by Mark Rippetoe Starting Strength
I have been asked about this so many times that I guess I should address it, so I can copy and paste the link when I need to. Saves time.
All squats – if heavy enough, and regardless the back angle – are initiated with the hips. If you are sufficiently adept at using the internet, slowing down any heavy squat video will show you that the first movement up out of the bottom is accomplished by the motion we refer to as hip drive. Sergey Kopytov’s 235kg front squat at the 1990 European Championships, performed with an almost completely-vertical back angle, was started out of the bottom with the hips. It’s just not optional, and it’s true for heavy snatches as well.
So if there are reasons that a lifter can’t get the bar in the lower position – and there may be on occasion a shoulder injury that prevents this from being possible – then the lifter has to squat with the bar on top of his traps. This lifter has to think about two things instead of the one thing most necessary: Keeping the chest up and driving the hips.
Normally, with a sufficiently-horizontal back angle, shoving your ass up in the air is the simple way to focus on the thing most responsible for the squat being finished. But if the bar is higher on the back, the potentially longer distance between hips and barbell must be carefully managed or it adds to the force that must be produced to squat the weight. Keeping the chest up makes the moment arm along the back segment manageable.
But the shorter moment arm uses less muscle mass than the low-bar position we prefer, and therefore requires the use of lighter weights, making it a less desirable choice for strength training. The use of the heaviest weight possible with the longest effective ROM allows for the most efficient development of strength. The chest-up position also places the knee angle in a more closed position, the consequence of shortening the moment arm with a more vertical back. This takes some of the hamstring function out of the kinetic chain, one of the reasons the weight must be lighter.
So if you have to high-bar squat, you do it the same way you squat: driving your ass up out of the bottom must be your primary focus, but if you don’t keep your chest up too you’re going to cost yourself reps at the end of the set. This is the price you pay for making your squat more like a front squat.