Functional assessment of athletes’ fitness includes a variety of variables. Serum creatine kinase (CK) and lactate dehydrogenase (LDH) give an indication of the degree of metabolic adaptation to physical training of skeletal muscles. Both enzymes are involved in muscle metabolism, and their serum concentration is normally very low, a result of physiological wear and tear of the cell. They increase considerably after intensive exercise and in muscle pathology.^1,2

Changes in serum activity of muscle enzymes have been reported in normal subjects and athletes after strenuous exercise.^3–6 The amount of enzyme efflux from muscle tissue to serum can be influenced by physical exercise.⁷ Also, there are ethnic differences,⁸ and the differences between the sexes have been attributed to the protective effects of oestrogen on muscle cell membrane.⁹

Muscle biopsy findings have evidenced different activity of total LDH and LDH isozymes in endurance and strength athletes. The former had lower total LDH with a prevalence of LDH1–2 isoenzyme activity compared with the latter, who showed higher total LDH and prevalence of LDH5 activity. In addition, LDH and CK activity measured by needle biopsy showed different behaviour before and after training, and changes due to different protocols and intensity and level of training.^10–13 This is especially true for CK, the isoenzymes of which can be identified in different organs: CK1(BB) is mainly from the brain, CK2(MB) is mainly cardiac, and CK3(MM) is mainly from skeletal muscle.

Monitoring of CK and characterisation of its isoenzymes is widely used in the diagnosis of cardiomyopathies, encephalopathies, and muscle disease.^14–16 CK activity may increase after treatment with cholesterol lowering …