When people want to build bigger biceps, they should focus on bicep curls rather than compound movements like pull-ups that work multiple muscle groups at once.

If you want bigger biceps, you should focus on exercises that only move your elbow (like bicep curls) instead of exercises that involve multiple joints (like pull-ups).

When untrained adults want bigger biceps, doing simple curl exercises works better than doing rows that use multiple muscles at once.

When men who hadn't trained before did bicep curls (a single-joint exercise) for 8 weeks, their arm muscles grew about 11%. But when they did dumbbell rows (a multi-joint exercise) instead, their muscles only grew about 5%.

Doing bicep curls leads to bigger arm muscles (11% growth) than doing dumbbell rows (5% growth) after training for 8 weeks, at least in men who haven't exercised before.

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.