A 35 Percent Increase in Muscle Mass in Three Weeks!!

The study that should turn the heads of all bodybuilders was a head-to-head comparison of the two titans of anabolic growth: MGF versus muscle-liver type IGF-1. LET’S GET READY TO RUMBLE!!!!!!!!!! MGF was inserted into rat leg muscles by intramuscular injection. This resulted in a 35 percent increase in the weight of the injected muscle within three weeks, and the analyses showed that this was due to an increase in the size of the muscle fibers.12 Additionally, there was a increase in muscle strength of 25 percent.
Similar experiments by other groups have also been carried out using a viral construct containing the liver-muscle type of IGF-1, which resulted in a 15 percent increase in muscle mass, but this took over four months to develop.11 Hence, the dual role MGF plays in inducing satellite cell activation as well as protein synthesis suggests it’s much more potent than the muscle-liver type or IGF-IEa for inducing rapid hypertrophy.


Eccentric Contractions Stimulate MGF Greater than Concentric Contractions

MGF is stimulated by muscle damage. When a muscle is placed on stretch or for even greater effect, stretch combined with muscle contraction, it leads to a rapid increase in MGF.6 I know I’m beginning to sound like a broken record, stressing the need for bodybuilders to emphasize eccentric contractions in their training, but a new study suggests that eccentric contractions have far superior growth potential than concentric or isometric contractions. In a recent issue of the Journal of Applied Physiology, researchers reported that not only will performing eccentric contractions reduce myostatin expression (myostatin suppresses muscle growth), but heavy eccentric contractions also increase both muscle-liver type IGF-1 and MGF to a greater extent than concentric contractions.9 The researchers concluded that eccentric training appears to have the greatest potential for inducing muscle hypertrophy than any other training type.

MGF— The Key to Reversing Muscle Loss?

Older men can increase muscle mass, but it takes a whole hell of a lot longer than a teenager! Well, MGF might be the answer why. The resting levels of MGF in muscle are 100-fold lower than muscle-liver mediated IGF-1.7 Interestingly, resting levels of MGF aren’t different between young and older men at rest, but MGF isn’t increased in aging muscle after resistance exercise compared to young men. For example, young and elderly men performed 10 repetitions of leg extensions in which muscle biopsies were taken immediately post-exercise and examined for MGF gene responses. In young subjects, MGF mRNA levels were significantly increased in response to resistance exercise, but there was no significant change in older subjects. Furthermore, the muscle-liver type IGF-1 was unchanged in both groups.7

There was a huge variation in the response among subjects— some young subjects increased MGF levels by as much as 864 percent after resistance exercise. In another study comparing MGF responses in older and younger men, 20 young men (20-35) and 18 older men (60-75) performed 3 sets x 8-12 repetitions to volitional fatigue of squats, leg presses and knee extensions. MGF increased by 49 percent in the young group of men, however, MGF didn’t increase at all in the older men.16 These human studies are in agreement with animal studies showing that older rats have a blunted MGF response to mechanical damage of muscle. For example, Owino, Yang and Goldspink studied the ability of muscle to express MGF as well as muscle-liver type IGF-1 at different ages of rats (young, middle-aged and old) in response to muscle overload. In young rats, when muscle was overloaded, MGF was rapidly increased in muscle. In middle-aged rats, the increase in MGF was moderate and in old rats, the MGF response was very low and attenuated.14 The young rats increased MGF levels by about 1,000 percent after five days of muscle overload, as opposed to older rats who had only an approximate 250 percent increase. These studies suggest that MGF is needed to initiate the hypertrophy response of muscle by activating satellite cells. But as we age, muscle MGF responses are blunted.

GH is responsible for increasing IGF-1 in the liver, but the effects of GH on muscle hypertrophy have been controversial. Peripheral levels of IGF-1 may not be as important for muscle growth as growth factors produced in muscle as MGF. For example, mice that have been genetically engineered to not produce liver-mediated IGF-1 have normal bodyweights and muscle mass, despite low plasma levels of IGF-1.15 The study demonstrates the importance of locally produced IGF-1 in muscle— including MGF— for muscle growth and tissue maintenance.

Most of the studies so far have been investigating the role of MGF as a treatment for muscle loss with aging. A recent study by Hammeed, et. al.,10 reported that GH administration can increase MGF levels. In that study, elderly men were assigned to three groups: 1) GH administration alone; 2) 12 weeks of resistance exercise alone, and 3) resistance exercise plus GH administration. After 12 weeks, GH alone caused MGF mRNA to be increased by 80 percent compared to baseline. Twelve weeks of resistance training alone significantly increased the mRNA expression of MGF by 163 percent and muscle-liver IGF-1 by 68 percent. However, after 12 weeks of training combined with GH treatment, MGF mRNA increased by 456 percent and muscle-liver IGF-I-E by 167 percent. The results of the study concluded that GH administration alone to elderly men increases mainly muscle-liver IGF-1 levels in muscle by 238 percent in elderly men, with small increases in MGF. The muscle-liver IGF-1 levels were found to be two- to three-fold higher than that of MGF after GH administration.

Interestingly, the group that received GH combined with resistance exercise had larger increases in MGF compared to the group receiving GH or resistance exercise alone. The results seem to indicate that GH results in the upregulation of the IGF-1 gene in muscle. When GH is combined with resistance exercise, there’s a greater increase in MGF gene expression. When the researchers examined muscle hypertrophy, there was only a correlation between the changes in muscle MGF and muscle size, while there was no correlation between muscle-liver IGF-1 and muscle size. It could be that greater changes in MGF need to occur for older men to achieve substantial increases in muscle hypertrophy.