And before we had a chance to calm down from the 7-minute craze, Auburn University fitness expert Michele Olson, PhD, released [research showing that a 4-minute Tabata workout (which involved alternating between 20 seconds of max-effort jump squats followed by 10-seconds of rest, repeated for a total of 8 sets) is equivalent to walking briskly for 20 minutes. But how short can your workout really be to still reap the rewards? [sidebar]Reality Check TimeWhile “The 7-Minute Workout” is a great hook, the workout’s creators specify that fit people should complete the circuit two or three times for maximum benefit —bringing the total time to 14 to 21 minutes. What the headlines of the quickie workouts also fail to mention is that the length of each workout only refers to the time you’re performing high-intensity effort. However, investing a few minutes pre- and post-workout for a proper warm-up and cool-down is essential—especially because you need to push your body to its max in order to get the benefits. That’s right: Slack off for even a minute or two and the benefits drop considerably. Every minute has to count and you need to be working hard enough that you’re breathless and uncomfortable. “With high-intensity intervals it’s about kicking some butt mentally and physically. They are hard to maintain, and that’s why you only need to do a few,” says Prevention’s fitness expert Chris Freytag. Here’s Freytag’s advice on how to know if you’re working out hard enough. Are Short Workouts Right For You?If you only have a few minutes to spare, using that time for a quick, high-intensity work out is better than lounging on the couch. That’s a no-brainer. But don’t discard your longer workouts workout just yet. “One seems to reach peak benefit, at least from a major health standpoint (lowering mortality), when the exercise reaches approximately 40 minutes,” says Carl J. Lavie, PhD, professor of medicine and medical director of cardiac rehabilitation and prevention at John Ochsner Heart and Vascular Institute. Bottom line: Short workouts are effective if you’re time-crunched, but only if you really push yourself to your limit. And don’t forget to tack on a few extra minutes for a warm-up and cool-down, and, if you can, repeat the circuit a few times. “I’m a big fan of interval training, but the total bout of interval training needs to be within at least a 15- to 20-minute period to get maximum cardio and calorie benefits. You’re not going to burn a lot of calories in a 3-minute workout, no matter how hard you train,” Wayne L. Westcott, PhD, Prevention fitness advisor and fitness research director at Quincy College told Prevention.com. For a simple plan anyone can do, try our 20-Minute Walking Interval Workout, or crank up the intensity with this Belly-Blasting Boot Camp Workout. More from Prevention: The Only 4 Exercises You Need