Type I fibers (sometimes called slow-twitch or slow-oxidative fibers) are better designed for prolonged exercise performed at a lower intensity. In comparison to Type II fibers, Type I fibers will have more mitochondria and rely more heavily on the aerobic generation of ATP. The primary energy molecules used to generate ATP in these muscle cells will be fatty acids and glucose. Since ATP production in mitochondria requires oxygen, proper function of these muscle fibers is very dependent upon oxygen supply via the blood. Luckily, Type I muscle cells always seem to have many capillaries around them to deliver oxygen-endowed blood. In addition, Type I fibers contain a substance called myoglobin which is an iron-containing protein that binds oxygen and serves as an oxygen reserve for these cells during exercise.
Type II muscle fibers (sometimes called fast-twitch or fast-glycolytic fibers) can execute a much faster speed of contraction than Type I muscle fibers. This is to say that Type II muscle fibers are designed to generate force more rapidly, thereby allowing them to be more powerful. This will allow a job to be performed in a shorter amount of time. Meanwhile, Type II muscle fibers are relatively limited in their ability to generate ATP by aerobic means. So, when these cells break down glucose to pyruvate andgenerate a couple ATP in the process, much of the pyruvate that is formed will then be converted to lactic acid (lactate). This is because these muscle cells have less mitochondria and receive less oxygen as they are served by fewer blood vessels.