If healthy guys train one arm with heavy weights three times a week for three months—whether they push to total muscle exhaustion or not—they’ll get stronger, their muscles will grow, and their brains will activate those muscles better.

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.

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.

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 regular gym-goers train their biceps by lifting in a way that stretches the muscle more, they’ll likely gain more muscle size over two months.