Red muscle cells, most commonly found in slow twitch fibers, have abundant mitochondria in order to produce adenosine triphosphate (ATP) used for muscle metabolism.

Muscle is a bundle of fibrous tissue that can contract, producing movement in or maintaining the position of parts of the body. There are three major types of skeletal muscle fibers: fast twitch, slow twitch, and intermediate. Slow twitch fibers respond much more slowly and generate lower amounts of force than fast twitch fibers. They are capable of sustaining long contractions and, consequently, contain a large number of mitochondria in order to obtain their adenosine triphosphate (ATP) from aerobic metabolism. Slow twitch fibers are surrounded by capillary networks that supply oxygenated blood for use in the aerobic energy systems. They also contain a red protein called myoglobin. Myoglobin can bind oxygen (like hemoglobin) and provide a substantial oxygen reserve. Due to the reddish color of myoglobin, these fibers are often called red muscle fibers.

Key Points

• There are three major types of skeletal muscle fibers: fast twitch, slow twitch and intermediate.

• Slow twitch fibers are capable of sustaining long contractions and, consequently, contain a large number of mitochondria in order to obtain their (ATP) from aerobic metabolism.

• Slow twitch fibers are also surrounded by capillary networks that contain a red pigment called myoglobin and supply oxygenated blood for use in the aerobic energy systems.

• Due to the reddish color of myoglobin, slow twitch fibers are often called red muscle fibers.

Key Terms

Adenosine triphosphate (ATP): the molecule containing high-energy bonds used to transfer energy between systems within a cell

Slow twitch fibers: Of, relating to, or being muscle fiber that contracts slowly especially during sustained physical activity requiring endurance.

Fast twitch fibers: Of, relating to, or being muscle fiber that contracts quickly especially during brief high-intensity physical activity requiring strength

Intermedia fibers (also known as fast oxidative-glycolytic fibers): Fast twitch muscle fibers which have been converted via endurance training.

Myoglobin: A protein found in the muscle cells of animals; an oxygen-storage unit.

Aerobic: A combination of glycolysis and the Krebs cycle, an efficient but slow way of producing ATP.