Hip joint lever

[paddy]

Hi Rip. I'm a long time lurker, first time poster...

I have a question (hopefully not a stupid question) about your description of the hip joint while pulling - specifically about Figure 4-18 from SSBBT3. I thought the direction of force from the hamstrings pulling behind the hips would be perpendicular to the thick black line, so that the moment arm IS the thick black line drawn from the muscle attachment to the hip joint. The picture makes it look like the direction of the force behind the hip is straight down towards the ground (going on the length of the lever arm drawn above the lifter). If the force behind the hip were pointing straight down to the ground, the ratio of the two moment arms would be constant, regardless of the angle the hip makes with the ground. But that can't be right. So what direction are the muscles at the back of the hip pulling at - perpendicular to the black line, or straight down to the ground?

Thanks for clearing that up, much appreciated.

Paddy

[Mark Rippetoe]

I don't have the book with me now -- I'm in a hotel room. Maybe Steve can help.

[chaloney]

The moment arm in this drawing is related to the force which the lifter must overcome--gravity. The fat arrow is the direction the force of the hamstring-from the bottom of the pelvis to just below the knee. As the lifter rises up with the bar, the torso (the thick black line representing the lever) becomes more vertical but gravity is still pulling straight down so the moment arm length is still measured perpendicular to the direction of the force gravity.

[paddy]

hi, thanks alot for the reply.

i hope i don't seem like an arse with this question, just want to make sure i understand this picture properly.

cool so I agree the direction of force at the shoulder is vertical, so the length of that moment arm is measured perpendicular to gravity (i.e. horizontal, as it is drawn in the picture). the part i am confused about is the other moment arm. the length of the other moment arm in the picture is shown as the horizontal distance from the muscle attachment at the hip to the fulcrum hip joint. that implies that the direction of force at this moment arm is vertical (i.e. so that the length of the moment arm drawn above is the cosine of the angle the torso makes with the horizontal). but as you say the force at the hip is not vertical, but instead is in the direction of the big arrows.

if the force is in the direction of the fat arrows (NOT vertical), then the moment arm length drawn above the picture doesn't seem right. on the other hand, if the force at the hip is vertical then the moment arm length drawn above the picture looks right. but if the force is vertical then leverage can never improve regardless of the angle the torso makes with horizontal. so that must be wrong. i hope my question makes sense?

thanks again for your help! appreciate it!

[Steve Hill]

The force vector of the hamstrings continuously changes as the hip extends. It is also dependent upon individual anthropometry. An individual illustration cannot convey this concept, but what it can do (and what it is meant to do) is show the concept of a small segment length and large force being necessary to overcome a longer segment length with a smaller force applied to it, and also the concept of large force/short segment length/less distance of displacement/lower radial velocity vs small force/long segment length/more distance of displacement/higher radial velocity. It also serves to illustrate a what a class I lever is, which is an important concept in understanding human movement.

Trying to read any more into it would be pedantic.

[paddy]

cool, thanks both of you for your replies. i was worried that i was sounding pedantic - i didn't mean to be if i was. as a beginner its hard to know if im being pedantic, or if i have just completely missed the point so thanks again.