Sit down and cut out the cheating.
Sitting kills momentum, so keep the reps strict and controlled.
Sitting down removes the hip drive and torso lean that quietly steal work from your biceps on standing curls. With your arms hanging straight and elbows pinned by your sides, every rep is a strict elbow flexion from full stretch to full squeeze, which is exactly the recipe for biceps growth. It is the simplest way to make a familiar movement more honest.
They are stricter. Sitting removes leg drive and most of the body swing, so the biceps do all the work but you will curl a little less weight. Standing lets you go heavier, seated makes the reps cleaner. Both have a place.
Curling together saves time and keeps the set dense, alternating lets you focus on one arm and sneak a breather between reps. Either is fine as long as your torso stays still and the reps stay strict.
Because you were cheating standing up, even if only a little. The seat exposes how much your hips and lower back were contributing. Take the small drop in load, keep full range, and your biceps get more out of every set.
