Anatomical wise the characteristics of these muscle fibers are acertained by their twitch. Fast twitch is the majority of skeletal muscle, is large in diameter and is densely packed with myofibril (fibrous strands of muscle), is capable of a lot of tension. They also have the least amount of capillaries and mitochondria (which is explained in their metabolic process). Metabolically Fast twitch muscles rely more on anaerobic metabolism (without oxygen), but require a greater amount of Glucose and Glycogen, this is why they fatigue easy.
Slow twitch is just the opposite, their diameter is much smaller, they aren't capable of much tension, they have more capillaries and mitochondria, and are sower. They require less glucose but more oxygen, and thrive on aerobic metabolism.
Intermediate twitch, are similar in appearance to fast twitch muscles, but have the endurance offered by slow twitch.
