Creatine kinase (CK)

Creatine kinase (CK) is an enzyme that catalyses the reversible transfer of a phosphate group from phosphocreatine to ADP, regenerating ATP in tissues with high and fluctuating energy demands, principally skeletal muscle, cardiac muscle, and brain. Serum CK is released when these tissues are damaged, with isoenzyme fractionation — CK-MM (skeletal), CK-MB (cardiac), CK-BB (brain) — guiding organ-level attribution; markedly elevated CK-MB or troponin-confirmed CK-MB fraction remains a diagnostic criterion for myocardial infarction in some protocols. Transient CK elevations after unaccustomed strenuous exercise are expected and benign, whereas persistent elevation suggests myopathy, rhabdomyolysis, or statin-induced muscle toxicity. In sarcopenia research, resting CK and its trajectory are explored as indirect indices of muscle membrane integrity.