Muscle Hypertrophy


Muscle hypertrophy is the increase in size (mass and cross-sectional area) of muscle cells. Muscle hypertrophy is different from muscle hyperplasia, which refers to an increase in the number of muscle cells.


Muscle cells are genetically programmed to increase in size to adjust to additional stresses placed upon them. Individual skeletal muscle cells accustomed to lifting no more than 44 lb. (20 kg) have a particular size and shape. If those muscle cells were then used to lift somewhat heavier masses, such as a 55-lb. (25-kg) mass the cells grow in mass and crosssectional area in an amount proportional to the new stress placed upon them. This process is known as hypertrophy. Scientists do not yet understand all the details as to how these changes in muscle cells occur. It appears that additional stress on muscle fibers triggers a change in the rate at which actin and myosin proteins are formed in muscle cells. The additional actin and myosin proteins formed by this process add to existing myofibrils, increasing either their width or length, or both. The larger muscle fibers thus formed are able to exert greater forces of contraction than were their predecessors.

Some authorities believe that damage to muscle fibers is an essential part of hypertrophy. They point to the fact that muscle tissue contains a number of biological and chemical agents, such as various types of growth hormone and steroid hormones, that are programmed to respond to torn or damaged muscle fibers. If a person intentionally damages muscle tissue by exercising it too much, those natural agents respond by building new muscle tissue that results in bulkier muscles. The extent to which tissue damage is a necessary component of muscle hypertrophy is still a matter of some dispute.


Participants in almost every sport are interested in increasing their strength, although that objective differs among sports. For football and ice hockey players, for example, strength is an essential part of being a top-notch player. Although strength is important in other sports, such as golf and tennis, it probably does not rank as high as in contact events. For this reason, muscle hypertrophy exercises are an important aspect of training and conditioning regimes in most sports. The precise exercises included in such a regime are, however, important. The reason is that some forms of exercise result in tissue damage that leads to an accumulation of the cytoplasmic fluid within muscle cells called sarcoplasm. Sarcoplasmic hypertrophy, then, occurs when muscle cells swell up because they contain more fluid, not because their structure has changed to increase muscle strength. Sarcoplasmic hypertrophy is common among bodybuilders whose regime consists of many lifts in a single session. After the first few lifts, protein production has already increased to its maximum level for some given lift mass, and continuing to lift only increases muscle damage associated with the accumulation of fluid. By contrast, limiting lifts to just the number of repetitions required to initiate additional protein synthesis results in an actual increase in muscle size, a process known as myofibrillar hypertrophy.

A protein that occurs in muscle fibers. Atrophy—Wasting away of tissues.
A long, thin, spaghetti-like strand of actin and myosin that constitutes the basic structure of muscle tissue.
Myofibrillar hypertrophy—
An increase in muscle size because of the addition of actin and myosin proteins to myofibrils.
A protein that occurs in muscle fibers.
Sarcoplasmic hypertrophy—
An increase in muscle size resulting from an accumulation of fluid in the interior of muscle cells.

Role in human health


Hypertrophic training is recommended for the elderly. As a person grows older, his or her muscles have a tendency to atrophy, decrease in size. Loss of muscular strength has some real disadvantages, such as reducing one's ability to maintain a stable posture, which can lead to falls and other accidents. Muscular atrophy can also lead to a kind of series of health problems as one is less able to move about and care for herself or himself easily, leading to further atrophy. Trainers do not generally expect that the elderly will become enthusiastic body builders or weight lifters, but even modest levels of hypertrophic activities can lead to significant improvement in muscle strength that contribute to leading a healthier and longer life.

Common diseases and disorders

There appear to be no significant health risk associated with the proper execution of a well-designed hypertrophic exercise program. Individuals who go beyond the limitations of a hypertrophic plan, however, may experience problems resulting from over-exertion of their bodies.



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American College of Sports Medicine, 401 W Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, .

David E. Newton, AB, MA, EdD

  This information is not a tool for self-diagnosis or a substitute for professional care.