The presence of disorganized areas called cores in the center of muscle fibers. There is a typical appearance of the biopsy on light microscopy, where the muscle cells have cores that are devoid of mitochondria and specific enzymes. Cores are typically well demarcated and centrally located, but may occasionally be multiple and of eccentric. The phenotype is Central core regions in muscle fibers (Human Phenotype Ontology id HP_0030230). Also known as: Central core regions in muscle fibres.