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A muscle fiber (AE) or muscle fibre (BE) (also technically known as a myocyte) is a single cell of a muscle. Muscle fibers contain many myofibrils, the contractile unit of muscles. Muscle fibers are very long; a single fiber can reach a length of 30cm.
Muscle fibres can be grouped according to what kind of tissue they are found in -- skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle fibers can be further divided into two basic types, type I (slow-twitch fibers) and type II (fast-twitch fibers). The muscle cells of heart muscle tissue are called cardiomyocytes.
Type I
Type I muscle fibers (slow-oxidative fibers) use primarily cellular respiration and, as a result, have relatively high endurance. To support their high-oxidative metabolism, these muscle fibers typically have lots of mitochondria and myoglobin, and thus appear red or what is typically termed "dark" meat in poultry. Type I muscle fibers are typically found in muscles of animals that require endurance, such as chicken leg muscles or the wing muscles of migrating birds (e.g., geese).
Type II
Type II muscle fibers use primarily anaerobic metabolism and have relatively low endurance. Due to their low-oxidative demand, these muscle fibers have low levels of mitochondria and myoglobin, and thus appear white. These muscle fibers are typically used during tasks requiring short bursts of strength, such as sprints or weightlifting. Type II muscle fibers cannot sustain contractions for significant lengths of time, and are typically found in the white meat (e.g., the breast) of chicken.
There are two sub-classes of type II muscle fibers, type IIa (Fast-Oxidative) and IIb (Fast-Glycolytic). Type IIb (Fast-Glycolytic) tire the fastest, and are the prevalent type in sedentary individuals. Some research suggests that these subtypes can switch with training to some degree.
The biochemical difference between the three types of muscle fibers is their myosin heavy chains.
See also
Electrically responsive cells | Contractile cells
Links
http://www.coachr.org/fiber.htm - Great Article Explaining muscle fibers, especially in relation to training.
"Muscle fiber"