When lifting weights, the amount of weight you should be lifting depends on your goal and on your maximum capacity. (HANDOUT / June 14, 2010)
Weight lifting is an effective — if often overlooked — exercise for achieving a number of different fitness objectives. It can help you lose weight, build endurance and strength, or serve as an aerobic workout.

The ideal weight that you, in particular, need to lift depends on a combination of two factors: which goal you have set your sights on, and the maximum amount of weight you can currently lift, just once.

The amount of weight you can lift only once is referred to as your one-repetition maximum, or 1RM. And it sounds scary. But you can think of your 1RM as a theoretical number; you don't ever have to lift your one-time maximum weight to discover your 1RM with a fair degree of accuracy. In fact, we never ask our clients to determine their 1RM by actually lifting their true maximum, or working at 100 percent of their ability. Maximum lifting makes sense for people who are power lifters or competitive lifters, but it does not make sense for athletes seeking weight loss, or greater endurance, strength and muscle tone. Instead, we recommend you estimate 100 percent of your maximum by lifting much less, then using the number of times you can lift it to determine how close you are to maxing out. This method is not precise, but it's precise enough. Here's how it works.

Start with a weight that seems friendly. Lift it. If you cannot control the motion in good form, you have too much weight. Once you have a safe and comfortable weight, count how many times you can lift it easily. If you can lift it easily and smoothly 10 times, in all likelihood you are lifting a weight that is 75 percent to 80 percent of your one-repetition maximum (1RM). If you can lift it 15 times, it's probably 50 percent of your 1RM. Six times? Most likely it's 85 percent to 90 percent of your 1RM. In other words, if you lift 50 pounds and can do so five or six times, most likely 50 pounds is 90 percent of your 1RM. Thus, you can estimate that your 1RM is probably 55 to 56 pounds. If you can lift it 15 times, your 1RM is around 100 pounds.

Now think about your goals. The speed at which your muscle contracts when you lift is very important in determining how your muscle will change and improve. When you try to lift a weight that is close to your maximum, the speed of contraction you develop is slow, so the power you are producing is not very high (remember, power equals force times speed). On the other hand, when you lift a weight much lower than your maximum, the speed of contraction you can develop is much higher.

• If your goal is to get an aerobic workout that also develops muscular endurance, do numerous repetitions using a weight or load that is in the 30 percent to 40 percent range of your 1RM.

• If you are targeting muscular strength, pump around 75 percent of your one-repetition maximum 8 to 12 times, then wait one to three minutes to recover before you start a new set.

• If you want to lose weight, use light weights and do what we call super sets: Pump three sets or more of 25 to 50 repetitions each at a fast speed of contraction. Allow only a short period (60 to 90 seconds) for recovery between sets.

Once you have a sense of your strength, progress gradually, by 5-pound increments. Remember: When you reach a point where you cannot control the motion or lift in good form, you're using too much weight. And if you lift too much weight, you not only forfeit the myriad potential benefits of weight lifting, but you also risk injury.

Eric Heiden, M.D., a five-time Olympic gold medalist speed skater, is an orthopedic surgeon in Utah. He co-wrote "Faster, Better, Stronger: Your Fitness Bible" (HarperCollins) with exercise performance physician Max Testa, M.D., and DeAnne Musolf. Visit fasterbetterstronger.com.