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.

Using momentum to help lift weights builds arm muscles just as well as lifting slowly and carefully for beginners doing bicep curls and triceps pushdowns over 8 weeks.

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.

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.

Doing push-ups with light weights until you can't do more doesn't make your biceps bigger, but doing the same kind of light-weight bench presses does — so your arms respond differently to these two exercises.

If you're lifting weights regularly, doing more reps and sets might make your biceps grow bigger, but it doesn't seem to help your triceps grow as much—your arms might just respond differently to extra work.