The coordinated contraction and relaxation of muscles that produces movement, maintains posture, and powers vital processes. Skeletal muscles pull on bones to enable walking, lifting, and breathing movements, while smooth muscle propels food through the gut and regulates blood vessels, and cardiac muscle pumps blood tirelessly. Contraction consumes ATP generated from food energy, with calcium signals triggering the sliding of actin and myosin filaments. Protein intake, resistance exercise, and adequate calcium, magnesium, and vitamin D all support healthy muscle function. Loss of muscle with ageing, sarcopenia, impairs strength and independence, making lifelong activity essential.