High-intensity interval training (HIIT), also called high-intensity intermittent exercise (HIIE) or sprint interval training (SIT), is an enhanced form of interval training, an exercise strategy alternating short periods of intense anaerobic exercise with less-intense recovery periods. HIIT is a form of cardiovascular exercise. Usual HIIT sessions may vary from 4–30 minutes. These short, intense workouts provide improved athletic capacity and condition, improved glucose metabolism, and improved fat burning. Compared with other regimens, HIIT may not be as effective for treating hyperlipidemia and obesity, or improving muscle and bone mass. Researchers also note that HIIT requires "an extremely high level of subject motivation," and question whether the general population could safely or practically tolerate the extreme nature of the exercise regimen.