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.

If regular gym-goers train their biceps by lifting in a way that stretches the muscle more, they’ll likely gain more muscle size over two months.

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 someone who works out casually, lifting weights with your muscles stretched might give you slightly bigger arms compared to mixing stretched and shortened positions — but the difference is small and we're not sure if it really matters in real life.

When young men who haven't trained before do either multi-joint exercises (like curls) or single-joint exercises (like bicep curls) for 10 weeks, their arm muscles grow by about the same amount - there's no meaningful difference between the two types of workouts.

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.