Muscle fibers are the basic units of skeletal muscle tissue. They can be broadly classified into two main types: type 1 and type 2 fibers. Type 1 fibers are known for their endurance and are often referred to as slow-twitch fibers. They contain a high number of mitochondria and are rich in myoglobin, which gives them their red color. Type 2 fibers, or fast-twitch fibers, are designed for quick, powerful movements and have less endurance. They can be further divided into type 2A and type 2B fibers, with type 2A being somewhat more resistant to fatigue than type 2B.
