Even though people who swing weights use their whole body more, their upper arms don’t get any thicker than those who lift strictly—so the extra movement doesn’t help build more arm muscle.

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.

When people who have never trained before start doing resistance exercises - whether they use strict form (slow, controlled movements) or add external momentum (like swinging weights) - both methods make the muscles in their upper arms bigger.

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.

If you're already strong and you train your arms using only the part of the movement where your muscles are stretched (like holding the bottom of a bicep curl), you'll build just as much arm muscle as if you did the full movement — the stretch alone seems to be all you need.

Even if you cheat a little to lift heavier, your muscles still grow just as well—so maybe the key to building muscle isn’t perfect form, but just getting your muscles to work hard enough.