The next time you are at the gym sweating through an hour on the elliptical machine or going for a long run to improve your fitness, think about this: you may be able to get the same benefits with just a few minutes of exercise.


It's not as easy as it sounds, though. The exercise has to done at a very high intensity, often in short intervals.


This type of exercise is called high-intensity interval training, which involves multiple bouts of very intense exercise separated by periods of rest or light exercise.


I have written about this type of training previously, but new research and the popularity of HIIT training programs warrant revisiting this topic. And, although high-intensity training is effective for improving fitness and burning calories, it may not be right for you.


Exercise to improve cardiorespiratory fitness typically involves 20 to 60 minutes of moderate to vigorous exercise done three to five days per week.


This type of exercise is common for people who are training for an event like a 10K run since it leads to improvements in maximal exercise capacity (called VO2max) and endurance by increasing heart function and promoting changes in the muscle.


This type of training is also followed by most people who are interested in losing weight or getting in shape, even if they don't plan to compete in a race.


Research and practical experience have shown that shorter HIIT sessions can be effective, too. The length and intensity of the intervals vary both in the research and in practice.


For example, in one study these intervals were as short as 30 seconds of all-out, maximal exercise separated by rest periods, for a total of just 6 minutes of exercise per day.


Other studies employ slightly less intense (still 90 percent of maximal heart rate) intervals for a total of 20 minutes of exercise per session. The results show that HIIT leads to adaptations in the muscle and improvements in VO2max that are greater than that of more traditional, lower intensity exercise.


A study published last month showed that even one bout of high-intensity exercise can promote changes in the muscle that lead to improved endurance.


This study compared the effect of four 30-second bursts of very intense exercise separated by four minutes of recovery with a single 4-minute bout of vigorous exercise.


Both promoted a similar effect on blood and muscle markers that lead to improvements in fitness. This suggests that both sustained and interval exercise can be effective, as long as the intensity is high enough.


Does this mean that high-intensity training is right for you? It depends on several factors.


First, the risk of injury during intense exercise is greater than during more moderate exercise. At the very least, exercise of this intensity is likely to be uncomfortable.


Second, exercising at a high intensity may not be a good idea if you are not already in good shape or have other health problems like diabetes or high blood pressure.


Third, HIIT may not be the best way for you to meet your exercise goals. If you exercise to lose weight your emphasis should be on duration, not intensity, to burn calories.


If you are trying to build endurance for a marathon or long distance bike ride, you really do need to focus on longer duration exercise.


For most people, there is little harm in trying some higher-intensity exercise, even just one day per week.


In fact, many group exercise classes are designed to be a high-intensity workout, so this might be a good way to add more intense training sessions to your exercise routine.


Brian Parr, Ph.D., is an associate professor in the Department of Exercise and Sports Science at USC Aiken where he teaches courses in exercise physiology, nutrition and health behavior.