Should Creatine Kinase be tested at baseline in athletes?

Creatine kinase (CK) level depends on muscle mass, age, race, physical activity and various pathologies involving the muscles or the heart. Resistance training elicits the greatest release of CK, and is the best way to attain muscle hypertrophy. The highest post-exercise serum enzyme level is found after prolonged exercise such as ultradistance marathon running or weight-bearing exercises and downhill running, which produce eccentric muscular contractions. Persistently elevated CK levels in apparently healthy subjects may indicate a late-onset underlying condition. The decrease in the serum enzyme level depends on the period of rest after exercise, as short-term physical inactivity may reduce the lymphatic transport of CK and the release of the enzyme from the muscle fibres. We discuss the source and level of CK in various situations, the different CK isoenzymes and also when to test for an underlying myopathy or other pathologies which may impact the long-term athletic performance. It is likely safe for athletes with suspected myopathy to continue physical activity at a lower intensity to prevent muscle damage, and allow appropriate rest to favour recovery. Thyroid dysfunctions and drug-induced myopathies should be ruled out. History and clinical examination could help clarify whether other muscle-directed investigations are required.