Muscle hypertrophy requires tension, the availability of proteins and amino acids, calories, anabolic hormones such as growth hormone, IGF-1, testosterone and insulin, and adequate recovery between training sessions.  While all are important, optimizing muscle tension is the most critical concern to bodybuilders and strength athletes.  Brad Schoenfeld from Global Fitness Services in Scarsdale, New York concluded that drop sets, supersets, heavy negatives and forced repetitions are effective for maximizing muscle tension and triggering muscle growth in fast and slow twitch muscle fibers.  These Training methods are intense, so athletes should ensure adequate recovery to prevent over training.  (Strength and Conditioning Journal, 33 (4):  60-64, 2011)