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.