If the mature red blood cell has no nucleus, the opposite is true for the mature skeletal muscle cell - it has several nuclei per cell. How did this happen?
Well, the myoblasts or young muscle cells start out as uninucleated cells. Sometime during their development however, these cells fused with each other and become surrounded by connective tissue. The fused cells then elongate and develop into what is now called muscle fiber or the mature muscle cell. Thus, a single muscle fiber is equivalent to one muscle cell with multiple nuclei.
Muscle fibers are also bundled together by connective tissue and form what we recognize as our muscles. So our triceps or biceps and other muscles actually consist of bundles of muscle fibers.