How does the release of calcium ions regulate muscle contraction?

Tropomyosin is a protein that covers the actin binding site. This prevents a cross-bridge forming between the myosin and actin filaments whilst the muscle is at rest. Calcium ions bind to troponin molecules, changing their shape. This change in the troponin moves the tropomyosin away from the actin binding site enabling the myosin head to attach. When the muscle relaxes, calcium ions are removed and the tropomyosin returns to its original position.