Nephritis is the most common cause of blood in the urine of children and young adults but it can occur at any age. If your GP suspects nephritis he will usually refer you to a nephrologist (kidney doctor).
Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. These fast twitch fibers can be further categorized into Type IIa and Type IIb fibers, which are also known as fast twitch oxidative and fast twitch glycolytic, respectively.
There are two important types of skeletal muscle. Type I, also called slow-twitch, is composed of small fibers and is used primarily for carrying light loads. This type of fiber predominates in a runner who is very thin and whose body contains a lot of lean muscle.