Creatine kinase (CK), one of the most characteristic muscle tissue enzymes, catalyzes transfer of phosphate from phosphocreatine to adenosine 5′-diphosphate (ADP). The CK activity during exercise depends largely on the duration and intensity of exercise. The physiological range of activity of this enzyme in the blood plasma of clinically healthy horses is in the range from 90 to 275 U/L. Common phenomenon, especially in endurance horses, is the occurrence of a much larger post-exercise CK activity. However, the increase in CK activity in the blood plasma is a frequent phenomenon in endurance horses and should not be interpreted as an undesirable symptom, unless it is accompanied by clinical signs of myopathy or is still persisting over a longer period of time to increase the activity of this enzyme in the blood. It is recommended to collect blood from the horse directly before and after (up to 5 min.) exercise and 15, 30, 60 minutes or 24 hours after the exercise. Due to the various analytical methods applied and various units in which the results are presented, it seems that it would be appropriate to present variability of post-exercise CK activity in percentages, which will facilitate the assessment of this phenomenon in different individuals, often subjected to different loads. In horses especially important for diagnostic purposes is a muscle isoform (M-CK), present only in striated muscle, which probably determines to a large extent, their potential aerobic. The paper discusses creatine kinase function and usefulness for determining the activity of this enzyme during the physical effort of horses.

Key words: Creatine kinase, Horse, Physical effort