Total creatin kinase (CK) and lactate dehydrogenase (LDH) serum levels depend on age, gender, race, muscle mass, physical activity, and climatic conditions. High CK serum levels in athletes following rest and without any further predisposing factors should prompt a full diagnostic workup, with special regards to signs of muscle weakness or other signs not always evident. In subjects who have silent myopathy, repeated intense prolonged exercise may produce negative effects, because given the continuous loss of muscle proteins, it does not induce the physiological muscle adaptations to physical training. Serum total LDH and specific isozyme activities change with the training status of the athlete. Variation in LDH isozymes profile might have a role in studying muscle response to training.