Lifting weights with a barbell over your head works the side and back parts of your shoulders more than using a shoulder press machine, because your muscles have to work harder to stay balanced.

If you're a male bodybuilder, doing the back barbell shoulder press lights up your side shoulder muscles more than the front version — both when lifting and lowering the bar, and the difference is really big.

When competitive male bodybuilders do overhead presses with a free barbell instead of a machine, their side and rear shoulder muscles work harder because they have to balance the weight themselves.

For competitive male bodybuilders, doing the back barbell military press fires up the rear shoulder muscles way more than the front version — in both the lifting and lowering parts of the move.

When bodybuilders do overhead shoulder presses with the bar behind their head, their rear shoulder muscles work a lot harder than when they do the same move in front — at least according to this claim.

For competitive male bodybuilders, lowering the bar behind the head during shoulder presses seems to work the side of the shoulder muscle harder than pressing in front, based on muscle activity readings.