Why does muscle contraction occur?

Muscle contraction occurs due to the interaction between actin and myosin filaments within muscle fibers. When a muscle is stimulated by a nerve impulse, calcium ions are released, triggering the binding of myosin heads to actin, forming cross-bridges. This interaction pulls the actin filaments closer together, shortening the muscle fiber and generating force. ATP provides the energy required for this process, allowing the myosin heads to detach and reattach for continued contraction.