An abnormally reduced activity of the enzyme cytochrome C oxidase in muscle tissue. The phenotype is Human Phenotype Ontology identifier HP_0003688 (Cytochrome C oxidase-negative muscle fibers). Also known as: Cytochrome C oxidase-negative muscle fibres, Cytochrome c oxidase deficiency in skeletal muscle, Decreased activity of cytochrome C oxidase in muscle tissue, Decreased skeletal muscle cytochrome c oxidase activity.