Any of the cells of skeletal or cardiac muscle tissue. Skeletal muscle fibres are cylindrical multinucleate cells containing contracting myofibrils, across which run transverse striations, enclosed in a sarcolemma. Cardiac muscle fibres contain one or sometimes two nuclei and myofibrils and are separated from one another by an intercalated disk; although striated, cardiac fibres branch to form an interlacing network.
2. fast-twitch muscles. Skeletal muscle fibres having high myofibrillar atpase activity, high glycolytic enzyme activities, and an intermediate glycogen content which produce a fast twitch. There are two types. Fast fatigable fibres, also called white fibres, have a low myoglobin content, and a small mitochondrial content, and fatigue rapidly due to their limited glycogen content and low capacity for oxidative metabolism. Fast fatigue-resistant fibres, also called red fibres, have a large mitochondrial content and a high myoglobin content, related to their resistance to fatigue.
3. slow-twitch muscles. Skeletal muscle fibres having low 63b myofibrillar atpase activity, low glycogen content, and high myoglobin content, high mitochondrial oxidative enzyme activities, and an intermediate mitochondrial content which produce a slow twitch and are fatigue-resistant.