No anabolic reference guide would be complete without the mention of the most anabolic of all hormones: testosterone! GH and testosterone work synergistically together when produced in conjunction with intense exercise.  It seems the increases in both GH and testosterone results in the greatest increases in muscle mass and reductions in bodyfat.  For example, when the effects of GH or a combination of GH and testosterone on lean muscle mass and fat mass were compared, a combination of GH and testosterone resulted in superior increases in lean muscle mass and reduced fat mass compared to GH or testosterone alone^{26, 27}.  The effects of GH and testosterone on lean body mass appeared additive, suggesting that both GH and testosterone are synergistic yet increase muscle hypertrophy thru different mechanisms. 

Testosterone: The King of Muscle Building

Testosterone is the king of anabolic hormones, don't expect much increases in size or reductions in bodyfat without increases in testosterone.  For example, one study administered a drug that suppresses natural testosterone production to a group of healthy men which reduced circulating testosterone levels to sub-normal levels and found that testosterone decreases in protein metabolism, decreased lean muscle mass, decreased strength, and increased adiposity.   If that's not bad enough, there was a decrease in gene expression for IGF-1 in muscle²⁸.  One of the major mechanisms that testosterone is suggested to increase protein synthesis and subsequent muscle growth is through the stimulation of satellite cell number and an increase in the actual binding of satellite cells to muscle fibers. Testosterone administration has been shown to increase satellite cell number. Several factors appear to influence the acute serum total testosterone responses to resistance exercise. The magnitude of elevation of testosterone during resistance exercise has been shown to be affected by the muscle mass involved (i.e. exercise selection), intensity and volume, nutrition, and training experience. A bout of resistance exercise produces acute changes in testosterone have been linked to those cellular processors involved in protein turnover and muscle growth^{2, 29}.  In brief, elevated testosterone concentrations produced during intense exercise increase testosterone-receptor interactions, thereby initiating a cascade of events leading to the acute (e.g. protein metabolism) and chronic (e.g. muscle growth) adaptive response to training. Acute elevations in serum free testosterone occur after high intensity exercise in both young and elderly men after resistance exercise. However, the magnitude of elevation was greater after 10 weeks of progressive resistance exercise stimulus; thereby suggesting that a resistance training base may enhance the acute response to a workout. In addition, a significant elevation in resting serum free testosterone was observed in the young men³⁰. Testosterone is thought to be the predominant factor that produces greater hypertrophy in men than women when placed on identical training regimens.  Strength protocols which take prolonged rest periods produce smaller testosterone responses than short rest period hypertrophy schemes. Such a notion is supported by the larger muscle fiber sizes of bodybuilders versus power-lifters/Olympic lifters, which may be attributed to the training methods employed by these athletes (bodybuilders =hypertrophy schemes, power/Olympic lifters = strength schemes) and associated testosterone responses.