Yesterday I did 40 mins of HIIT on the treadmill. Instead of using the timer to say when to stop and start, I used my HRM. I ran until my heart beat got to 182bpm, stayed there for 10 secs, then slowed to a walk until I got down to 135bpm, then ran back up to 182bpm. Did this for 40 mins. Was a fantastic workout. Instead of feeling tired I felt energised and full of adrenaline. I read a book over the w/e about HIIT, saying how much more you burn over normal cardio and judging by the state of my wet clothes I would certainly agree!!! Also made a nice change.