In order to improve body composition, it is important to see muscle growth as part of your exercise program. Weightlifting is seen as a necessary part of personal training sessions for this reason.
Muscle growth basics
Muscle growth usually occurs due to an increase in volume, and therefore mass, as a result of long-term strength training. There are two theoretical ways in which muscles can increase in volume. These come down to the fibrous make-up of muscle. The first way, called hyperplasia, is when the number of fibres in a muscle increase. The second way, fibre hypertrophy, is when the volume of the current fibres increases.
Both of these methods involve an increase in the volume of the muscle as a whole, this is known as hypertrophy.
Research in rodents has found increases in fibre number after mechanical loading. Greater increases were observed after exposure to higher forces at longer muscle lengths. The new fibres have been observed to be smaller than the original ones. Research suggests that this could be because the fibres split to create new ones.
In humans, researchers have observed signs of fibre splitting after very strenuous programs of voluntary strength training. To date, there are no solid indications that long-term strength training causes increases in muscle fibre number.
It has even been suggested that fibre splitting could actually be bad. It has been known to be a side effect of muscle contusions. Therefore fibre splitting could just be a symptom of severe muscle damage.
Increases in the protein content of individual muscle fibres can occur either because they increase in cross-sectional area, or they increase in length.
It can seem odd to think about muscle fibres increasing in length after training because the locations of the origin and insertion of the whole muscle cannot change. Even so, the whole muscle can increase in length after training, by bulging out slightly in the middle, even while its starting and ending points are fixed.
Many studies in humans have shown that muscle fascicle length (fascicles are bundles of muscle fibres) increases after long-term strength training. This happens particularly often when the strength training program involves eccentric-only contractions.
Similarly, researchers have found that the diameter of individual muscle fibres also increases after long-term strength training. Increases in diameter are sometimes greater in type II fibres, likely because type I fibres are more commonly (but not always!) linked with lowest threshold motor units. Generally, only the higher threshold motor units increase in size after strength training.