A muscle fiber (American usage) or muscle fibre (British usage) 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.

Skeletal muscle fibers can be divided into two basic types, type I (slow twitch fibers) and type II (fast twitch fibers).

Type 1 muscle fibers (slow oxidative fibers) use primarily oxidative metabolism 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 1 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 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 burts of strength, such as sprints or weightlifting. Type II muscle fibers cannot sustain contractions for significant lenghts 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 Oxidative 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.