Which uses more muscles: smiling or frowning? Life processes - Body systems - Joints and muscles Learning Points Muscles are vital for movement. Skeletal muscles are those that are attached to bones. Muscles often work in pairs, contracting and relaxing, to move bones. A Twig Curriculum Film. Delivering key learning points. Get straight to the facts in just three minutes – directly linked to core learning, Twig's curriculum films will fit neatly into your lesson plans.