Muscle fibers comprise muscles. They are made of myofibrils, which are strands of protiens, most notably actin and myosin. Myofibrils, actin and myosin are responsible for the muscle's ability to contract. Muscle contraction is accomplished by the placement of actin in relationship to the myosin, and the fact that the myofibrils run the length of the muscle. Additionally, a chemical called ATP must be present, along with an impulse from the CNS
An interesting and useful fact about muscle fibers is that they run in specific directions, relating to the function of that muscle. The direction of muscle fibers influences the line of action made when the muscle contracts.