The muscle underneath your bicep gets just as strong and big whether you curl with your arm in front or behind you.

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.

If you're new to lifting, changing your arm position during cable curls doesn't make one part of your biceps grow more than another — both positions give you the same kind of muscle growth over 10 weeks if everything else is kept equal.

In untrained men performing cable curls with the same resistance, movement range, and effort, changing the shoulder angle between neutral and fully extended positions leads to similar increases in elbow muscle thickness after 10 weeks of training.

If new guys do a specific arm curl workout for 10 weeks — going until their muscles are totally tired — their arm muscles get about 7% to 9% thicker, no matter how their shoulders are positioned, as long as the movement and resistance stay the same.