i don't know if anyone else knows about this, but the MHR scale was devised by cardiologists who were working with people with cardiac problems; it is a guideline based upon people who have had heart attacks etc for rehabilitation; it's much more dangerous for someone with a history of cardiac problems to push their HR really high eg. "90%" than for someone who is in good health. It's pretty hard for anyone to actually reach their 100% MHR; there's been times where i've really pushed myself, felt a bit faint and sick with the effort and my HR never went above 183 (my supposed MHR is 201). Basically, i'd recommend you to ignore the HR monitor for the most part (except maybe if you wanted to use it as a test to see how you've progressed cardiovascularly) and go with your instinct; what feels right for you? instead of measuring your intervals in terms of HR percentages, measure them in terms of effort percentages; this will change with your fitness so you'll be pushing yourself continuously so you're less likely to hit a plateau. if your judging it by your heart rate (which will decrease with increased fitness) you'll eventually end up limiting your potential.