Plasma creatine kinase after the marathon--a diagnostic dilemma.

The mechanism of the protein leak from exercised muscle remains obscure, but may be related to depletion of intracellular high-energy phosphate and/or to mechanical disruption. The high levels of creatine kinase (CK) and other muscle proteins found in plasma for several days after marathon running, especially downhill running, are due to protein efflux from skeletal muscle. There is no evidence that marathon running damages the healthy, well-perfused myocardium, despite the fact that the plasma levels of total creatine kinase (CK), the isoenzyme CK-MB, CK-MB/total CK (%), myoglobin, aspartate transaminase, lactate dehydrogenase and tropomyosin may be the same as after myocardial infarction. These indices must be interpreted with the greatest caution when found in anyone who habitually undertakes strenuous exercise, especially if they have done so within the previous week.