Hypertrophy is the enlargement of a body part due to an increase in the size of its cells, most commonly used to describe muscular hypertrophy.  A recurring misconception is that hypertrophy is a result of an increase in the numbers of muscle fibers (hyperplasia), however, hypertrophy is defined as enlargement of muscle fibers. It is most commonly brought about with resistance training; the muscular work done against a progressively challenging overload leads to increases in muscle mass and cross-sectional area. The process of muscular hypertrophy involves a complex interaction of satellite cells, the immune system, growth factors, and hormones with the individual muscle fibers of each muscle.
Following training, the body adapts by increasing muscle cross sectional area through an increase in the size of actin and myosin filaments. This results in the addition of sarcomeres to existing muscle tissue. Hypertrophic increases occur in a selective manner primarily to fast twitch muscle fibers and may take as long as eight weeks to become evident. There also appears to be some limit to how large a myofibril can become; at some point, they split.
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.
- ↑ Donche, Dan (2008). FF Trainer Certification Guide. USA: Fatal Fitness.
- ↑ 2.0 2.1 2.2 2.3 Hernandez, R.J. and Kravitz, Len (n.d.). The Mystery of Skeletal Muscle Hypertrophy. UNM.edu. Retrieved on 2008-10-17.
- ↑ 3.0 3.1 Brown, Lee E. (n.d.). Muscle Hypertrophy Training Program = Volume. TrulyHuge.com. Retrieved on 2008-10-17.
- ↑ University of California San Diego (n.d.). Hypertrophy. UCSD.edu. Retrieved on 2008-10-17.
- ↑ Morgan, JE; Partridge, TA (Aug 2003). "Muscle satellite cells.". Int J Biochem Cell Biol. 35 (8): 1151-6. Retrieved on 2008-10-17.