If you're a young guy just starting out, doing bicep curls with your arms in front of you or behind you gives about the same strength gains after 10 weeks — even if one seems to improve a bit more, the difference isn’t really meaningful.

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 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.

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% growth in 10 weeks.

If you're new to lifting and do cable curls the same way each time, it doesn't really matter whether your arms are stretched far back or in a neutral position — your biceps will grow about the same amount after 10 weeks.

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.