The document describes the process of muscle contraction. It begins with the muscle fiber being stimulated, which causes calcium ions to be released from the sarcoplasmic reticulum. The calcium ions then attach to troponin and tropomyosin, exposing active sites on the thin filaments. Myosin cross-bridges attach to these active sites and pivot, pulling the thin filaments toward the center of the sarcomere and causing the muscle fiber to contract. This cycle of cross-bridge formation and pulling of the thin filaments continues as long as calcium ions are present.