How to Harness Your Body's Growth Factors

Get on the fast track to greater size by taking advantage of these key mechanisms



MGF, which is produced locally in the muscle cells, appears to stimulate muscle growth through an entirely different process than circulating IGF-1. It does so by stimulating satellite cells, which are dormant muscle cells, or muscle stem cells, that sit outside muscle fibers. When the muscle fiber is damaged, such as during an intense workout, these satellite cells move into the muscle fiber and mature, growing bigger and stronger. Since they are actual cells,
they each contain a nucleus, and
so they bring along new nuclei to the muscle fiber. Because the nuclei are where the process of muscle protein synthesis is initiated, the more nuclei a muscle has, the bigger it can become. The increased number of nuclei in muscles has recently been found to be the reason for “muscle memory,” or the ability of someone who has previously trained consistently and built up muscle mass, to gain it back rapidly after a layoff. IGF-1Ea produced locally in muscle fibers, on the other hand, appears to act in a similar manner to the IGF-1 produced by the liver. That means it also instigates the process of muscle protein synthesis and decreases muscle breakdown.

(L) Bettmann (M) Sunset Boulevard (R) Steve Granitz / Getty


Some recent research on IGF-1 suggests that circulating IGF-1 is not critical for muscle growth. And some bodybuilding experts out there are latching on to these few and limited studies and stating that circulating IGF-1 is inconsequential to muscle growth and that only locally expressed IGF-1s from the muscle are important. As one of the few bodybuilding experts who has actu
ally worked with both circulating IGF-1 and muscle IGF-1s in the laboratories at the University of Connecticut and Yale University School of Medicine, I feel that I am well qualified to tell you that it is just too early to discount circulating IGF-1. The truth of the matter on muscle hypertrophy and growth factors is that no one knows with certainty how muscle grows. No one. Not me, not any scientist at Yale University or Harvard, or any bodybuilding expert or guru out there. What is known is that the IGF-1s appear to be players in the process of muscle growth. Locally produced MGF and IGF-1Ea likely play a very important role in muscle growth. After all, if your muscle fibers have a limited number of nuclei, they can grow only so big. The more nuclei the muscle fiber has, the greater its growth potential.

A few limited studies done in either untrained individuals or in animals suggest that circulating IGF-1 doesn’t play a key role in muscle growth. But other studies suggest that circulating IGF-1 levels may be important, especially for trained lifters, such as most FLEX readers. And medical research shows that patients with GH insensitivity (Laron syndrome) increase muscle mass when their circulating IGF-1 levels are increased.

It is still way too early to interpret the data. So don’t disregard circulating IGF-1 just yet. Currently, both circulating IGF-1 and the IGF-1s produced in the muscle seem to play roles in building muscle. While MGF brings more nuclei into the muscle fiber, circulating IGF-1 and IGF-1Ea can increase the protein synthesis that occurs from those nuclei, not to mention decrease muscle breakdown. This tag-team effort can lead to bigger and stronger muscles.

Close the books and put this new information to good use with this breakdown of the proper reps and rest

For access to exclusive fitness advice, interviews, and more, subscribe on YouTube!