During an exercise session, vigorous cardiovascular workouts such as running or biking can typically torch more calories than resistance or strength training.
But what happens once the workout is over?
Exercise scientists have long debated the wondrous notion of an exercise afterburn, or the body's ability to keep burning calories even after you've showered and returned to your desk. Meanwhile, if such an effect exists, it is not clear which form of exercise — cardio or strength training — has a greater metabolism-boosting potential.
Studies have shown post-exercise calorie burn varies quite a bit, largely because of differing study designs and methodologies. Some research has suggested that moderate exercise of any type has little, if any, effect on fat burning after a workout, in part because it doesn't push the body far enough from its comfort zone, which would then require an increase in metabolism.
By contrast, a recent, carefully controlled study by North Carolina researchers showed 45 minutes of intense exercise boosted the metabolic rate in male participants for a whopping 14 hours.
Researchers don't exactly know how post-exercise calorie burning can occur. It is calculated by measuring the increase in oxygen consumption (or metabolism) after a bout of exercise. If your oxygen consumption is above your normal level after exercise, you're burning more calories.
The secret to triggering the effect may lie in the workout's intensity and duration, according to the North Carolina study, which was published in the journal Medicine & Science in Sports & Exercise in 2011.
"What matters is exercising at a high, unrelenting intensity for a prolonged period of time," said study co-author David Nieman, a professor of health and exercise science at Appalachian State University. Nieman, a cardio proponent, believes the study results help settle the debate. "Vigorous sweat gets the hormones cycling and can alter the body's temperature and ability to store fuel. It takes a long time for the body to get back to normal," he said.
Short-duration, high-intensity exercises, meanwhile, have a high post-exercise bump in oxygen consumption that quickly falls to normal, Nieman said.
The study differed from most other research in that Nieman and his team asked volunteers to spend two 24-hour periods in a metabolic chamber, a small lablike room, large enough to house a desk, bed, toilet, laptop, telephone and bike or treadmill. The chamber, which had two air locks, allowed tight control over the environment, including spontaneous activity, sleep, diet and other factors that could influence the results.
During one day, the participants sat, ate and slept; during the second day, they remained inactive with the exception of a vigorous 45-minute cycling exercise. On the exercise day, they were given extra food to keep their energy levels in balance.
Based on previous work, the researchers expected metabolism to be elevated for an hour or two after the workout. To their surprise, "every single subject had an extended increase in their metabolism after their vigorous cycling, an average 14.2 hours," said Nieman, director of the Human Performance Lab at the North Carolina Research Campus in Kannapolis. The net energy expenditure was about 193 extra calories above the rest stage. This increase could have implications for weight loss and management, the study found, especially when combined with the more than 500 extra calories burned during the 45-minute cycling bout.
Still, others say it is important not to overlook the hidden benefits of strength or resistance exercise, which builds muscle and greatly improves body composition.
Both resistance training and high-intensity exercise causes normal, small-scale damage to muscle tissues. Repairing this damage requires energy, which increases metabolism. Moreover, simply sustaining a larger amount of muscle mass raises your metabolic rate.
There are also hormone and inflammatory responses, said Mark Schuenke, an assistant professor of anatomy at the University of New England College of Osteopathic Medicine.
"In the early post-exercise stages, you also consume extra oxygen in an attempt to bring your body temperature, heart rate and blood oxygen levels back to resting levels," he said.
Schuenke co-wrote a 2002 study that showed that the body's oxygen consumption rate stayed elevated for at least 36 hours after an intense, whole-body resistance workout. The study used several multijoint exercises — power cleans, squats, bench press — that were so intense that some subjects became nauseated. Other resistance-training studies have used more sustainable workouts that resulted in less but still significant excess post-exercise oxygen consumption, he said.
"In terms of body adaptations, excess post-exercise oxygen consumption is all about recovery and growth," said Schuenke. "The more strenuous — and the more novel — the exercise, the more (muscle) microtears (that) need repair."
Another benefit of strength training: Each additional pound of muscle increases your metabolism a slight 15 calories a day, which works out to be a pound and a half per year.
Experts say if you're trying to lose weight and have 20 minutes a day, try to burn as many calories as you can in that time period. "But energy intake cannot be ignored," warned Edward Melanson, an associate professor in the division of endocrinology, metabolism, and diabetes at the University of Colorado. His research has shown that while exercise does burn fat, the effect doesn't carry over if the calories expended during exercise are replaced.
"It's not a good idea to think that you can eat anything you want just because you exercised for 20 minutes," he said. "Even with 30 to 40 minutes of exercise, most people only expend 200 to 400 calories, and this can be easily undone by one extra high-calorie snack per day."
What you can do
Associate professor Edward Melanson says more studies need to be done before the post-exercise calorie burn question can be answered definitively. In the meantime, if you want to improve your overall health and body composition, experts suggest focusing on a combination of more rigorous cardio (three days a week) and resistance training (three days a week).
The bottom line on exercise intensity depends on your goals, Melanson said. "There are certainly many health benefits to low- to moderate-intensity exercise, but higher-intensity training expends more calories and has greater effects on aerobic capacity," he said.