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.

Whether you do full or half curls, the upper part of your bicep grows about the same amount—so where you curl doesn’t matter much for that part of the muscle.

Doing bicep curls only halfway up might help you push harder when your arm is bent at 100 degrees, even though you didn’t train that angle—possibly because your muscle got a bit bigger in the stretched part.

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 long as you get that deep stretch.

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.

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.