Testosterone is a highly anabolic hormone that acts by binding with androgen receptors. This signals the cell nucleus to augment the synthesis of new proteins. Until recently, scientists and physicians looked at the role of testosterone in muscle metabolism at a very superficial level. They suppose that a normal blood testosterone level meant that the system was functioning normally and even optimally. Advances in cellular biochemistry showed that the system is much more complicated.
Activation of specific genes and hormone receptor sites are more imperative than circulating levels of hormones and chemical growth factors. Besides, hormones such as testosterone, human growth hormone, and IGF-I must work together to make the optimal effect. A Korean study showed that androgen receptors in the gene controlling them were activated best in the presence of IGF-1. IGF-1 is a potent growth factor that works in combination with growth hormone to promote muscle hypertrophy. Intense training can optimize growth hormone and IGF-1 levels, which also assists testosterone metabolism, especially at the receptor level. The study shows that anabolic hormones work together to promote enhancements in muscle growth and strength.