Abstract
Creatine kinase (CK) activity has been generally considered as reliable blood marker for assessing muscle function, damage, and repair. However, the greatest challenge in the interpretation of CK activity remains the high variability in CK increase in relation to degrees of muscle cell damage or disturbance. Several known contributors to CK variability have been identified. The most important include the type of training, exercise intensity, gender differences, body composition, intra- and interindividual biological variability, as well as preanalytical and analytical considerations. Creatine kinase variability following different types of exercise reflects the complex interplay between exercise modality, intensity, individual physiology, and recovery strategies. High-intensity exercises, especially those involving eccentric muscle contractions, tend to produce more significant CK responses due to greater muscle fiber disruption. Gender differences in CK variability are pronounced, with men generally exhibiting higher CK activities following exercise compared to women. Creatine kinase variability is also closely linked to body composition, with muscle mass generally leading to higher CK activities post-exercise, while higher body fat may correlate with lower CK responses. Regarding preanalytical and analytical considerations, perhaps the greatest challenge in CK measurement is the limited sample stability, which should always be taken into consideration when analyzing CK activity in stored samples for research or clinical purposes. This review, through exploring all of the above-mentioned sources of CK variability, could facilitate the development of evidence-based practices for preventing overuse injuries, and promoting long-term athlete health and well-being.