Satellite cells function to facilitate growth, maintenance and repair of damaged skeletal muscle tissue (but not cardiac muscle). These cells are termed satellite cells because they are located on the outer surface of the muscle fiber, between the sarcolemma and basement membrane of terminally-differentiated muscle fibres. Satellite cells are mononucleated; with the nucleus making up most of the cell volume.
Satellite cells are normally quiescent (dormant) in adult muscle, but act as a reserve population of cells, which proliferate in response to trauma or injury, such as from training overload, and give rise to regenerated muscle and to more satellite cells.  The new cells are drawn to the damaged muscle site, where they then fuse to the existing muscle fiber, donating their nuclei to the fiber, which helps to regenerate the muscle fiber. It is important to emphasize that this process is not creating more skeletal muscle fibers (in humans), but increasing the size and number of contractile proteins (actin and myosin) within the muscle fiber.
The amount of satellite cells present within in a muscle depends on the type of muscle. Type I fibers can have five to six times greater satellite cell content than Type II fibers due to an increased blood and capillary supply. This may be due to the fact that Type I muscle fibers are used with greatest frequency, and thus, more satellite cells may be required for ongoing minor injuries to muscle.