The repeated bout effect: is the blunted creatine kinase response an effect of an altered enzyme inactivation...

If unaccustomed lengthening contractions are repeated within a certain period of time, muscle damage symptoms are blunted. This observation, often referred to as the repeated bout effect (RBE), also holds true for the response of muscle damage markers like creatine kinase (CK). However, measuring plasma enzyme activity rather than the concentration of enzyme protein might conceal the actual amount of damaged tissue. Therefore, the primary aim of the study was to investigate if the RBE of CK can partially be explained by enzyme inactivation.

Ten healthy male subjects performed two bouts of 100 drop-to-vertical jumps (DVJs) from a 70-cm high platform at an interval of three weeks. CK activity, CK concentration, and neutrophils were measured prior to, and on four consecutive days after the interventions.

Besides significant main effects, there was a significant group by time interaction for the specific CK activity (CK activity in blood [U/L] divided by the enzyme concentration [ng/mL]). Higher values following the first bout (133.1±99.4 U/µg) than the second bout (94.7±63.0 U/µg) indicate that the ratio of inactive to active CK molecules increased. Neutrophil levels were similar following both bouts and differed only at 8 hours (7.0±2.5 bout 1, 5.1±1.6 bout 2).

The findings of the present study support the hypothesis that the blunted response of CK activity after a repeated bout of eccentric exercise is not solely the result of tissue protection, but can be at least partially attributed to enzyme inactivation.