You burn additional calories after a workout session without even trying -- which is sort of like a buy one, get one free deal at the supermarket. This concept of continuing to burn extra calories long after you've left the gym is called "afterburn." The amount of calories you burn depends on how long and how hard you work out in the first place.
You're likely to get the greatest afterburn from a long, hard weight-training workout. In one study, fit men who completed a super-high intensity, 90-minute strength workout, performing 60 sets with little rest in between, experienced an 11 percent metabolism increase for about two hours after the workout. The next morning, 15 hours after the workout, their metabolism was still elevated by a substantial 9 percent, which translated into about 150 extra calories burned. While these numbers are impressive, the workouts were far longer and more strenuous than most people can handle or tolerate on a regular basis without injury or burnout. During a more typical workout -- say, three sets of ten exercises -- researchers say a generous guesstimate for afterburn add up to no more than 50 to 75 calories total.
Still, that's a decent return on investment, and it's a considerably greater afterburn bonus than most people appear to enjoy from cardio workout. For the amount of aerobic exercise most people do -- about 30 to 45 minutes -- the afterburn appears to be less than 50 calories. Still, even such a miniscule boost is nice; five days a week could theoretically add up to a 3-pound weight loss over the course of a year with zero additional effort.
Weight training produces a greater afterburn than cardio exercise because it's a more intense type of workout and possibly because it elevates hormones, such as epinephrine, that simulate metabolic rate. It's also possible that the microscopic tissue damage caused by weight lifting may compel the body to expend extra energy for tissue repair.
So what about a more permanent elevation in metabolism? Is it possible that exercise can actually shift the number of calories you burn all the time by changing your body composition?
Lifting weights builds muscle, and the more muscle you have, the more calories your body will burn at rest. All true, but it takes a lot of extra muscle to dramatically rev up your resting metabolic rate. Although you may have heard that you burn an extra 50 or 100 calories per day for each pound of muscle you build, that figure isn't supported by science. The number is probably closer to 10 to 15 calories, so you really can't expect to see much of an increase in metabolic rate with the typical 3- to 4-pound increase in muscle mass.
It's actually unclear what you can expect to see. Weight-training studies have produced radically different findings. For example, some show an increase in fat-free mass of up to 4 pounds over 12 to 16 weeks, along with a boost in resting metabolism of up to 10 percent, or 160 calories a day. Others show no increase at all in resting metabolism over three to four months, despite increases in fat-free mass. Still, other studies show a slight increase in metabolism despite no increase in fat-free mass and no detectible increases in fat-free mass or metabolism.
One possible reason the data is all over the place is that most studies last just a few months, which isn't enough time for many people to build enough muscle to result in a significant metabolism spike. In many cases, the increase may be so small as to be within the margin of error for measurement. Also, fat-free mass isn't the same thing as muscle; increases in fat-free mass may, in part, reflect increases in water content, not muscle.
You're most likely to get a dramatic metabolism boost if you lift weights consistently for a couple of years and gain more than the typical few pounds of muscle. It's worth it, too, since many experts believe that even a paltry extra 40 calories burned a day could make a real difference in weight control.
But say you do get a metabolism boost from packing on muscle. That doesn't mean you can pat yourself on the back and park your butt for the rest of the day. In some research, a metabolic boost proved to be irrelevant because subjects became less active during the rest of the day. Even though their resting metabolic rates increased, the total number of daily calories they burned did not.
Besides, exercise probably plays its greatest role as you age -- something that's far more important than any temporary boosts or small long-term increases. Sedentary people tend to lose at least one-third of their muscle from age 30 to age 70, but consistent weight lifting may offset most of the decline. With more muscle and a resting metabolic rate than doesn't grind to a halt with advancing age, you're less likely to gain fat.