If you're lifting weights and stretch your biceps and triceps really far—like at the bottom of a curl or overhead extension—you'll build muscle just as well as if you move all the way up and down, as...

If you're already experienced with lifting weights, doing exercises where your muscles are stretched out—like lowering a dumbbell all the way down or just partway—gives you about the same muscle growth as going all the way to the extreme stretch. So you don’t need to stretch your muscles to their absolute limit to get big.

When people who regularly lift weights perform exercises that stretch their muscles fully, they gain more strength in movements that involve those stretched positions, such as bending the elbow while the muscle is lengthened.

Doing bicep curls only halfway up (keeping the arm stretched) might make the lower part of your bicep grow a tiny bit more than doing full curls, but the difference is very small.

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.

Whether you do full or half curls, your arms still get stronger and a bit bigger—so you don’t need to do one over the other to see results.