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 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 you're someone who works out a little but not super hard, just keeping up your usual routine for 3 months can actually make your biceps bigger and stronger — you don’t need to go all out.

If untrained adult men lift weights twice a week for 8 weeks, doing 4-6 sets of 8-12 reps until they can't do any more, their upper arm muscles will get bigger - this is measurable with ultrasound or other imaging.

When young men who haven't trained before do arm exercises twice a week for about 2 months, their biceps and related muscles grow by about 6% on average.

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.