WOW!! I just decided to start doing some HIIT after my weight w/o in the mornings. I started yesterday morning. My plan was to do about 15 minutes of HIIT on the stair stepper after my w/o. Well, after 5 minutes!!! I was toast. I could hardly breath and my legs felt like complete jello. I could barely even move. I guess I will have to work my way up to 15 minutes! he he he...I wish I would have started this sooner. It sure beats sitting on a cardio machine for 30-45 minutes...much better of a workout, too!