Muscles are always attached at two ends by tendons - an origin and an insertion - which allow them to pull against bones and create movement. They work in pairs, with an agonist muscle contracting to cause movement and an antagonist relaxing to prevent injury. Inside, muscles are made of fascicles containing bundles of fibers called myofibrils. Myofibrils contain sarcomeres, the contractile units, which shorten when cross-bridges on myosin filaments pull actin filaments inward. This process occurs along the entire muscle fiber, causing contraction.