1. The present study examined if the presence of creatine kinase (CK) inhibitors might explain the large variability in blood levels of CK among subjects after exercise-induced muscle damage.

2. Twenty-four women performed an eccentric exercise with the forearm flexors and were then classified as no CK responders, low CK responders and high CK responders. High CK responders repeated the exercise two weeks later (bout two).

3. Sera from high CK responders were mixed with sera from no CK responders or low CK responders. Also, serum from high CK responders obtained after bout one was mixed with the same subject's serum from after bout two.

4. In all cases, the differences between the expected and observed CK activity for the mixes were within the expected variability for the assay.

5. Although CK inhibitors have been previously observed in sera from patients with muscle injury or disease, we were unable to demonstrate the presence of CK inhibitors in the sera from subjects who showed evidence of exercise-induced muscle damage.

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