Changes in serum creatine kinase and hexose phosphate isomerase activity with exercise duration.

In 18 groups of trained healthy male subjects (n = 166; age 17--42 years) the influence of type and duration of long-lasting exercise on the changes of creatine kinase (CPK) and hexose phosphate isomerase (PHI) activity in serum was studied. Above 65--70% of maximal aerobic capacity, a serum enzyme activity increase proportional to the exercise time was found with a different slope in each of the three exercise types examined [a) bicycle ergometer; b) ski race; c) impact-type exertion]. Work duration of more than 300 min induced in the third type of exertion an accelerated increase in serum CPK. A third degree equation best describes the changes in serum CPK for nine of our groups and five others from the literature. To ascertain the possible influence of intravascular hemolysis, sera were mixed with autologous erythrocyte hemolysates and enzyme activity measured. The findings are discussed with reference to critical aspects of the analytical methods, possible causes of serum enzyme changes, and possible use of enzyme measurements in evaluating adaptation of the organism to prolonged exercise.