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.

If young men lift weights for 15 weeks, their biceps get significantly stronger—by about 20%—no matter if their body releases extra muscle-building hormones during the workout or not.

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.

If you're a guy who already lifts weights, doing full-body workouts five days a week will make your forearm and thigh muscles grow bigger than if you only work each muscle group once a week, after eight weeks of training.

After doing bicep curls 24 times over 12 weeks, women’s upper arm muscles got noticeably thicker, no matter which way they did the curls.

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.