Testosterone may be an outdated drug for increasing muscle hypertrophy compared to myostatin inhibitors. Researchers examined the effect of a one-time gene administration of myostatin-inhibitor-proteins to enhance muscle mass and strength in normal and dystrophic mouse models for >2 years.  All animals treated with the myostatin inhibitors demonstrated an increase in body mass with an observable gross enhancement of muscles when analyzed at 725-days of age compared with-treated controls. Evaluation of individual muscle weights showed an increase in muscle mass for all myostatin inhibitor-treated animals, with the greatest increase in follistatin-treated animals. The enlarged muscle mass was accompanied by functional improvement demonstrated by an increase in hindlimb grip strength. There was no effect on heart mass or histological appearance of cardiomyocytes, indicating that myostatin inhibition was selective to skeletal muscle tissue. Effects were not restricted to the injected muscles; they were also found at sites remote from directly targeted muscles. Increased muscle mass translated to a dose-dependent improvement in muscle strength in the hindlimbs and forelimbs of treated animals compared withtreated controls. They found that follistatin injection at 210 days of age increased muscle strength ~60 days after administration and that the increased strength persisted long-term throughout the 560 days evaluated in the study. The amazing aspect about this study was that it was with a single dose of myostatin inhibitors! 

Haidet AM, Rizo L, Handy C, Umapathi P, Eagle A, Shilling C, Boue D, Martin PT, Sahenk Z, Mendell JR, Kaspar BK. Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors. Proc Natl Acad Sci U S A. 2008 Mar 18;105(11):4318-22.