Examining 44 published studies involving some 300 recipients of growth hormone, researchers found that although the substance increased lean body mass, biceps and quadriceps strength did not improve. In fact, elevated lactate levels - a measure of exhaustion and decreased stamina - were more common in those taking growth hormone.
The researchers say that the paradox of decreased exercise capacity in the face of increased lean mass may be explained by the fact that measures of lean mass may actually be capturing fluid retention and not increased muscle. They comment that the doses used in real-world settings may be much higher than those studied in the literature, and they warn about the drug's documented adverse effects, even at research-level doses.
You can read the full report here. The study authors' conclusion is worded thusly:
Conclusion: Claims that growth hormone enhances physical performance are not supported by the scientific literature. Although the limited available evidence suggests that growth hormone increases lean body mass, it may not improve strength; in addition, it may worsen exercise capacity and increase adverse events. More research is needed to conclusively determine the effects of growth hormone on athletic performance.