My trainer got me started on HIT about 2 years ago. Before that time I was in the gym 5 days a week doing all sorts of exercises and sets but not really getting anywhere. I started training with HIT and really liked it, started making some great gains. He started me off once every 7 days and compared my group with another control group that exercised traditionally 4 days per week. We made steady gains while the other group got did not (actually gained body fat), and were more prone to injuries and sickness. As I understand it, muscle fiber grows when it is stressed to failure, therefore you only need to lift to failure once in order to release the stimulus. Not only are multiple sets not needed but you may damage the tissue. You also need to allow the muscle to obtain adequate rest between workouts or you'll damage it further. For me it seems to be about 5 days. I go to the gym and make weight increases every time. Also, you need to time the exercise instead of counting reps. The time under load is what you want to measure, not reps. That's what I do. Any HIT guys out there wanna comment?