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Thread: Flaw in the starting strength model of moment arms (aka EPIC PHYSICS POST)

  1. #11
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    cool, I'm still trying to wrap my head around some of the wrinkles. I've made some edits. In retrospect, the angular momentum stuff is not relevant to the main argument I'm making. Consider the first post a cool physics tutorial on conservation of angular momentum, and how it can illustrate that joint rotations can be powered by some counterintuitive sources (e.g. lateral head of triceps can abduct the shoulder). The second post has nothing to do with angular momentum, but deals with the moment arm issue, so you can skip the first post if you like (and by "you", I mean anyone).

  2. #12
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    Hi,
    I appreciate, nay, *applaud* the effort by spacediver here, but just one bit I'd like to point out:

    There are limits to how much useful (i.e. relevant to the technique of the lift) information you can glean from a single moment 2D model. What is worse, even a full 3D-model built from these ingredients would be inaccurate due to the mechanics of the shoulder not really conforming to these models in terms of force production. I agree fully that it is somewhat important, highly relevant and most of all very interesting to analyze the mechanics of every model in detail. The detailed analysis, however, is on only useful in one of two ways: Firstly, insofar as the model accurately describes the actual situation, and, in particular, if two actual situations are compared, the model accurately describes the relevant parts that make them different; in such a case the model and analysis are useful because they tell you something that can improve technique. Secondly, the detailed analysis of a model, when it does *not* describe an actual situation, may be useful if it reveals that we should not use the model in the said situation.

    To achieve one of the two, we must be able to compare the results of the analysis to something that actually happens. Mind you, I am not taking the stand here that one should refrain from making analysis such as spacediver is doing, quite the opposite. But I am pointing out that perhaps such enthusiasm would serve a more useful purpose if directed towards finding criteria for validating the findings, or, alternatively, enrichment of the model to account for the 3D-nature of the press.

    I sadly do not have time to analyze this model in detail myself. But from what I can say, it is impressive.

    Don't give up and never stop.
    -H.

  3. #13
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    Seeing as how I also went to Stanford, but mostly on the dummy admission tract, I'd like to go through this and offer comments/criticisms.

    I agree with Tiedemies regarding the comment about 2D versus 3D analysis, but I think that's smallish potatoes.

    What I will say is that in just a skimming of this material I see no free body diagrams and no summation of force/moment equations. If you're going to try and prove that the application of a force at varying locations requires no appreciable change in the applied force in a mechanical system, then you might want to actually calculate the forces and see if they balance at various points. That beings said, it's probably in here, but I just don't see it.

    Also, angular momentum is irrelevant in this case for two reasons; one, these are effectively statics problems because of low velocities and two, you're looking at linear translation of the mass, i.e., the weight moved by the system.

    Again, I'm probably wrong.

  4. #14
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    I'm not sure if this has been mentioned before in this or the old thread, but if you are talking about a barbell movement (= not a dumbell movement), you would have what my mechanical engineer friend calls a "symmetry constraint", which could be modelled as the load moving in a vertical "rail" with all the reaction forces this implies.

    This also means infinite solution using only two muscles. Either muscle could take all or none of the load. ...According to the model that is.

    (I'm an E.E. but had a little mechanics in university, my understanding of the subject is OK but my terminology might not be correct)

  5. #15
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    Quote Originally Posted by Satch12879 View Post
    Also, angular momentum is irrelevant in this case for two reasons; one, these are effectively statics problems because of low velocities and two, you're looking at linear translation of the mass, i.e., the weight moved by the system.

    Again, I'm probably wrong.
    I think you are probably right, we will know for sure when someone take the time to measure/calculate the angular acceleration and moment of intertia of their arms !

  6. #16
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    spacediver, good stuff! I have minor quibbles here and there (you still aren't drawing proper free-body diagrams which leads to JC Superstar's comments and Satch's complaint, I'm not entirely sure of your undetermined claim, and a few other minor things), but in my opinion your main points still stand! That's why I said I don't think the (SS) statics analysis (torque about knee and hip in the squat, for example) is thorough enough. Also, now you get to think about the "uses the most muscle mass" statement. Hahaha. Aw hell, I really don't want to get dragged back into all of this. I have very strong opinions about the lack of scientific rigor coupled with the arrogance of many of the claims made in these forums. Some of the things are very difficult to simply call falsehoods.

    I'm swamped this week, but for whatever it's worth maybe I'll take a stab at a dynamics model this weekend, and work out what approximations are necessary to show when a (proper) statics treatment is good enough. It's something I've been meaning and wanting to do. Maybe I'll find time for it. Maybe somebody else will beat me to it!

  7. #17
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    Quote Originally Posted by Savs View Post
    spacediver, good stuff!
    Flipping heck, is this the Second Coming?

    :-)

    IPB

  8. #18
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    Thanks for the very interesting post. Similar to John, Tom, etc. I think I will have to come back and re-read everything a few times in order to fully grasp the information, but I really appreciate the effort your guys put into trying to explain the mechanics behind the lifts as I find it to be very educational. Similarly, while I understand why some of you guys are frustrated with some of the responses that you get from members on these forums, I don't think we should lose sight of the fact that SS is a book aimed at helping beginners gain a basic understanding of a handful of barbell lifts, and while I agree that there is room for improvement in the model (like there is with all theoretical models), I still feel that the book does a more than adequate job of presenting a sound rationale for the performance of the lifts as they are described.

  9. #19
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    I think we need to be more clear on what the objective is here.

    Is it lifting as much weight as possible with the largest ROM?

    If yes, what would limit the amount of weight lifted?

    Is it exceeding the maximal force capability of a any given muscle? No, not necessarily so, because in principle we don't have to use every muscle, as an example the 1-2 fiber in the model can be left out.

    There exist an optimal solution of course. But we would need to specify the force capabilities of the muscles relative to each other to find it. This involves some guesswork, but so does everything else in this model.

  10. #20
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    Quote Originally Posted by JC View Post
    There is something about your initial descriptions that bother me, and i think it affects your analysis, but i'm not sure if i'm "getting" it so i'll try and refute your explanation and see if it sticks i guess!

    In your first part, you describe the system as under no gravity and as internal-only forces. This is fine, and describes the movement in free floating space (for simplicity i imagine).

    You then go on to say that because of this, there can be no torque at A because G is also free floating and operating as an internal force, which is still fine, you've added a torso to your space arm model.

    Where i think my problem lies is here; there is an additional component missing underneath G, the bench itself. And without the bench, the torso rotates about point A (free floaing in space and all), which is why it has no external forces or torques acting there. But by adding a bench underneath G, it can no longer rotate at A and this creates the external force required for the "floating space arm" to push up at what becomes point C.

    Also, it means that in your initial model, either you need to add in the bench and it's external force at A, or else in diagram "time 2" you need to alter G to show it sloping down from left to right as it is operating under internal forces only with no torque at A.

    The diagram was referring to the press, not bench press. In the bench press, I think the situation is more complex, because the location of A isn't as fixed, and the bench isn't directly touching the shoulders.

    In my model, G certainly exerts an external force at A. I never meant to imply that the only forces operating on the system were internal. In fact, in a zero G environment, if G didn't exist, and the limbs were massless, then the barbell (point C) would never be able to move at all (another way of thinking about this is that if you are floating in space, and you are massless but your hand is attached to a barbell, then nothing you do with your body can cause the barbell to move at all).

    However, the argument that I was making is that the force of G upon the system cannot produce a torque on the system (and by system, I mean everything from the shoulders up to barbell) around A. One of the things I learned is that when specifying torques, it's useful to specify the object and the point of rotation, and I *think* this might help answer your question. For example, suppose an external force was applied upwards at the blue G support, but off centre, say a metre to the left. This would cause a torque on G around A, but it would not cause a torque on the limbs+barbell around A.

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