Creatinine kinase is elevated in MI after

CK-MB remains a valuable marker of myocardial injury, second only to the cardiac-specific troponins . Total CK activity is not a reliable marker of cardiac injury since various isoforms of CK are also found in brain, myocardium, and skeletal muscle. However, the CK-MB isoform--principally derived from myocardium, but also present at low levels in skeletal muscle--is the more specific indicator of hea damage. CK-MB activity begins to rise within 2 to 4 hours of MI, peaks at 24 to 48 hours, and returns to normal within approximately 72 hours. (Robbins Basic pathology,9th edition.pg no.382)