"Instead of doing extra sets or more exercises for the same muscle groups, do back-to-back training," says Wayne Westcott, PhD, fitness research director at the South Shore YMCA, in Quincy, Massachusetts. That means making your muscles work to fatigue — twice. For example, if you’re doing a chest press and you normally do 60 pounds for three sets, try reducing the weight by about 10 percent, to 50 pounds, after the first set; without resting, do as many reps as you can until you work your muscles to fatigue again. "When you do the second set immediately, you’ll target the slow-twitch fibers of your muscle, because you’ve already worked the fast-twitch ones to exhaustion," Westcott says. "This stimulates better muscle growth."