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. As mentioned in Chapter 10, myoglobin is an iron-containing protein that binds oxygen and serves as an oxygen reserve for these cells during exercise.
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