A stupid question about the deadlift?

[tertius]

I don't understand this, sorry.

Savs is saying that noting that the COM of the lifter must approximately be in line with the COM of the bar, if the bar is to travel vertically, and thus, efficiently. Because if it wasn't, the system COM wouldn't be over the midfoot, unless the bar is very heavy indeed.

Since you implied that the COM of mass is actually behind the bar at some light weight, Savs is wondering how the lifter stays standing when he lifts the bar off the floor (with a system COM behind the midfoot), instead of falling over backwards. Which is what humans do when their COM isn't over the point of balance on their feet. Thus his question regarding whether the muscles of the lower leg can balance out the tendency to fall backwards.

At least, I think that's what he's saying.

The answer, of course (he says, hoping he's not wrong), is that the lifter doesn't fall over because the COM of the lifter--and thus that of the system--shifts over midfoot pretty shortly after the bar starts to move.
This seems to be why everyone passes through the position you described pretty soon after the bar comes off the floor. They can't help it.

The exact biomechanics of it might be complicated, but it's a basic fact that a human being can't maintain any position in which their COM (and/or the COM of the system they are part of) is NOT over the balance point(s) of their feet (or hands, or whatever part of the body is touching the object supporting their weight). Same as how you can't balance a four legged chair on one leg unless you can line it up it just so. It's just how things work. Gravity will yank on anything that isn't lined up over its balance point correctly, until it assumes a position where there is no moment arm between the COM and the point of contact with the supporting surface. There's no avoiding it.
I bet our local australian weirdo acrobat could expound on this point at length.

It might be that people have a weird feeling that they need to lean back to lift up the heavy thing, in order to "balance it out", but their bodies know that this is stupid, and will put itself in the proper position once the movement starts, whether they like it or not.

Which is why you should just start there in the first place, even though it may feel like you are leaning way forward until the bar is actually moving.

So, yes, you could avoid assuming The Position by trying to deadlift like an idiot (and keep your COM behind your midfoot, with the weight of the bar keeping you from falling down), but you'd either fall backwards on you ass or never be able to get the bar past your knees and have to set it back down. And the bar could never be actually heavy.

I bet there are videos of this happening, of someone failing a pull from the floor because they get themselves in a biomechanically stupid position where once they've lifted the bar a little bit, resist getting their shoulders in front of the bar and the COM of the system is now in a position they simply can't apply force to effectively (because they can't pull the bar through their lower leg), and so they fail the lift and fall over backwards.

[Savs]

Thanks, tertius.

I don't understand this, sorry.

I wrote too many disjointed questions, and there are a few misunderstandings on my end.

Begging your patience, and limiting the questions to a 3x BW deadlift, I've attached a diagram that shows a vertical line drawn through the center of the foot, and three cases (out of the continuum) for the bar's, body's, and system's COM at the instant the bar leaves the floor. The questions are: (1) Is A ideal but not possible? (2) Is B (bar centered, lifter back of center, so system COM is not centered) what really happens? (3) Is it possible that C (lifter back, bar slightly forward, system centered) is what really happens?

More-importantly at this stage for me, this thread and your replies forced me to reread the deadlift chapter and re-evaluate my setup for the pull.

[tertius]

Okay, some of that doesn't make sense, sorry.
It was latish (though I laid in bed for three hours, unable to sleep, shortly after posting it).

[Mark Rippetoe]

Begging your patience, and limiting the questions to a 3x BW deadlift, I've attached a diagram that shows a vertical line drawn through the center of the foot, and three cases (out of the continuum) for the bar's, body's, and system's COM at the instant the bar leaves the floor. The questions are: (1) Is A ideal but not possible? (2) Is B (bar centered, lifter back of center, so system COM is not centered) what really happens? (3) Is it possible that C (lifter back, bar slightly forward, system centered) is what really happens?

It will help your understanding if you remember that the SYSTEM COM and the BAR dots are proportionally very large relative to the BODY COM dot at 3x bodyweight.

[tertius]

Thanks, tertius.



I wrote too many disjointed questions, and there are a few misunderstandings on my end.

Begging your patience, and limiting the questions to a 3x BW deadlift, I've attached a diagram that shows a vertical line drawn through the center of the foot, and three cases (out of the continuum) for the bar's, body's, and system's COM at the instant the bar leaves the floor. The questions are: (1) Is A ideal but not possible? (2) Is B (bar centered, lifter back of center, so system COM is not centered) what really happens? (3) Is it possible that C (lifter back, bar slightly forward, system centered) is what really happens?

More-importantly at this stage for me, this thread and your replies forced me to reread the deadlift chapter and re-evaluate my setup for the pull.

My understanding is that Rip wants us to do (A), which is actually easily attainable (after all, we balance over our midfoot all the time, don't we?), that (B) is what I was talking about as leading to failing the lift and/or falling over, and (C) is how certain individuals teach pulls from the floor, for reasons I simply cannot figure out, as it introduces a bunch of horizontal movement that is extraneous and wasteful.

[Mark Rippetoe]

As it turns out, figure B is most correct. BODY COM will be behind both BAR COM and SYSTEM COM.

[Savs]

It will help your understanding if you remember that the SYSTEM COM and the BAR dots are proportionally very large relative to the BODY COM dot at 3x bodyweight.

The COM is a point. I drew the dots larger than points so they would be easily visible. The fact that the masses (and therefore weights) are different at the different points is understood.

As it turns out, figure B is most correct. BODY COM will be behind both BAR COM and SYSTEM COM.

I believe you (for whatever that is worth). I'll share a story at the risk of feeling silly (because it is so obvious). Many of the sciences are divided into theoretical and experimental work. As a young experimentalist, I went to seek advice from a theoretician for whom I, and many others, had great respect. His advice was two sentences. He said, "You can sit in your office and mathematically prove, without a doubt, cows can jump over the moon. However, you must never forget, cows can not jump over the moon." The theory must agree with the experiment, not the other way around. So, when a distinguished experimentalist tells me he has done the experiment many times, and has seen it done many more times, and he tells me what happens, I will accept that as the truth, until proven otherwise.

My take: If the system COM is located in front of the ankle, a moment is created about the ankle. The muscles of the feet and calves act to counter the moment. I don't see anything (again, for whatever that is worth) mechanically impossible with B.

[tertius]

As it turns out, figure B is most correct. BODY COM will be behind both BAR COM and SYSTEM COM.

At all phases of the pull?

[Mark Rippetoe]

The position illustrated is at lockout.

[Instinctkiller]

Sorry for the late reply. But as I see it, at lockout the body COM and the bar COM are over the midfoot, isn't it? when I stand up without a bar in my hands, I'm balanced without using any muscles, and when I stand up with a bar in the hands (which is over the midfoot), I'm also balanced, using only my grip muscles and my core to keep my spine healthy.
Isn't this the case?

And about why we wouldn't be able to pull it if the lats aren't in 90 degrees - I really think that my brother solved the problem.
He accept that nothing is pushing the bar away, and that the only problem of pulling with vertical arms is that we just won't be able to pull it if it's heavy enough.
He think that our body shifts forward (meaning shoulders & hips, when the bar is still over the midfoot, creating more horizontal arms) in order to minimize the moment arm between the hips and the bar --- that the heavier the weight, the shorter moment arm between those two we can handle.
Problem is, that too horizontal arms, when the lats are no longer in 90 degrees means that when we lift the lats are literally lifting the weight up --- which they can't do. So the "balanced" point is only at 90 degrees, where they have the biggest leverage.

Vertical arms, i.e shoulders are over the bar mean that the hips have a bigger moment arm to overcome, since they're horizontally far from the bar (compared to the more horizontal arms, when the soulders are in front of the bar).


So, the tendency is to try and minimize the distance between the hips and the bar, but too much forward shoulders (as a "side effect" of more forward hips) means that the lats won't really be able literally lift the bar UP (not only BACK).



What do you think about this analysis, Rip?