in the article?
    The guy to ask about this is Dr. Keith Baar. I am a nobody who took one mechanics of materials class 40 years ago and forgot how to calculate the area of a circle. I would love to hear Dr. Baar as a Starting Strength podcast guest. In general it seems that the physical therapy people are slowly catching up to what Mark has been saying for years. The old advice to put an injured ankle in a boot and totally immobilize it has turned out to be wrong. An injury is healed by a load stimulus. The type of load matters. There is a reason that we don't jerk the bar from the floor during a deadlift. Dr. Baar's recommendation of sustained isometrics is based upon his laboratory studies with human tendon samples. It seems to me that the last rep of a set of five should also be a sustained load which produces a similar result. For all I know the last rep of a set of five may produce a better result since the isometric method relies on the patient to contact forcefully for at least 30 seconds and not all patients will be able or willing to do this effectively. All I know is that squats have made my knee better, deadlifts fixed my back, and presses helped my shoulder.

    Here is another link:

    Stress Relaxation and Targeted Nutrition to Treat Patellar Tendinopathy in: International Journal of Sport Nutrition and Exercise Metabolism Volume 29 Issue 4 (2019)

  6. #16
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    Quote Originally Posted by RayK View Post
    During the last rep of a set of five squats the tendon is loaded for enough time, and with enough stress over the whole cross section, that even the weaker part of the tendon is loaded.
    I believe you, but how do you know this? I mean like, from a mechanical standpoint, what's happening inside the tendon that allows the weaker part of the tendon to pick up some of the load, and how do we/you know that that's what happens?

  7. #17
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    Quote Originally Posted by RayK View Post
    The guy to ask about this is Dr. Keith Baar. I am a nobody who took one mechanics of materials class 40 years ago and forgot how to calculate the area of a circle. I would love to hear Dr. Baar as a Starting Strength podcast guest. In general it seems that the physical therapy people are slowly catching up to what Mark has been saying for years. The old advice to put an injured ankle in a boot and totally immobilize it has turned out to be wrong. An injury is healed by a load stimulus. The type of load matters. There is a reason that we don't jerk the bar from the floor during a deadlift. Dr. Baar's recommendation of sustained isometrics is based upon his laboratory studies with human tendon samples. It seems to me that the last rep of a set of five should also be a sustained load which produces a similar result. For all I know the last rep of a set of five may produce a better result since the isometric method relies on the patient to contact forcefully for at least 30 seconds and not all patients will be able or willing to do this effectively. All I know is that squats have made my knee better, deadlifts fixed my back, and presses helped my shoulder.

    Here is another link:

    Stress Relaxation and Targeted Nutrition to Treat Patellar Tendinopathy in: International Journal of Sport Nutrition and Exercise Metabolism Volume 29 Issue 4 (2019)
    The linked article mentions that supplementing with Gelatin improves whole-body collagen synthesis and thus benefits tendon quality. I'm too lazy to search for myself so, is anyone aware of other literature that supports this benefit of supplementing with collagen or gelatin? I've always been of the understanding (likely erroneously) that ingested collagen is simply broken down into its constituent amino acids and added to the amino acid pool in the body -- not that it possesses a mechanism for direct action in the body? What's the truth?

  8. #18
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    Quote Originally Posted by Oberon View Post
    I believe you, but how do you know this? I mean like, from a mechanical standpoint, what's happening inside the tendon that allows the weaker part of the tendon to pick up some of the load, and how do we/you know that that's what happens?
    Here's a better question: if all parts of the tendon are intact between the bones, how would all parts of the tendon NOT be loaded?

  9. #19
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    Quote Originally Posted by newguyray View Post
    I've always been of the understanding (likely erroneously) that ingested collagen is simply broken down into its constituent amino acids and added to the amino acid pool in the body -- not that it possesses a mechanism for direct action in the body? What's the truth?
    This is my assumption as well. Collagen is synthesized at the site of deposition, like all other proteins, and collagen is not the only protein in a tendon or ligament.

  10. #20
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    Quote Originally Posted by Mark Rippetoe View Post
    Here's a better question: if all parts of the tendon are intact between the bones, how would all parts of the tendon NOT be loaded?
    I have no idea. My understanding of the mechanics of a tendon is basically zero. I'm thinking of it like a bundle of ropes, where some of the ropes are shorter than others. Pulling on the bundle adds strain to the shorter ropes, but not the longer ones, at least until the shorter ones snap. If a tendon's got a stronger/stiffer part and a weaker/more malleable part, wouldn't increasing load just add stress to the stiffer part until it snaps? But it sounds like you're saying that, as the load increases, the softer/weaker parts begin to take up part of the load. I don't understand how that would happen.

    I really should take an anatomy class.

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