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
Several biological factors such as age and nutrition can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an increased rate. Usually natural hypertrophy stops at full growth in the late teens. Some methods used to increase muscle hypertrophy include strength training, nutritional supplements, and anabolic steroids.
Muscle hypertrophy is most effectively done by undertaking strength training, though it can also occur during other short duration, high intensity anaerobic exercises such as interval training, rowing, cycling and sprinting.  Lower intensity, longer duration aerobic exercise generally does not result in organ hypertrophy, instead causing greater storage of fats and carbohydrates within the muscles, as well as neovascularization.
For hypertrophy to occur in the skeletal muscles, the muscle must be directly stimulated. Hypertrophy can be pathological in many organs; for example in the heart, non-exercise based hypertrophy of the left ventricle can be associated with up to a four fold risk of dying over the following 5 years.  In skeletal muscle, it is usually helpful and increases strength.

## Types of hypertrophy
Two different types of muscular hypertrophy are common; sarcoplasmic hypertrophy, in which sarcoplasmic fluid in the muscle cell increases rather than the contractile protein, and hence no increase in contractile strength, and myofibrillar hypertrophy, in which there is an increase in myofibrils, and hence increase in the potential muscular contractile strength.

### 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
Strength training typically produces a combination of the two different types of hypertrophy; contraction against 80-90% of the one repetition maximum for 2-8 repetitions 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
Finally, the message filters down to alter the pattern of protein expression. It can take as long as two months for actual hypertrophy to begin. 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.

## Anaerobic training
Experts and professionals differ widely on the best approaches to specifically achieve muscle growth (as opposed to focusing on gaining strength, power, or endurance); it is generally considered that consistent  anaerobic strength training will produce hypertrophy over the long term in addition to its effects on muscular strength and endurance.  As testosterone is one of the body's major growth hormones, on average men find hypertrophy much easier to achieve than women. Taking additional testosterone as in anabolic steroids will increase results, but the psychological and physiological side-effects can cause serious health issues.  It is also considered a performance-enhancing drug, the use of which can cause competitors to be suspended or banned from competitions.  In addition, testosterone is also a medically regulated substance in most countries, making it illegal to possess without a medical prescription.
In order to get the best gains out of training sessions, experts agree on some basic principles, however some are contradicted by other research:
Progressive overload is considered the most important principle behind hypertrophy, so increasing the weight, reps and sets will all have a positive impact on growth. Some experts create complicated plans that play around with weight, reps and sets, increasing one while decreasing the others, to constantly shock the body into growing. Keeping the sets and reps the same while just increasing weight will lead to growth, but will focus more on developing muscular strength; keeping the weight the same but doing more sets, or doing a few extra reps, may be more effective at stimulating growth for a few weeks, before a rise in weight is necessary. It is generally believed that with more than 15 repetitions per set, the weight will be too light to stimulate growth. Also leave about 45-60sec rest between sets.

## 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.
Because microtrauma is physical damage to the muscle, rest and recovery are just as important as training. Leave at least 48 hours before training a muscle group again. Also stretch after training, as well as on rest days, to maintain/improve flexibility and range of motion.

## Nutrition
Experts agree that nutrition is very important to hypertrophy, especially a diet high in protein, as protein is the main non-water constituent of muscle. There is disagreement over the necessity of artificial protein supplements (such as shakes and bars) versus maintaining a less processed diet high in lean meats such as fish or chicken.
Some scientific research on hypertrophy training is contradictory. For instance some studies have found that the anabolic stage lasts only 36-48 hours after a workout, while others have found that the body is still making adaptive changes after 1-2 weeks. Which study one chooses to believe will affect the training regime, for instance working a muscle group once a week in a 3 day split, or 3 times a week in a full body program. Therefore trying both types of programs for a few months each may help one determine which is better for the individual.