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Thread: Starting Strength 3rd Ed: The Best Middle School Science Textbook Ever (No, Really)

  1. #21
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    Quote Originally Posted by Mark Rippetoe View Post
    Dustin is pretty smart too. Geek wars in progress. Enjoy!
    I have been on the internet since before the September That Never Ended, and there was never a time that technical discussions on the internet did not involve male dominance displays. It seems to be some kind of law of nature. Is it territorial dominance for purely virtual territory? Courtship fitness display gone awry? There is probably literature on this subject.

    Dustin

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    Quote Originally Posted by Mulgere Hircum View Post
    Is it territorial dominance for purely virtual territory? Courtship fitness display gone awry?
    Try asking yourself for answers since you were the one that started it. Search your feelings.

    The good thing about the Internet is that everything on it stays there forever. The bad thing about the Internet is that everything on it stays there forever.

  3. #23
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    Quote Originally Posted by Mulgere Hircum View Post
    I don't remember thinking of that approach last time I mulled this over, it's probably one of the best ways to treat it theoretically. That's a really good extra credit problem for Junior year mechanics, and should be solvable for anyone at that level. Unless I am missing something due to a few decades of rust on the relevant tools....
    I don't know what your talking about with boundary conditions. You don't solve these problems using f=ma. You work backwards in a sense. You start with constraint functions and differentiate them if you want to find out the forces required to satisfy the constraints.

    Looking at how a point on the body goes from p to p' is solving the IK pose problem. It tells you what you joint angles must be to be in a specific pose. This in general is non unique.

    If your net torque is not zero at the end of the lift you are falling over, end of story.

    No, your net torque does not need to be zero throughout the lift. You can fall forward at the beginning of the lift as long a you fall backwards at the end of the lift and vise versa. I'm sure there is some critical angle though past which you couldn't recover and would end up falling over completely.


    The path that minimizes work is the one that lifts the weight straight up.

    The path that maximizes weight lifted MIGHT be the same path but it depends.


    Imagine a rod with a point fixed at the origin so you can rotate around the origin. Its starts at reference angle of 0 deg. Lets say you can apply 1 unit of torque CCW and 2 units of torque CW. Now lets say you want to rotate the rod to the 90 deg position and to rotate the rod takes X units of torque.

    If X <= 1 then going CCW minimizes work done. But if X > 1 then you can't supply enough torque to go CCW. Going CW maximizes X. Cool, they're not the same.

    I think keeping the bar over your mid foot is a great HEURISTIC and you should probably do it when you squat.

    But you would have to know about the muscles in addition to the joints to really analyze it and that makes the problem super hard.

  4. #24
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    Sometime last night I decided there is a much simpler argument that I think supports my position directly from coaching observation. Since it is not a direct argument from principles it needs critique from more experienced coaches (hi, Rip) than I to evaluate its strength.

    Consider a novice lifter. They will not stumble onto the correct technique on their own, and in particular will have bar paths that don't stay over the mid-foot and will also do strange things with the degree of freedom that measures back angle/knee position/hip position, the one I said can be freely optimized. But I think the mechanical significance of those two errors is quite different in a way that illustrates my contention. The bar path deviating significantly from mid-foot is clearly an immediate mechanical error--it kills the drive and, if done badly enough, is dangerous. The novice may need practice in getting the right path in motion, but should easily perceive that *something* went wrong with the rep. On the other hand, saying the back angle is wrong seems to require a value judgment, because depending on the lifter the "wrong" technique may be most efficient in the short run--in other words, the naive lifter may be doing it wrong *because* he has optimized over the available degree of freedom, not because he has failed to do so.

    To me the most obvious case of this is with the adductors, so we'll need to include both degrees of freedom at the hip in our model rather than fix one by the assumption that the lift is done correctly with the knees out. We tend to start with weak adductors, so the brain automatically brings the knees in at the bottom. As Rip's skeleton-and-tape-measure demonstration shows, this pre-contracts the adductors before coming out of the hole so that they are not the weak point in the lift. Consider what that means--the brain is *not* "doing it wrong" by failing to optimize the lift. It is doing so precisely because it is *correctly* optimizing the lift. Our novice can in fact lift more with the knees in. We have to explicitly teach knees out, and until the adductors strengthen (or for some of us, apparently forever) the novice has to consciously override the optimization and do a lift that is sub-optimal in terms of weight lifted for the sake of future gain (stronger adductors, and presumably eventually stronger lifts--but the brain doesn't optimize for next month's reps, it optimizes for *this* rep right now). I think this is generally true--if we don't impose symmetry as I've been doing and include the left and right degrees of freedom separately, those of us who struggle to not shimmy around in the hole are trying to override the brain's short-term optimization to lift *this* weight right now for the sake of the future. An experienced lifter may well not do this, but I claim the reason is that he's trained the correct technique until it is the strongest path, not because it necessarily was the first time he squatted. (Having too open a hip angle could be this as well, perhaps the trainee has overdeveloped quads, though my observation is that it's often that they're not even aware that they can lean at the hips and get the knees back so I'm not relying on this as a good illustration.)

    What I'm trying to argue is that direct observation shows that keeping the bar over the mid-foot is optimal even for the immediate lift, because it's never optimal to be out of balance with a heavy weight on your back. The novice can get it wrong, but that's because they haven't trained enough to have real control over the path. By contrast, keeping the knees and hips back is not necessarily optimal in the short run, and so teaching that aspect of correct technique is essentially prescriptive and in the beginning tends to require overriding the brain's attempts to optimize the path rather than teaching what the optimal path is. The trainee isn't experienced enough for it to yet be the optimal path, but we're training with the goal of strengthening the muscles in the proportions that will make it the optimal path in the future.

    I claim that this line of reasoning supports my argument about the relative roles of stability and efficiency quite well, in that I still don't see an alternative that explains what we observe as well, but we'll see if anyone can make a good argument to the contrary. If nothing else it brings the abstract argument much closer to practical lifting, which is a good thing.

    Dustin

  5. #25
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    Quote Originally Posted by Scaldrew View Post
    Try asking yourself for answers since you were the one that started it. Search your feelings.
    My feelings are that I can present the argument that Rip asked for, or I can respond to a lot of arguments that don't bear. I haven't the time for both. And in particular I'm not going to respond to stuff like this. You don't need my permission to formulate your opinions on whatever grounds you choose, if you don't see or care that dragging in physical considerations that don't bear on the question is pretty much a waste of everyone's time.


    Quote Originally Posted by Raymondo View Post
    I don't know what your talking about with boundary conditions.
    Hmm. Shouldn't have been obscure. The path is a solution to a differential equation whether we find the solution that way or not. Such solutions depend on integration constants as well as the equation itself, and those constants have to specified separately. Those are boundary conditions (initial conditions if we choose to specify them with initial positions and velocities). So if you're constraining the path's endpoints, you're specifying boundary conditions.

    You don't solve these problems using f=ma. You work backwards in a sense. You start with constraint functions and differentiate them if you want to find out the forces
    If the path is constrained, say a Smith machine squat, this would tell you what happened during a specific rep. We're asking what the constraints were on the squat that led to that path, so I don't think that's likely to be the right approach. Of course, you can produce a compelling solution and show me I'm wrong, I am a sucker for an elegant solution (no sarcasm).

    No, your net torque does not need to be zero throughout the lift. You can fall forward at the beginning of the lift as long a you fall backwards at the end of the lift and vise versa. I'm sure there is some critical angle though past which you couldn't recover and would end up falling over completely.
    I assumed it was clear that the model I'm using is quasi-static, and more or less said so with a side comment about excluding dynamic lifts or some such. That's how it's treated in SS (Rip doesn't say "quasi-static" because he's nicer than that to his audience, but there is no question that he knows how to use the tool), so when asked how I differ from the explanation there I have to begin with that model, and since I think the model is sufficient I don't need to extend it to dynamical behavior, tissue elasticity (note that if we were asking about stretch reflex that would not be true, the point is that it isn't relevant to *this* question), physical properties of the Earth's crust, the time differential between the lifter's head and foot, the tidal forces exerted on the lifter by the moon and planets, or any of an endless list of other things.

    Please, if you think the model is *insufficient*, make an argument as to what aspect of the original question is neglected. (Seriously--make the argument. No sarcasm.) Please don't play the game that keeps popping up on this thread, of constructing unnecessarily complex models to show off or simply because you can. Simplifying models isn't just useful for this thread, it is also practice in one of the key skills for mathematical modeling. It takes *far* greater skill and physical insight to know what to leave out than what you could possibly put in.

    The path that maximizes weight lifted MIGHT be the same path but it depends.
    A point I've been belaboring, so I assume we agree on this.

    But you would have to know about the muscles in addition to the joints to really analyze it and that makes the problem super hard.
    Certainly--you don't include things you don't need. As I said at some point, a sufficiently realistic model will have to model muscles and attachments, but we don't want to do that if we don't have to.

    Quote Originally Posted by Raymondo View Post
    Looking at how a point on the body goes from p to p' is solving the IK pose problem. It tells you what you joint angles must be to be in a specific pose. This in general is non unique.
    I should have added that this an example of why you don't complexify your models. You are talking about underconstrained systems, and yes they're quite ugly in the general case. One of the reasons to keep the model simple is that it reduces to a single unconstrained degree of freedom, which can be treated by optimization. Making the generality of the control problem go away is a *very* good thing, unless you're getting paid for your time.

  6. #26
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    Dude you keep talking about dynamics to solve what is essentially a kinematics question. You don't need newtons laws. You don't need Lagrangian dynamics.

    You know that a point on the bar goes from point p at the bottom of the squat to a point p' at the completion of the squat.

    Constraints are just systems of algebraic equations. The solutions to these equations form a differential manifold (note you would need to differentiate this and impose the principal of virtual work to exclude illegal velocities). Any continuous path on this manifold that takes p to p' would represent a successful lift. Of course the system is under constrained. If you impose one and only one legal path from p to p', why would we even talk about how to optimize that path? At that point you've simplified the model to the point of meaningless.

    You seem to be confused about the meaning of constraints in general. Constraints represent joints or contact, not the bar path.

    The inverse kinematics problem is very well studied and it's not that hard and it's definitely the correct approach to this problem.

  7. #27
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    starting strength coach development program
    I'm not going through all the spaghetti logic...

    Quote Originally Posted by Mulgere Hircum View Post
    I should have added that this an example of why you don't complexify your models. You are talking about underconstrained systems, and yes they're quite ugly in the general case. One of the reasons to keep the model simple is that it reduces to a single unconstrained degree of freedom, which can be treated by optimization. Making the generality of the control problem go away is a *very* good thing, unless you're getting paid for your time.
    You need to use complex models to move beyond a simple heuristic. In actuality a human is constantly training and optimising the 'bar-path' as it continually changes and the problem is underconstrained. Nice to see that you realise why your initial assertion was incorrect.

    I have two asks:

    #1. Can you plainly state your goal / intention? (what are you trying to show as it is quite muddled at this point)
    #2. Can you attempt to be concise in your statements? (you aren't being paid by the word)

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