Researchers have found a number of potential ways to spur growth of muscle tissue, and some of these might be used in attempts to fend off the loss of muscle mass and strength that occurs with aging - not by fixing the root causes, but by trying to compensate through another mechanism. Here is a recent example:

The protein is an isoform, or slight variant, of PGC-1 alpha, an important regulatory of body metabolism that is turned on by forms of exercise, such as running, that increase muscular endurance rather than size. [A rise in] PGC-1 alpha-4 with exercise increases activity of a protein called IGF1 (insulin-like growth factor 1), which facilitates muscle growth. At the same time, PGC-1 alpha-4 also represses another protein, myostatin, which normally restricts muscle growth. In effect, PGC-1 alpha-4 presses the accelerator and removes the brake to enable exercised muscles to gain mass and strength.

Several experiments demonstrated the muscle-enhancing effects of the novel protein. The investigators used virus carriers to insert PGC-1 alpha-4 into the leg muscle of mice and found that within several days their muscle fibers were 60 percent bigger compared to untreated mice. They also engineered mice to have more PGC-1 alpha-4 in their muscles than normal mice who were not exercising. Tests showed that the treated mice were 20 percent stronger and more resistant to fatigue than the controls; in addition, they were leaner than their normal counterparts.

Link: http://www.sciencedaily.com/releases/2012/12/121206121728.htm

Source:
http://www.fightaging.org/archives/2012/12/pgc-1-alpha-4-spurs-muscle-growth.php