University College London explains that skeletal muscle cells have more than one nucleus in each cell because each cell is actually a combination of several cells that have fused to each other. During the process of fusion, the combined cell retains the nuclei of its constituent cells. In addition to having an abundance of nuclei, skeletal muscle cells also contain multiple mitochondria.
Animals and humans consciously control their skeletal muscles. The University of California, San Diego, describes skeletal muscle cells as long, tube-like cells that work in conjunction with numerous other cells. A given muscle, such as the bicep, contains thousands of individual muscle cells that all work together.
Skeletal muscles function by contracting when stimulated. The University of California, San Diego explains that this stimulation usually takes the form of an electrical impulse from the nervous system, but other stimuli may also cause muscles to contract. Muscles do not have a discrete process for elongating after they contract other than the natural elasticity of the muscles, which pulls them back into their original shape after they contract. Additionally, as muscles often work as part of an opposing pair, when one muscle contracts, the other necessarily elongates.
According to About.com, mitochondria are the organelles that conduct most of the cellular respiration that takes place, and therefore, provide energy to the cell. As muscles must perform large amounts of work, this preponderance of mitochondria in the muscle cells makes sense.