When a new weightlifter admires her bulging bicep or developing deltoids, she might be thinking that her larger muscles indicate she has grown new muscle cells. But the cells in skeletal muscle -- the muscles attached to the skeletal system that enable voluntary movement -- have a surprisingly long life span.

Skeletal muscle, also called striated muscle, is one of three types of muscle tissue in the body, along with cardiac muscle tissue in the heart and smooth muscle tissue lining other hollow organs in the body. Many scientists believe that skeletal muscle cells, like nerve cells, do not reproduce once they are created during the process of development. Skeletal muscle cells can grow, however, by increasing the number of muscle fibers they contain, and they have a relatively long life span. Skeletal muscle cells near the rib cage, analyzed using carbon-14 dating techniques, were found to be as old as 15.1 years, according to Dr. Jonas Frisén of the Karolinska Institute in Stockholm and his team of scientists. And professional bodybuilders don't actually have more muscle cells than the average person; instead, their individual muscle cells contain many more fibers and mitochondria, the so-called "powerhouses" of the cell.