Troponin is a complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle. 

Troponin has three subunits, TnC, TnI, and TnT. When calcium is bound to specific sites on TnC, tropomyosin rolls out of the way of the actin filament active sites, so that myosin  can attach to the thin filament and produce force and/or movement. In the absence of calcium, tropomyosin interferes with this action of myosin, and therefore muscles remain relaxed.

Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm, also action potential. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction.

Both cardiac and skeletal muscles are controlled by changes in the intracellular calcium concentration. When calcium rises, the muscles contract, and when calcium falls the muscles relax.

Troponin levels can be used as a test of several different heart disorders, including myocardial infarction. When heart muscle is damaged, as in a myocardial infarction, troponins leak out of cells and into the bloodstream. Increased troponin levels indicate myocardial infarction or injury in a person with chest pain or pressure. Some MIs are silent, manifesting few if any symptoms. If infarction is ruled out in a person with continuing or recurring chest pain (unstable angina), an increased troponin level indicates the person has heart muscle ischemia and is at an increased risk for a future serious heart event.