Mixed

When resistance profiles, elbow range of motion, and effort are matched, altering shoulder extension angle (neutral vs. maximally extended) does not produce meaningful differences in elbow flexor hypertrophy in untrained men.

For most recreational lifters, whether you perform bicep curls with your arms by your side (neutral) or slightly behind your torso (extended shoulder) makes no meaningful difference to muscle growth, provided you train to failure and manage the weight similarly. Choose the variation that feels best for your shoulders and allows you to lift consistently. Don't overcomplicate your exercise selection based on shoulder angle alone.