For guys new to lifting, doing bicep curls with your arms in front of your body works just as well for building arm muscle as doing them with your arms behind you — both give about the same 6% to 9%...

If you're a guy new to lifting, doing bicep curls with your arms in front of you or behind you doesn’t really make a difference in how much your biceps grow — both ways work about the same.

If you're a guy new to lifting and do bicep curls with a cable machine for 10 weeks, your biceps grow evenly from top to bottom — not more in one spot — no matter how your shoulder is positioned.

If you're a guy new to lifting, doing bicep curls with your arms in front of you or behind you gives your brachialis muscle (a hidden arm muscle) about the same growth — around 8–10% after 10 weeks. It doesn’t matter much where your shoulders are, as long as the resistance feels the same.

Doing bicep curls with your arm in front of your body or behind your body gives you the same muscle growth and strength gains if you're using the same amount of weight and doing the same number of reps.

Doing bicep curls on an incline bench might build your biceps muscle better than doing preacher curls, especially in the middle part of your upper arm.