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

If you're new to strength training, having more tiny blood vessels around your fast-twitch muscle fibers might help those muscles grow more when you start working out.

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.

If you're just starting to get fit, different types of workouts like steady jogging, intense intervals, and all-out sprints all help grow similar numbers of tiny blood vessels per muscle fiber. But jogging-style workouts boost blood vessel density more because they don’t make your muscles grow as much, so the vessels aren’t stretched thinner.

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.