Forget that accentuate the positive stuff. This is the gym, where accentuating the negative can earn you stronger muscles. Negative training, also called eccentric training, focuses on the lowering phase of a weightlifting repetition rather than the lifting phase, explains Donald Chu, Ph.D., P.T., former president of the National Strength and Conditioning Association. "You're actually stronger when doing negatives," says Dr. Chu, "because you can lower more weight than you can lift." What makes that added power possible? It is believed that more fast-twitch muscle fibers…