moment arm analysis in 3rd edition.

[spacediver]

page 33, last paragraph.

I understand that the torque on the hip joint is the force of the bar X the horizontal distance between the bar and the hip joint. This torque produces a tendency for the hip angle to decrease.

The hip extensors will feel, or "see", this torque.

I also get that the torque on the knee joint is the bar force X the horizontal distance between bar and knee joint, and that this torque produces a tendency for the knee angle to close.

The knee extensors will feel this torque.

What I don't get is that "The hip extensors "see" the femur moment arm between the hip and the bar..."

If I'm understanding you correctly, you seem to be implying that the torque along the hip joint due to the force of the bar can be analyzed as if the bar was resting on, or hanging from, the thighs themselves (in the same horizontal position as the position on the back). Indeed, this would produce a torque on the hip joint, although it would produce a tendency for the hip angle to increase (by snapping the femur in half).

I feel I'm misunderstanding you here, but I'm confused, since you seem to be decomposing the torques along two different moment arms:

1: the back segment
2: the thigh segment.

But I'm not sure what you mean when you say "...moment arms along the back segment..."

I'm taking "moment arm" to literally mean "the horizontal distance between the weight and the joint in question" (horizontal because the force due to gravity is what we're concerned with here). So when you talk about the "moment arm" along the back segment and the moment arm along the thigh segment, these should refer to the same distances (keeping the joint in question the same).

There seems to be no analytical rationale to talk about different segments when discussing moment arms. The only two things that seem relevant are the position (and magnitude) of the force, and the distance to the joint in question. The only purpose I can see of introducing the idea of things like the thigh segment, or the back segment, is that we can describe how these particular segments have a tendency to rotate around a particular joint, due to the calculated torques. But if this is the way in which you are using the terms, then how can the hip extensors "see" the femur moment arm? Surely we are talking about the tendency of the back segment to rotate around the hip joint here.

Also, on p 156:

"Essentially, all the force being generated...and the forearm parallel to the bar with elbow directly underneath..."

Did you mean to say that the forearm should be perpendicular?

[Mark Rippetoe]

There seems to be no analytical rationale to talk about different segments when discussing moment arms. The only two things that seem relevant are the position (and magnitude) of the force, and the distance to the joint in question. The only purpose I can see of introducing the idea of things like the thigh segment, or the back segment, is that we can describe how these particular segments have a tendency to rotate around a particular joint, due to the calculated torques. But if this is the way in which you are using the terms, then how can the hip extensors "see" the femur moment arm? Surely we are talking about the tendency of the back segment to rotate around the hip joint here.

You are probably right, in that the moment arm on the whole system is the force we're dealing with, and not really the moment arms on the segments themselves. But do you not see the value of explaining it this way to people who have never thought about moment forces in barbell training?

Also, on p 156:

"Essentially, all the force being generated...and the forearm parallel to the bar with elbow directly underneath..."

Did you mean to say that the forearm should be perpendicular?

Are you looking for typos? We don't pay you to find typos.

[Catecholamine]

While it's easy to figure out what you actually mean from context clues, "parallel" vs. "perpendicular" isn't exactly a typo.

[Mark Rippetoe]

I'm in Tampa right now, and I don't have the book with me. Relish your victory until I get back to my office tonight and can sort this out.

[spacediver]

You are probably right, in that the moment arm on the whole system is the force we're dealing with, and not really the moment arms on the segments themselves. But do you not see the value of explaining it this way to people who have never thought about moment forces in barbell training?

No. I found the way it was explained in the book in that particular section very confusing and inconsistent. I'm the type of person that reads slowly and deeply and tries to get a clear understanding of what the author is trying to convey. I've been over that section several times, and I still can't make good sense of it.

This isn't a nitpicking issue of "hah, he didn't use the term correctly, I found an error!"

Are you looking for typos? We don't pay you to find typos.

No, this is me reacting to what I think is overall a brilliant text, and being motivated to help improve it if I can.

[spacediver]

sorry, should have included this in my last post.

It's not that you've decomposed the moment forces with respect to the different segments that bugs me. It's that you seem to have done so erroneously. When you say that the hip extensors see the femur moment arm, the only way this seems to be an accurate statement is if you are using the term "femur moment arm" to mean "the horizontal distance between the bar to the hip joint" (i.e. it just so happens that the femur is conveniently parallel to the ground, so we can use it as a horizontal reference), and that the hip extensors have to resist the tendency of the back segment to rotate.

Related to all this, when you say "The force of gravity always operates straight down from the bar ... So we can calculate the moment arms along the segments as measured perpendicular to the bar."

you're not suggesting that the torque acting upon the hip joint due to a bar on the back is equivalent to the torque upon the hip joint if the bar was resting on, or hanging from, the thigh in the same horizontal position as the bar-on-back, are you? I can see how the torques upon the knee joint would be equivalent in both those cases, but certainly not upon the hip joint.

[Thomas Rohner]

page 33, last paragraph.

What I don't get is that "The hip extensors "see" the femur moment arm between the hip and the bar..."

If I'm understanding you correctly, you seem to be implying that the torque along the hip joint due to the force of the bar can be analyzed as if the bar was resting on, or hanging from, the thighs themselves (in the same horizontal position as the position on the back). Indeed, this would produce a torque on the hip joint, although it would produce a tendency for the hip angle to increase (by snapping the femur in half).

To my understanding this is not implied, as there is only one torque acting on the hip joint, the one created by the gravitational force of the weight on the back with the moment arm of the back segment. As the horizontal distance to the hip joint is the same, irrelevant if the point of attack of the force created by the weight is on the back or hypothetically the femur, the magnitude of the torque around the hip joint and its direction is the same => the one the hip extensors have to counter.
The analogy does not hold to your observation that the hip angle would increase as this would require, as stated, that the femur would be failing, which we all hope is not happening when one squats, but the load on the bone is essentially "trying" to do that.

[Iain]

While it's easy to figure out what you actually mean from context clues, "parallel" vs. "perpendicular" isn't exactly a typo.

How many times on that page does it say the forearms should be vertical, and that the elbow should be directly under the bar. Nitpick much?

[Mark Rippetoe]

While it's easy to figure out what you actually mean from context clues, "parallel" vs. "perpendicular" isn't exactly a typo.

Yes, it's a typo. The sentence should read " ...and the forearm perpendicular to the bar with the elbow directly underneath the bar..." It will be corrected in the next printing.

It's not that you've decomposed the moment forces with respect to the different segments that bugs me. It's that you seem to have done so erroneously. When you say that the hip extensors see the femur moment arm, the only way this seems to be an accurate statement is if you are using the term "femur moment arm" to mean "the horizontal distance between the bar to the hip joint" (i.e. it just so happens that the femur is conveniently parallel to the ground, so we can use it as a horizontal reference), and that the hip extensors have to resist the tendency of the back segment to rotate.

Related to all this, when you say "The force of gravity always operates straight down from the bar ... So we can calculate the moment arms along the segments as measured perpendicular to the bar."

When I say "femur moment arm", I mean the torque at the hip joint generated by the moment arm along the femur measured from the hip to the gravity vector. I think this is clear in the sentence.

you're not suggesting that the torque acting upon the hip joint due to a bar on the back is equivalent to the torque upon the hip joint if the bar was resting on, or hanging from, the thigh in the same horizontal position as the bar-on-back, are you?

How did I suggest that? The force is clearly being transmitted along the kinetic chain from the bar to the floor. Should I have stipulated that?

[spacediver]

Thanks for the responses.

When I say "femur moment arm", I mean the torque at the hip joint generated by the moment arm along the femur measured from the hip to the gravity vector. I think this is clear in the sentence.

To be clear, the torque here produces a tendency for the back segment to rotate around the hip joint, right? (i.e. you're not talking about the tendency for the femur to rotate around the hip joint).

As the horizontal distance to the hip joint is the same, irrelevant if the point of attack of the force created by the weight is on the back or hypothetically the femur, the magnitude of the torque around the hip joint and its direction is the same => the one the hip extensors have to counter.

No. It does matter if the weight is on the back or if it's on the femur. A weight on the back will produce a tendency for the hip angle to close. A weight hanging from the femur produces a stress at the hip joint in the opposite direction. If the femur was weak enough, it would snap in half, and the hip angle would increase.