As it relates to strength and muscle, hypertrophy is simply the enlargement of a muscle belly due to an increase in the size of muscle cells - particularly the muscle's fibres.
This differs from hyperplasia which would equate to an increase in the number of fibres.
Unlike hyperplasia, hypertrophy is a well-recognised and accepted physiological adaptation to skeletal muscle. But to really understand what happens we need to take a closer look at muscle anatomy...
Each muscle or muscle group (like the biceps) is made up of bundles of muscle fibres. Traditionally, researchers believed that the number of fibers we are born with doesn't change, regardless of any exposure to resistance training. This is contested by many proponents of muscle fibre hyperplasia, who suggest that training may induce a greater number of fibers to be formed.
According to the principle of hypertrophy, muscles become larger following a strength training routine, in part, because each fibre (usually fast twitch) becomes larger or thicker. One or more of the following adaptations cause the increase in fibre size (1):
The long-term increase in muscle size is referred to as chronic hypertrophy. Short-term or transient hypertrophy refers to the pumping-up of muscle that occurs during a resistance training session. This is predominantly due to fluid retention in the interstital and intracellular spaces of the muscle and is known as edema (1).
The increase in individual fiber size seems to be stimulated by an increase in muscle protein synthesis. During intense exercise, protein synthesis appears to decrease and then increase during the recovery period (2). The opposite is true for the breakdown or degradation of protein. This increases during and immediately following exercise and decreases in the recovery period. Taking a carbohydrate and protein supplement immediately after a resistance training sessions has been shown to reduce the rate of protein degradation (3)
Interestingly, eccentric muscle action appears to induce a greater amount of muscular hypertrophy compared to concentric muscle action (4,5). Comparing training regimens of only concentric or eccentric exercises, one study found hypertrophy in fast twitch fibers was ten times greater in the eccentrically trained group.