Originally Posted by Towlie
No, they are pulling. Your quads are on top of your leg, right? They PULL your lower leg with respect to your upper leg to straighten your leg. That is exactly like triceps pulling to straighten your arm.
Your legs as a whole may be supporting or "pushing" your body upward, but that is quite different from saying that your quads are pushing -- muscles can't push, they can just pull. Think about a rope -- a rope can't push a car; it can only pull a car when in tension.
The correct terms are "flex" and "extend". Sorry... med school beat this into me.
Quads extend the lower leg. Tricepts extend the forearm. Conversely, hamstrings flex the lower leg and bicepts flex the forearm. Technically, none of these actions are "pushing" or "pulling". Muscles only create a force by contracting. Note that you're explaination isn't wrong... you have it all right, I'm just a nerd for needing the terms to be correct.