Abnormal muscle fiber morphology (HP_0004303) can be described as follows. Any abnormality of the skeletal muscle cell. Muscle fibers are subdivided into two types. Type I fibers are fatigue-resistant and rich in oxidative enzymes (they stain light with the myosin ATPase reaction), and type II fibers are fast-contracting, fatigue-prone, and rich in glycolytic enzymes (these fibers stain darkly). Normal muscle tissue has a random distribution of type I and type II fibers. Also known as: Abnormal muscle fibre morphology, Abnormal skeletal muscle fiber morphology, Abnormal skeletal muscle fibre morphology, Abnormality of muscle fibers, Abnormality of muscle fibres.