Hypertrophy is the increase of the size of an organ. It should be distinguished from hyperplasia which occurs due to cell division; hypertrophy occurs due to an increase in cell size rather than division. It is most commonly seen in muscle that has been actively stimulated, the most well-known method being exercise.
This is most effectively done by undertaking resistance training, though it can also occur during other high anaerobic exercises such as interval training, rowing, cycling and sprinting.
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 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.
Two different types of 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. Myofibrillar Hypertrophy, in which there is an increase in myofibrils, and hence increase in muscular contractile strength.
Resistance training
Resistance training typically produces a combination of the two different types of hypertrophy; contraction against 80-90 percent of the one repetition maximum for a lower number of repetitions causes myofibrillated hypertrophy to dominate (as in powerlifters, olympic lifters and strength athletes), while several repetitions against a sub-maximal load facilitates mainly sarcoplasmic hypertrophy (professional bodybuilders and endurance athletes).
Neural Response
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.
Genetic Response
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.
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.
