Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that are integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Troponin I binds to actin in thin myofilaments to hold the actin-tropomyosin complex in place. Troponin (cardiac I and T) are sensitive and specific indicators of damage to the heart muscle (myocardium). Troponins are increased in around 40 % of patients with critical illnesses such as sepsis. In 2020, it was found that COVID-19 patients with severe disease had higher troponin I levels compared to those with milder disease.