Whether you lift your arm weights slowly or with a little swing, your biceps and triceps both get noticeably bigger after eight weeks of training—your triceps grow even more than your biceps.

Doing bicep curls one arm at a time makes your right arm stronger than doing them with both arms together, but doesn't make your biceps bigger than using both arms.

Whether you lift weights with one arm or both at the same time, your biceps grow about the same amount after 8 weeks of training.

When beginners do arm curls and triceps exercises with a little swing to lift heavier weights, their arms grow just as much as when they do the movements slowly and strictly—even though they lift way more total weight with the swing.

When untrained men do arm curls for 8 weeks, their biceps get thicker (about 11%). But when they do dumbbell rows instead, their biceps only grow about half as much (about 5%).

People who swing the weights while doing arm exercises can lift a lot more total weight over time than those who do it slowly and strictly—even though both groups end up with the same size arms.