Myosin is a substance that causes muscle filaments to thicken and shorten. Actin is a substance that forms thin filaments along which the myosin uses as a guide. The myosin causes the muscle filament to "ratchet" or move along, the actin substances thin filament, causing themuscle fiber to become shorter. It is important to understand this is done with smaller muscle fibers first, then working up to larger muscle fibers. It is also important to understand that skeletal muscles, shen they contract, only use about one-third of their total contractive power, so as to avoid fractures and breaks in the skeletal system. The myosin causes the muscle fibers to shorten, one by one, moving them in a pattern along the thin membranes created by the actin. Working in conjunction with each other, the muscle can get shorter, or can also lengthen out. The three things a skeletal muscle can do is get shorter, get longer, or do nothing.