Muscle Fiber Types
Endurance sports are all about movement. And to move our bodies through space and time we use muscles. Muscles operate when a signal is sent from the central nervous system (CNS) to individual cells (also called muscle fibers) in your skeletal muscles. As a sufficient number of muscle fibers are recruited for the task, they then contract to produce movement toward your goal.
Skeletal muscle fibers come in different flavors. When you see the difference between the leg meat and breast meat of a chicken, you know that this is literally true—dark meat tastes different than light meat. These different color patterns are a result of the chicken legs consisting of a higher percentage of slow-twitch muscle fibers while chicken breasts contain a higher percentage of fast-twitch muscle fibers.
Slow-twitch muscle fibers—also called Type I—contract more slowly, just as the name implies. They are built to bring in oxygen and maximize the production of energy through the aerobic pathway. To those ends, they contain a greater number of capillaries and proteins called myoglobin to carry oxygen into the cells, along with a greater number of mitochondria which act as cellular factories for aerobic energy production. The iron-rich pigments associated with myoglobin are responsible for the color of dark meat.