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.

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.

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.

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.

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 healthy guys do shoulder presses, standing with dumbbells makes their shoulder muscles work harder (based on electrical activity), even though they can lift heavier weights when sitting with a barbell.