Hitting the Wall? Could you be over-exercising? So, how do you know if you’re over-training? Creatine kinase (CK) is a type of enzyme that is located in several tissues in the body, mainly in muscle. In normal conditions, there is a small amount of creatine kinase circulating in the blood below 200 units per liter in men and women, but when muscle damage occurs, the amount of CK in the blood can rise substantially.