When the muscle fiber is electrically excited, calcium (Ca²⁺) ions released from the sarcoplasmic reticulum bind to the troponin C subunit of the troponin molecules on the actin filaments, with four calcium ions binding to each troponin molecule. Calcium binding causes allosteric changes in the configuration of troponin molecules that affect both the troponin T and I subunits, and subsequent changes in the troponin-tropomyosin-actin interactions ultimately allow tropomyosin to “unblock” the actin-binding sites for the myosin crossbridges. These sites are then bound by the closest myosin head groups, with the attached ADP and P_i. At this point, the thick and thin filaments are mechanically connected but no movement has occurred. Movement requires the head groups to change their angle and drag the thick and thin filaments past one another, and this process involves another conformational change of the myosin head groups that is tightly coupled to release of the P_i ion, followed by release of ADP. This results in a change in the angle of the head group, tightly bound to actin, causing the thin filament to be pulled toward the middle of the sarcomere, and the sarcomere is shortened.

The flexed position of the myosin head groups bound to the actin of the thin filament is called the rigor complex. It is so named after the term rigor mortis, because, after death, muscle fibers run out of ATP and all the myosin and actin molecules are tightly crosslinked in this configuration. However, in healthy muscle, ATP rapidly binds to myosin, causing release of the actin filament and the beginning of a new cycle. When electric activity ceases, excess calcium is rapidly taken up by the sarcoplasmic reticulum. Without calcium bound to the troponin, the head groups cannot bind actin. The cycle is interrupted, the sarcomeres lengthen, and the muscle once again relaxes.