How much weight should I lift when I'm strength training?

That all depends on the muscle group you're working, says Stephen Rice, M.D., Ph.D., director of the Primary Care Sports Medicine Fellowship at Jersey Shore Medical Center in Neptune, New Jersey and a fellow at the American College of Sports Medicine. "Different muscles can handle different amounts of weight." For instance, you can squat more weight (using your legs to lift the weight on your shoulders) than you can bench-press (raising the weight while lying on your back). "When strength-training the arms, the amount of weight you should use in lifting is 10 to 12 percent of your body weight," says Dr. Rice. The rule of thumb for quads is one third of your body weight; for hamstrings, it's one fifth. That means if you weigh 150 pounds, you should train with 15- to 20-pound weights in each hand (or bench-press about 40 pounds), 50 pounds for each quad on the leg press, and 30 pounds for each hamstring curl. As for repetitions, the moderate range calls for performing eight to 12 repetitions between three and six times, with about 90- second rests between each set. Dr. Rice's recommendation: "Use a weight that will tire you out in about five to seven repetitions." When you progress to 11 reps without becoming fatigued, it's time to increase your weight.