Myoglobin is a protein present in the heart and other muscles. When a heart attack occurs Myoglobin is one of the first cardiac biomarkers to rise in the blood. Levels increase within 2-3 hours of a heart attack or other muscle injury, reach their highest levels by about 8-12 hours, and generally fall back to normal by about one day after injury occurred. Consequently, myoglobin testing is used to help rule out a heart attack in conjunction with other markers such as troponin.