I was wondering how essential it is to squeeze your muscles when lifting. I'm a bit of a noob. On most lifts, it tells you to squeeze your muscles at a certain point - mainly the end of the rep. I don't think I've been nailing this down correctly, and may take some time for me to perfect this. How important is this part? For example, if I'm doing barbell bent over rows for my back, and don't squeeze my back muscles for 1-2 seconds when the bar reaches my chest, will the effectiveness of the exercise be 75% or something, assuming my form is perfect?