When people who aren't very fit start exercising, their muscles grow new blood vessels quickly in the first month, but after that, even if they keep working out, they don’t get many more new blood...

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.

If you've lifted weights before, it doesn't make a difference in how your muscles rebuild blood vessels during rest and recovery after not using them, compared to someone who hasn't trained before.

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.