Slight physics correction

[Mulgere Hircum]

I don't know if you want to address this in a Q&A thread, but this is how I know to get a message to you easily. You can do what you want with it.

I was re-reading the SS deadlift chapter and noticed a small physics error--I'd normally ignore it in a weight book but you're pretty careful about mechanics so you might care. On p120 (2nd ed) under "Pulling Mechanics," you make the general point about maintaining a straight bar path by citing the work done against gravity: "Work is defined as the force (the force of gravity acting on the mass of the loaded barbell) multiplied by the distance (the measured distance the barbell has to travel)..." and then in the next sentence say "The farther it has to move the more work must be done to move it...." As far as I can tell, because of the juxtaposition of the two sentences you mean "the farther it has to move the more work must be done *against gravity* to move it."

The problem is that work done against gravity is path-independent and depends only on the initial and final heights. But I think maybe it's better just not to talk about work done only against gravity, since you really care about physiological work and and they aren't the same (if they were, farmer's walks would be easy and we could do static holds forever). I suspect a curved bar path is more work because you body has to accelerate the bar perpendicular to gravity (and unlike gravity muscles aren't conservative), possibly because it takes more time, and perhaps it also takes extra effort to support it with changing leverage. Basically I think your muscles end up doing extra chemical work that ends up as heat, not gravitational potential energy.

Looking back it seems likely that similar statements are made elsewhere, this is just where I noticed it. Just so it doesn't sound like carping, SS is by far the most sophisticated mechanically of any weight lifting book I own. I wouldn't bother to make the point otherwise, because you're the only author I think might even really care and you have another edition in the works.

[Mark Rippetoe]

An excellent point. The work done against gravity is the work done parallel to its vector, and therefore the straight vertical bar path is the way to do the least possible work against the load. If we can arrange our pulling method to yield a vertical bar path, we have done the most efficient job possible of moving the load. Right?

The 3rd edition cleans up some of these loose ends, as well as going into detail about some other pulling mechanics topics.

Thanks. Thoughtful posts are always welcome.

[jurassic]

I actually had a similar question/correction regarding the physics discussed in ppst 1.
It could be that in the second edition it is corrected, or that I misunderstood something, but anyway here it goes.
Ion the episode about power production, you speek of the work done by the force exerted on the bar, but when you give numeric examples, you describe this force as m*g, which is only the gravity, applied by the earth, as if the bar free falls.
It could be that you are intentionaly talking about only the gravity's work, and I misunderstood it, but from my point of understanding you are talking about the work of the combined force that is exerted on the bar and causes it to accelerate, i.e m*a (mass and acceleration of the bar) and the work of this force. This force of course, is the force the lifter exerts minus gravity (if the vectors of both forces are in opposit directins and in a strait line), and not just the gravity.
Thanks for your time answering this, and perhaps for giving me a free physics lesson.

[Brenden]

The reason it takes more (physics) work with a curved bar path is because of Newton's first and second laws. You can separate out the acceleration into three vectors corresponding to the x, y, and z coordinate directions. In a straight bar path, a(x) = a(y) = 0, as the bar only travels in the vertical direction. Once you add lateral movement, the acceleration in the x or y direction is nonzero, and that requires work. There's first the inefficiency that causes the bar to deviate from vertical movement, which shifts some work from the vertical to the lateral direction, and then there's the work needed to correct the error.

[spiderman]

since you really care about physiological work and and they aren't the same (if they were, farmer's walks would be easy and we could do static holds forever).

Even though energy is being consumed to maintain a particular body position that exerts a force, no work is being done during a static hold because nothing is moving unless you are looking at the molecular level. At any scale no work is being done on the bar by the body during a static hold as the bar is not moved even in any direction.

[rpbrown]

Along these lines, the straight bar path minimizes the amount of force required to move the bar over that set distance. Once that bar path deviates from linearity, the forces (vectorial) are no longer parallel to the force of gravity and our old friend torque is introduced.

[crossley]

Physiological work is done to produce the mechanical work done against the barbell.

So really, an efficient pull is obtained by minimizing the ratio:

(Physiological Work) / (Mechanical Work).

Is this correct?

[Scrofula]

No, I don't think that's right. The mechanical work done on the bar is completely independent of bar path. i.e., mechanically speaking, moving the bar from point A to point B requires the same amount of mechanical work, no matter what path the bar takes.

The difference is in thermodynamic work, which is the amount of energy transferred from one thing to another. The thermodynamic work done on the bar is still pretty much independent of bar path, but a different amount of work might be done on the environment (i.e. you release a different amount of heat) if you choose a bar path that involves contracting more muscles.

So vertical bar paths do tend to be more efficient, but that's just because our muscles do not have to accelerate the bar back and forth in useless directions and thus burn up less fuel. Efficiency of a mechanical operation is useful mechanical work divided by total thermodynamic work. Mechanical work is the same, so the most efficient bar path is the one that requires the least thermodynamic work.

(My physics is rusty, but I think this is right.)

[David55]

I think one way to put it is this: Gravity will have to be fought, no getting around it. The straight up and down fights ONLY gravity. Moving it in any other path fights something else as well, meaning wasted effort.

[Scrofula]

No, I don't think that's right.

By the way, this was in response to Rip's post, not crossley's. Due to the moderation, a bunch of posts appeared in between.