If you do cable curls the same way every time except for where your shoulders are, it doesn’t really matter for building arm muscle—even if you can lift heavier when your arms are stretched out.

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 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.

If you keep the resistance the same at different arm angles during cable curls, you can better compare how muscle stretch affects growth, without other forces messing up the results.

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.

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.