High-intensity interval training, or HIIT for short, continues to be one of the top fitness trends in the world. So if you’re doing Tabatas (20 seconds of effort, followed by 10 seconds of recovery), you’re running full throttle for 20 seconds. So it’s no surprise HIIT training is outstanding for your cardiovascular system. But even if you’re already fit, you can still reap measurable benefits from adding HIIT to your training regimen, says exercise physiology professor and endurance coach Paul Laursen, Ph.D., author of The Science and Application of High-Intensity Interval Training (HIIT), and contributor to HIITscience.com. “If you fade out during longer efforts, do longer HIIT training intervals.”