For people who work out regularly, doing bicep curls with the muscle stretched versus using a mix of stretch and squeeze doesn’t really make a difference in strength gains when lifting in the shortened position.

If you're someone who works out regularly, doing certain types of arm exercises—especially those that stretch the muscle while lifting—can noticeably boost your strength. Both fully stretched and mixed-range workouts seem to help about the same.

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 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.