"BURN MORE CALORIES WHILE YOU SLEEP!"

If you compare the number of calories burned during endurance exercise with the number burned during strength training, endurance wins pretty easily. Let's say you weigh 140 pounds. If you ran five miles in an hour-a twelve-minute-mile paceyou'd bum an estimated 512 calories. (That's including the 1 00 or so calories you'd bum in that hour if you didn't go running, but that's the same no matter what type of exercise we 're looking at.) An hour of serious strength training would bum an estimated 384 calories, or 25 percent fewer. If you're a talented runner clocking eight-minute miles, you'd bum 800 calories, or more than twice as many as you'd bum in the weight room for that same hour. At first glance, it's easy to see why strength training doesn't slay calories the way endurance exercise does. You spend more time resting in between sets than you do actually lifting, and you certainly aren't burning fat while you're pushing and pulling weights. If you're challenging yourself at all, you 're shifting from your fat-using aerobic energy system to your anaerobic systems, which by design run on glycogen. However, there is more going on. First is the afterburn-the calories your body continues to bum after the workout is over. Intensity is the most important factor determining post-workout metabolism, so the harder you work in the weight room, the more calories your body will bum afterward. Let's say that afterburn accounts for an additional 50 calories. Calories aren't the only consideration. Serious strength training also signals your body to bum a higher percentage of fat calories for many hours after you leave the gym. An intriguing University of Colorado study, published in the Journal of Applied Physiology in 2003, measured post-workout fat oxidation. ("Fat oxidation" is what happens when your body uses oxygen to turn fat into energy, as it does when you're using your aerobic energy system.) The researchers had a group of men and women do a weight workout one day and an aerobic workout another, with each workout burning about 400 calories. Fifteen hours after the weight workout, the men and women were burning 22 percent more fat than they did fifteen hours after their aerobic workout. The researchers concluded that the exercisers would've needed to burn twice as many calories during their aerobic workout-800, instead of 400-to reach the level of post-workout fat oxidation achieved by the lifters.I haven't yet mentioned resting metabolic rate (RMR) , which is the speed at which your body burns calories regardless of whatever you happen to be doing at the moment. With men, it's pretty clear that weight lifting increases RMR. The workouts themselves speed up metabolism, in part because the body needs to work harder to repair and rebuild muscles, connective tissues, and bones. And then there 's a cumulative effect that comes from adding new muscle tissue. It isn't anything close to the "50 calories per pound of muscle" that some people claim (and I say that knowing full well I 've used that figure in articles going back a few years) . But muscle is metabolically active tissue, and having more of it certainly forces your body to burn more calories throughout the day and night. The real key, though, is the workouts. The harder they are, the more calories you burn in the next day or two as your body recovers. Unfortunately, it's difficult to make any of these claims with women. Some studies have shown a metabolic boost from strength training, and some haven't. Here are two examples: A study conducted at Colorado State University, published in 2000, showed that resting metabolic rates of young women were still elevated by 4.2 percent sixteen hours after lifting weights. It was a small study, with just seven women between twenty-two and thirty-five years old. But a University of Maryland study, published the next year, showed that women had no chronic increase in their metabolic rates after a twenty-four-week strength-training program. That study used two groups of women-nine who were in their twenties, and ten who were over sixty-five. There is a slight twist to this conclusion: The researchers found no statistically significant increase, but the women were in fact burning about 50 extra calories a day after six months of training. That was an increase of close to 4 percent. The men in the same study increased their metabolic rates by about 9 percent, so the women's gains, even if they had been large enough to pass statistical muster, were still a fraction of the men's. I do think it's worth noting, though, that the significant metabolic increase in one study and the insignificant increase in the other aren't so far apart. True, the two studies looked at separate issues-elevated metabolism the day after a workout versus resting metabolic rate removed from the context of a recent workout-but they showed something similar. Women do seem to get a slight increase in metabolism from lifting. It's still in the neighborhood of just 50 calories a day, which isn't even a fifth of a Snickers bar. But it shows that the weights are doing something that probably won't happen with endurance exercise. So if you add it all up, weight workouts give you two, and possibly three, important advantages over endurance exercise: 1. The afterburn, which might be an extra 50 calories. 2. A higher percentage of fat calories used for energy after the workout. 3. A possible increase in resting metabolic rate, in the neighborhood of 50 calories a day. Having said all that, I'll acknowledge that you could equal these benefits of resistance training simply by doing more endurance exercise, or doing it at a higher intensity. You'd burn more calories, you'd get a greater afterburn than you would by exercising at an easier pace, and you'd train your body, over time, to use a higher percentage of fat calories during your runs or swims or rides, and to tap into those fat stores earlier in the workout. Can strength training compete with that? Let me explain why I think the answer is yes.