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 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.

If young adults stop using a limb for two weeks and then do eight weeks of strength training, their muscle fibers get bigger and gain more blood supply.

In older adults who are healthy, muscle growth from consistent strength training is not affected by how many blood vessels are already present in the muscle before training begins.

If you're new to working out, doing leg exercises for 10 weeks can make one of your big thigh muscles grow bigger, even if the individual muscle fibers don't get thicker.

After about 10 weight workouts, your muscles don’t just look puffy from water anymore—they actually start getting bigger because the muscle fibers themselves are growing.