Disorders affecting skeletal (voluntary) muscle. Every skeletal muscle consists of a large number of muscle fibers, each of which is a multinucleated cell containing the usual types of cellular inclusions, but also containing the myofibrils, which are the actual contractile elements. The myofibrils have a banded structure which gives the muscle fiber its characteristic striated appearance on histologic examination. Muscle fibers have been separated into two main groups based on their histochemical reaction: type 1 fibers, which are rich in mitochondria and oxidative activity, and type 2 fibers.