When young men who don't work out start doing arm exercises twice a week for 10 weeks, their biceps get about 6% bigger.

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.

After 10 weeks of hard lifting, your arms only get a tiny bit bigger — less than the thickness of a penny — no matter which method you use.

If young men who haven't trained before do lat pull-down exercises two times per week for about 2 and a half months, their arm muscles get about 10% stronger.

If young men who haven't exercised before do either multi-joint exercises (like pull-ups) or single-joint exercises (like bicep curls) for 10 weeks, their arm strength increases by about the same amount - neither type of exercise is clearly better than the other.