Question about compound exercises

[loneswaneast]

I have a physics questions that perhaps someone can answer. When we do exercises using two parts of the body to lift a weight, such as doing chest flies while moving your hips up and down on the step (forgive me, I forgot what that is called ...we do it at the end of High step advanced workout), is each muscle lifting whatever the weight is you are using or is each lifting half the weight? Similarly, when doing lunges with tricep exercises, do the legs take some of the work away from the triceps?
Thanks.
By the way, I just got my 11 new DVD's...I'm so excited!!!!!!!!!!!!:+

 

[kathryn]

Technically, "compound exercises" are those that include movement at more than one joint, and work more than one muscle in the same area. An example would be chest presses (which work the chest, but also the anterior shoulder and triceps) vs. isolation exercises, which involve only one muscle group and movement in one joint (like flyes that focus on the chest).

Regarding the moves you are asking about: when doing lunges with double arm triceps extensions with a 10# dumbbell, let's say, your triceps are lifting 10#, but the legs are moving the weight of your upper body plus 10# (the weight of the dumbbell). But, if you do the tricep move while doing the positive portion of the lunge (the "up" part of it), momentum from the legs can give the triceps a bit of help, but I wouldn't think it is enough to make a lot of difference (if you want to get rid of any momentum, then wait till you are at the top of the move, then do the tricep extension).

On the hip lift/flye, the chest lifts the weight of the dumbells, and the glutes/hamstrings lift the weight of part of the body.