Break point of serum creatine kinase release after endurance exercise.

We investigated whether there is a break point of creatine kinase (CK) release after daily endurance exercise and whether CK response depends on individual physical characteristics. Fifteen healthy young men performed 90 min of bicycle exercise for 3 consecutive days. Body composition, properties of the quadriceps femoris muscle (QFM), and aerobic and anaerobic capacities were estimated before the test. Blood samples were obtained 22 times during the experimental period. Endurance exercise significantly elevated serum CK from 3 h after the first exercise session (P < 0.05) and gradually increased thereafter. Subjects were classified into two groups according to their peak CK values: high responders (HR; >500 IU/l of CK) and low responders (LR; <300 IU/l of CK). Peak CK values during the experimental period correlated (P < 0.01) with workload/cross-sectional area of the QFM (r = 0.658), workload/volume of the QFM (r = 0.648), and knee extensor strength/body mass (r = -0.634); however, the HR and LR groups were separated in each variable. Thus the break point of CK release after endurance exercise under these conditions is 300-500 IU/l, two or three times higher than in the resting condition, and is associated with properties of the QFM.