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.

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.

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 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.

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 resistance exercises with the elbow muscles stretched out or in varied positions leads to a measurable increase in the size of the elbow flexor muscles in people who regularly work out but are not elite athletes.