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.

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 extra arm isolation exercises like bicep curls and triceps extensions doesn't help you get stronger or build more arm muscle than just doing big compound moves like bench presses and pull-downs — at least not for beginners after 10 weeks.

If young men who have never weight trained before do bicep curls twice a week for 10 weeks, their arm strength increases by about 12% on average.

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.