I am deeply involved in research (not supplement, but psychology) and I can actually give an educated reply for once!

In response to OTIS, there is no way to experimentally determine a causal relationship between supplements and muscle gains. This is entirely due to the impossibility of isolating supplements as the only independent variable, there are simply too many confounding variables to control (i.e., life stressors, motivational fluctuations, effort, etc.).

However, studies can come to a statistically significant correlational relationship, showing that when supplements are used, whichever they may be researching, a positive correlational relationship occurs (when supplement intake increases, so does muscle mass, or when supplement intake decreases, muscle mass decreases). THIS DOES NOT MEAN SUPPLEMENTS CAUSE, in ANY way, MUSCLE MASS INCREASE.

I would be very interested in a meta-analysis of supplement studies, if anyone has found one.