Normal levels of creatinine in the blood are approximately 0.6 to 1.2 milligrams (mg) per deciliter (dL) in adult males and 0.5 to 1.1 milligrams per deciliter in adult females. Muscular young or middle-aged adults may have more creatinine in their blood than the norm for the general population.
Types of muscle fibers. Skeletal muscle, the type that is responsible for moving our muscles when we run, is comprised of three different muscle fiber types, each with its own advantages, disadvantages, and specialty.
Humans have three different types of muscle fibers: slow-twitch (Type I), fast-twitch A (Type IIa), and fast-twitch B (Type IIb). Type I muscle fibers are recruited for aerobic activities, so therefore contract slowly, but are very resistant to fatigue.