One. It's an approach that's simple and effective. The reason is that you obtain most of the benefits of weight lifting from the first exercise you do, when your muscles are fresh. For instance, let's say you complete three sets of each of the dumbbell bench press, the incline dumbbell bench press and the dumbbell fly. By the time you reach the last exercise, the amount of weight you can handle is far lower than had you done that movement first. You can see for yourself by trying the routine in reverse order. You'll find that you'll be able to lift far less than usual for the dumbbell bench press—a weight you'd normally consider too light. So the benefit to your muscles has vastly diminished. That's why most of the time, sticking with one exercise per muscle group makes the most sense, especially if you have a limited amount of time to work out.
Now it's okay to break the one-exercise rule if there's a good reason to it. For example, if a muscle group has been lagging, you may want to work it a little harder for a four-week period by doubling the total number of sets you do for that area. This is called "prioritizing" a muscle group. So instead of doing all of your sets with one exercise, you might use two or three different exercises, as in the example of the dumbbell bench press, incline dumbbell bench press and dumbbell fly. While you won't be able to use as much weight in the second two exercises as would if you had done them when your muscles were fresh, you will increase the total amount of work the muscle group has to perform. This can help you break through plateaus and spark new muscle growth.
One caveat: If you try this method and find you're getting weaker, the workload is too high for you. Dial it back so that your muscles can better recover between workouts. What's more, prioritizing one muscle group may mean you have to cut back a little on other muscle groups. That's because the total-set per workout recommendation still applies. (See "How long should my workout last?")
Learn more about The Women's Health Big Book of Exercises and where to buy the book.
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20 reasons you should be lifting weights