• As an action potential propagates along the sarcolemma it invades the transverse tubules and causes the rapid release of Ca2+ from the terminal cisternae into the sarcoplasm.• Once in the sarcoplasm Ca2+ diffuses among the filaments and binds reversibly to troponin, which results in the displacement of the troponin tropomyosin complex and activates the sliding of the thick and thin filaments. • Because a single action potential is insufficient to release enough Ca2+ to bind all available troponin sites in skeletal muscle, the strength of a contraction increases with the action potential rate.