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.

In untrained young women, performing bicep exercises through a shorter range of motion (from fully extended to halfway bent) leads to more muscle growth near the elbow than using a longer range (from halfway bent to fully bent), but both approaches result in similar muscle growth in the middle of the bicep.

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.

Doing arm exercises that shorten and lengthen the muscles might help build arm size a little, but the evidence isn't super strong—there's only about a 55% chance it's better than just lengthening exercises.