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.

When untrained men do arm curls for 8 weeks, their biceps get thicker (about 11%). But when they do dumbbell rows instead, their biceps only grow about half as much (about 5%).

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%.

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

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 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.