Creatine kinaseCK · Creatine phosphokinase · CPK
A muscle enzyme that rises when muscle is damaged, including by hard training.
Creatine kinase is an enzyme that helps muscle cells regenerate energy rapidly. It sits inside muscle fibres, so when those fibres are damaged it leaks into the blood.
It matters as the standard marker of muscle injury, and because normal exercise raises it substantially. Unaccustomed or eccentric training can push it many times above the usual range in a perfectly healthy person, peaking a day or two afterwards.
In practice it is interpreted with recent training in mind, which is why lab reference ranges built on sedentary populations often flag trained people. Very high values with dark urine after extreme exertion are a medical emergency, not a training badge.