Muscles are composed of cylindrical muscle fibers, and muscle fibers contain many longitudinally arranged myofibrils, which are the devices for muscle contraction. Myofibrils are composed of sarcomeres. In each sarcomere, thick filaments composed of myosin and thin filaments composed of actin, F-actin, are arranged interspersed with each other, and the two are closely contacted by the cross-bridges at the head ends of the thick filaments. Muscle contraction is the result of the relative movement of thick filaments and thin filaments. This process is regulated by Ca and requires the hydrolysis of ATP to provide energy. When the muscle is in a resting (relaxed) state, the cytosolic Ca concentration is low (<10moL/L), and the calcium ion-binding subunit (TnC) does not bind to Ca, so the binding of TnC to TnI and TnT is loose. At this time, TnT binds tightly to tropomyosin, causing tropomyosin to cover the binding site between actin and myosin, preventing the binding of actin and myosin; at the same time, TnI binds tightly to actin , also prevents the interaction between actin and myosin, and inhibits the ATPase activity of myosin, so the muscle is in a relaxed state. When the intracytoplasmic Ca concentration increases to 10moL/L -10moL/L, Ca binds to TnC. Afterwards, the conformation of TnC changes, thereby enhancing the binding force between TnC, TnI, and TnT, causing the three to tightly combine. The binding force between TnI and actin is weakened, causing actin to detach from TnI and become activated. At the same time, TnT moves tropomyosin to the depth of the actin spiral groove, eliminating the obstacle to the combination of actin and myosin. Therefore, actin combines with the head of myosin, Actomyosin with cross-bridges is produced. In this protein, actin greatly increases the ATPase activity of myosin, so myosin catalyzes the ATP hydrolysis reaction. The energy generated causes the cross bridge to change its angle, and the release of the hydrolyzate restores the position of the cross bridge, which is then combined with another ATP. In this cycle, the thin filaments slide along the thick filaments, and the muscles contract. When the cytosolic Ca concentration decreases (<10moL/L), Ca separates from TnC, and TnI binds to actin, thereby returning actin to its static state. At the same time, tropomyosin also returns to its original position, so that actin and myosin cannot combine, and the muscle cannot switch to a relaxed state.