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Why do slow-twitch muscle fibers contain many mitochondria?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Slow-twitch muscle fibers, also known as type I fibers, contain a high density of mitochondria because they rely primarily on aerobic metabolism to generate energy for sustained activity.1

These organelles are essential because they house the enzymes required for the Krebs cycle and oxidative phosphorylation. Through these processes, the muscle fiber efficiently produces ATP—the immediate source of energy for muscle contraction—using oxygen. This aerobic capacity allows slow-twitch fibers to resist fatigue, making them highly effective for endurance tasks and maintaining posture over long periods.21