This document discusses the structure and function of skeletal muscle fibers. Muscle fibers contain myofibrils, which are made up of repeating contractile units called sarcomeres. Sarcomeres contain thin filaments of actin and thick filaments of myosin that slide past each other during muscle contraction, powered by the hydrolysis of ATP and the release of calcium ions. The arrangement and interaction of these contractile proteins allow for muscle contraction and relaxation.