Creatine kinase is an enzyme that catalyses the breakdown of phosphocreatine into creatine and phosphate, releasing the energy muscles use for short, intense bursts of activity. The phosphocreatine system is the fastest energy pathway, powering sprints and heavy lifts for seconds before glycolysis takes over. Clinicians measure creatine kinase in blood as a marker of muscle damage, since injured muscle leaks the enzyme; very high levels follow heart attacks, strenuous exercise, or statin-related muscle injury. Athletes supplement with creatine to enlarge phosphocreatine stores, one of the few supplements with solid evidence for strength performance.