Lifting weights for three months makes the muscle fibers in young men's biceps get bigger — both the endurance-type and power-type fibers.

Lifting weights for three months didn’t increase the number of muscle fibers in young guys’ biceps, so their muscles got bigger without adding new fibers.

Lifting weights for three months increases the number of tiny blood vessels in arm muscles of young men, and the growth keeps up with how much the muscle fibers get bigger.

Lifting weights for up to 6 months probably doesn’t increase the number of muscle fibers in your arms or legs — instead, your muscles grow because the existing fibers get bigger, not because you grow new ones.

When young men who don't normally exercise do eight weeks of weight training, their fast-twitch muscle fibers get bigger, but their slow-twitch fibers stay the same size and the overall mix of muscle types doesn't change.

When people lift weights, their arm and leg muscles show similar changes in muscle fiber numbers — and in both, we don’t see clear signs of new muscle fibers forming.