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Thread: What is stress?

  1. #21
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    Quote Originally Posted by Oberon View Post
    I really should take an anatomy class.
    The whole tendon is loaded, and the loading is determined by the force of the load. Take the class.

  2. #22
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    Quote Originally Posted by Oberon View Post
    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.
    Why would the ropes in your mental experiment snap rather than stretch? It would be pretty strange for the weaker tendon fibers to just be hanging around, never having been stressed at all their entire lives until their big brothers snap.

    I am also an anatomy ignoramus, but I would assume it must be more akin to stretching a stack of resistance bands of different elasticity. The least elastic bands take up most of the load at the beginning of the range of motion. As they stretch and everything lengthens, the weaker but more elastic fibers take up more and more of the load.

  3. #23
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    Quote Originally Posted by anticausal View Post
    Why would the ropes in your mental experiment snap rather than stretch? It would be pretty strange for the weaker tendon fibers to just be hanging around, never having been stressed at all their entire lives until their big brothers snap.

    I am also an anatomy ignoramus, but I would assume it must be more akin to stretching a stack of resistance bands of different elasticity. The least elastic bands take up most of the load at the beginning of the range of motion. As they stretch and everything lengthens, the weaker but more elastic fibers take up more and more of the load.
    I dunno man, I'm basing my mental model on what the other guy said about low load conditions being taken up entirely by a small portion of the tendon. I obviously drew some incorrect conclusions. For now I'm binning it in the "things I don't understand" category.

  4. #24
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    Thanks RayK, that’s a very interesting topic.
    Reading the article you cited, my understanding is that stress-shielding is something that happens to an injured and scarred tendon. It means that the part of the tendon that was ruptured scarred in a longer configuration than the fibers around it. Therefore it does not get stressed until the other fibers are sufficiently under tension that they stretch a lot.
    That would also mean that mechanically, the fibers that were still intact are at a higher risk of rupturing later on, because they get a disproportionate amount of tensile stress.
    It goes right along with what Rip says: just don’t let the damn thing scar by resting it, it’s not going to heal in a mechanically efficient configuration!

  5. #25
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    Barr’s claims are a little weird, and are based on anatomy rather than mechanics. They are detailed in this presentation, particularly slides 12 and 20. He says that during isometric exercise, the tensile stress on the tendon reduces over time (“stress relaxation”). This supposedly exposes more of the tendon (particularly the “shielded” parts) to the stress.

    He suggests collagen/gelatin due to its glycine content, which is supposedly lacking in milk proteins. (But not meat).

  6. #26
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    Thanks Sam. I kinda wish I had not mentioned tendons at all since I do not now anything about tendons. I do think it is wise to use the accepted definition of stress and strain from engineering mechanics since the blue book is derived from the fundamentals of physics, knowledge of physiology, and thousands of hours of observation about what works in people.. Why invent new definitions? Stress is force per area so it has the same units as pressure. Stress can vary in a material under load and can be resolved into shear and normal stress. Strain is basically the differential deformation resulting from a stress. I think I understand stress shielding as follows: Suppose I have a copper wire and a steel wire, each of the same diameter hanging from the rafters of my barn. I hang a ten pound weight plate from each, and the average stress in each wire is the same, but the strain is greater in the copper. If the wires are the exact same length, and I attach both wires to a twenty pound plate and hang it, the stress is greater in the steel wire--it has stress shielded the copper wire. I have the impression that stress shielding is better understood in bone than in tendon.

  7. #27
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    Quote Originally Posted by Shiva Kaul View Post
    He suggests collagen/gelatin due to its glycine content, which is supposedly lacking in milk proteins. (But not meat).
    Those slides are really interesting, thanks for the link.

    Meat makes sense, but milk is surprising. Amino Acid Composition of Milk from Cow, Sheep and Goat Raised in Ailano and Valle Agricola, Two Localities of ‘Alto Casertano’ (Campania Region) - PMC backs him up. It's a single study, but the glycine content they found is so low (scroll down to table 1, under non-essential amino acids) that, even if we assume there's significant variation across populations, it still wouldn't get very high.

    I've always thought of whole milk as being a good source of pretty much every amino acid, but I must have just looked at the essentials and gone "yep looks good!"

  8. #28
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    Quote Originally Posted by Oberon View Post
    Those slides are really interesting, thanks for the link.

    Meat makes sense, but milk is surprising. Amino Acid Composition of Milk from Cow, Sheep and Goat Raised in Ailano and Valle Agricola, Two Localities of ‘Alto Casertano’ (Campania Region) - PMC backs him up. It's a single study, but the glycine content they found is so low (scroll down to table 1, under non-essential amino acids) that, even if we assume there's significant variation across populations, it still wouldn't get very high.

    I've always thought of whole milk as being a good source of pretty much every amino acid, but I must have just looked at the essentials and gone "yep looks good!"
    Given that what makes essential amino acids essential is that they are the ones that the body cannot synthesize on its own, and glycine is not one of those, how does it matter if you don't ingest it directly, so long as you get enough overall protein?

  9. #29
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    I'm pretty shaky on the details, but if I understand correctly there's a limit to how much glycine you can produce in a day, and that limit is or may be lower than the demand you're creating re: muscle development.

    Hopefully someone who's more knowledgeable will see this and correct me if I'm wrong.

  10. #30
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    Quote Originally Posted by Oberon View Post
    I'm pretty shaky on the details, but if I understand correctly there's a limit to how much glycine you can produce in a day, and that limit is or may be lower than the demand you're creating re: muscle development.

    Hopefully someone who's more knowledgeable will see this and correct me if I'm wrong.
    That would make sense. I'll admit that I'm currently ignorant on how far down the body breaks proteins during digestion...I'm guessing it's no further than the amino acid level, for the idea of essential vs. non-essential to make any sense.

    I therefore share your hope of a better qualified opinion.

    Looking briefly on my own, I found this: Multifarious Beneficial Effect of Nonessential Amino Acid, Glycine: A Review - PMC

    It doesn't really answer the question, but it is entertaining that this piece of "the literature" appears to have spell-checked/autocompleted a hilarious substitute for threonine in section 3.2:

    In infants, threonine is not converted to glycine. Soy-bean meal based and conventional corn diet is given to postweaning pigs to supply good amount of heroin, and in milk-fed piglets lysine is synthesized from the heroin. If heroin is not supplied in adequate levels then we cannot find significant source of lysine in the body.

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