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Serum creatine kinase (CK) levels are commonly used to judge the severity of muscle damage and to determine when to hospitalize patients who present with symptoms of exertional rhabdomyolysis in order to prevent renal failure. However, no CK standard exists because of the limited information available regarding exercise-induced CK elevation and renal function. This study determined the magnitude of CK elevation and the effect on renal function produced by exercise in a large subject group.Blood samples were obtained from 203 volunteers who performed 50 maximal eccentric contractions of the elbow flexor muscles. The samples, taken before and 4, 7, and 10 d after exercise, were analyzed for markers of muscle damage (CK, myoglobin (Mb), lactate dehydrogenase, alanine aminotransferase, and aspartate aminotransferase and for measures of renal function (creatinine, blood urea nitrogen, phosphorus, potassium, osmolality, and uric acid).All indicators of muscle damage increased significantly after exercise (P < 0.01). CK levels were 6420, 2100, and 311% above baseline on days 4, 7, and 10 after the exercise, respectively (P < 0.01), and Mb was 1137, 170, and 28% above baseline on days 4, 7, and 10 after exercise, respectively (P < 0.01). Of the 203 participants, 111 had CK values at 4 d postexercise > 2,000 U·L−1 and 51 had values > 10,000 U·L−1, levels used to diagnose myopathy (e.g., statin myositis) and rhabdomyolysis, respectively. There were no significant increases in any measure of renal function. Despite marked CK and Mb elevations in some subjects, none experienced visible myoglobinuria or required treatment for impaired renal function.Exertional muscle damage produced by eccentric exercise in healthy individuals can cause profound CK and Mb elevations without renal impairment.