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Why do some muscles appear red and others appear white?
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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.
The color of a muscle is primarily determined by its concentration of myoglobin, an oxygen-carrying protein, and the presence of cytochromes involved in oxidative metabolism.123
- Red muscles: These muscles contain a high concentration of myoglobin and many mitochondria. They are typically composed of slow-twitch fibers, which are specialized for steady, prolonged activity and rely on oxidative metabolism to produce energy.1243
- White muscles: These muscles have lower concentrations of myoglobin and fewer mitochondria. They are primarily composed of fast-twitch fibers, which are adapted for short, intense bursts of activity. These fibers rely more heavily on anaerobic glycolysis to generate energy, a process that does not require oxygen.123
The distinction is clearly visible in many animals; for example, the dark meat of a chicken's legs consists of red, fatigue-resistant fibers used for standing and walking, while the pale breast meat consists of white fibers used for occasional, rapid bursts of flight.23