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.
Muscles usually 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 fiber number after mechanical loading, with greater increases being observed after exposure to higher forces at longer muscle lengths. The new fibres have been observed to be smaller than the original ones, with researchers suggesting that this could be because the fibres split to create new ones.
In humans, researchers have observed signs of fiber splitting after very strenuous programs of voluntary strength training, but to date there are no solid indications that long-term strength training causes increases in muscle fiber number.
It has even been suggested that fibre splitting could actually be bad, as it is 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 fibers can occur either because they increase in cross-sectional area, or they increase in length.
It can seem odd to think about muscle fibers 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 fibers) increases after long-term strength training. This happens particularly often when the strength training program involves eccentric-only contractions, or when the peak contraction of the exercise occurs at long muscle lengths.
Similarly, researchers have found that the diameter of individual muscle fibers also increases after long-term strength training. Increases in diameter are sometimes greater in type II fibers, likely because type I fibers are more commonly (but not always!) linked with lowest threshold motor units, and generally only the higher threshold motor units increase in size after strength training.