Originally Posted by Garrett Smith
Back to your original question, the leg extension does nothing for hip flexion training as the hips don't really move and there is no appreciable isometric load being imposed on them. The point of the leg extension is to attempt to remove the hips from the movement.
Ah -- but the whole point of the GHR example is that it seems like training one attachment of a muscle also strengthens the other one. Training the hamstrings via knee flexion -- which nobody cares much about -- also strengthens the hamstrings for hip extension -- which everyone cares about. Shouldn't we be able to apply this lesson elsewhere too?
Anyway, that's not so important. I guess my main point here is that the GHR requires compound stabilization, moves the body, and is safe for the joints, blah blah blah... but in terms of the prime movers, it's still, essentially, a single-joint exercise. So why is it that we've all shunned single-joint exercises from our strength programs, on the principle that they don't contribute to larger compound strength? Within the right context (adequately strong stabilizers and auxiliary muscles, for instance), the entire lesson of the GHR seems to be that this DOES work.
Unless, of course, those other elements (the stabilization element, for instance) are fundamental to the efficacy of the exercise, and without them, there is no benefit. But I don't see why that should be the case. We could all try doing leg curls, to remove those auxiliary benefits and see if it STILL improved our deadlifts, I suppose...
But yeah, my broader point is something like, "are we avoiding curls because they don't make us stronger, or are we avoiding curls because they're not the perfect exercise?"