1. Calcium is released from the sarcoplasmic reticulum into the cytoplasm and binfs to troponin, which changes troponin's shape.
2. Troponin "nudges" tropomyosin into the actin grooves, thus uncovering the myosin binding sites.
3. The energized myosin heads thus bind to the actin and perform a "power stroke" that pulls actin toward the center of the sarcomere.
4. ADP is released from the heads and a new ATP takes it place, coming from the mitochondria, which is quickly broken down into ADP + P, but which allows the myosin to detach from the actin filament, perform a "recovery stroke", and be ready to contract again.
5. If calcium is still present this process simply repeats.