Muscle hypertrophy

**Muscle hypertrophy** is a scientific term for the growth and increase of the size of muscle cells. It differs from muscle hyperplasia, which is the formation of new muscle cells.

## Muscular hypertrophy

 File:Lukáš Osladil.jpg
Several biological factors such as age and nutrition can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an increased rate. Natural hypertrophy normally stops at full growth in the late teens. Muscular hypertrophy can be increased through anabolic steroids, strength training and other short duration, high intensity anaerobic exercises. Lower intensity, longer duration aerobic exercise generally does not result in tissue hypertrophy, instead causing greater storage of fats and carbohydrates within the muscles, as well as neovascularization. Though an adequate supply of amino acids is essential to produce muscle hypertrophy and the consumption of carbohydrates and amino acids can transiently increase anabolism within muscle cells, it is not known if consuming protein immediately after exercising can result in long-term increases in muscle size.

### Types of hypertrophy

There are two different types of muscular hypertrophy: sarcoplasmic hypertrophy and myofibrillar hypertrophy. During sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength. During myofibrillar hypertrophy, the myofibrils, comprised of the actin and myosin contractile proteins, increase in number and add to muscular strength as well as a small increase in the size of the muscle.

#### Types of myofibrillar hypertrophy

Myofibrillar hypertrophy can, in theory, arise through two processes:

* Increase in the number of nuclei within each muscle fiber, or
* Increase in the amount of contractile material supported by each nucleus.

The latter is the usual means of muscle hypertrophy.

### Strength training

*Main article: Strength training*
Strength training typically produces a combination of the two different types of hypertrophy: contraction against 80 to 90% of the one repetition maximum for two to eight repetitions (reps) causes myofibrillated hypertrophy to dominate (as in powerlifters, olympic lifters and strength athletes), while several repetitions (generally 12 or more) against a sub-maximal load facilitates mainly sarcoplasmic hypertrophy (professional bodybuilders and endurance athletes). The first measurable effect is an increase in the neural drive stimulating muscle contraction. Within just a few days, an untrained individual can achieve measurable strength gains resulting from "learning" to use the muscle. As the muscle continues to receive increased demands, the synthetic machinery is upregulated. Although all the steps are not yet clear, this upregulation appears to begin with the ubiquitous second messenger system (including phospholipases, protein kinase C, tyrosine kinase, and others). These, in turn, activate the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response.

Muscle hypertrophy due to strength training does not occur for everyone and is not necessarily well correlated with gains in actual muscle strength: it is possible for muscles to grow larger without becoming much stronger.

### Protein synthesis

*Main article: protein biosynthesis*
Ultimately the message filters down to alter the pattern of protein expression. The additional contractile proteins appear to be incorporated into existing myofibrils (the chains of sarcomeres within a muscle cell). There appears to be some limit to how large a myofibril can become: at some point, they split. These events appear to occur within each muscle fiber. That is, hypertrophy results primarily from the growth of each muscle cell, rather than an increase in the number of cells.

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### Microtrauma

*Main article: Microtrauma*
Microtrauma, which is tiny damage to the fibres, is seen as the basis for hypertrophy. When microtrauma occurs (from weight training or other strenuous activities), the body responds by overcompensating, replacing the damaged tissue and adding more, so that the risk of repeat damage is reduced. This is why progressive overload is essential to continued improvement, as the body adapts and becomes more resistant to stress.

### Skeletal muscle hypertrophy at a cellular level

*Main article: Skeletal muscle*
 File:Skeletal muscle.jpg
Skeletal muscle hypertrophy is initiated by microtrauma occurring in the exercised muscle tissue. The cells that make up the tissue (myofibers) are polynucleated, gaining additional nuclei from activated satellite cells, which fuse to the already mature muscle cell. The satellite cells, and the signaling that activates them, are believed to be the secret behind muscle hypertrophy. In an effort to prevent future trauma, the nuclei, whose number has increased due to the signaling created by the exercise and integration of satellite cells, increase synthesis of sarcomeric proteins such as actin and myosin, increasing the size of the myofibrils that make up the sarcomeres contained in the muscle cell. Increased contractile proteins increase the strength of the muscle, contribute toward increased sarcomeric size, and make the muscle, as a whole, look larger. Skeletal muscle cells do not divide; size increases occur only at the sarcomeric level.

## References

de:Muskelaufbau (Bodybuilding)
es:Hipertrofia muscular