55-65%MRH is best for losing weight. Above this value and it is easier for the body to utlise the energy from the carbohydrate zone. This point is also usually associated with noticable change of breathing. As for max heart rate (MRH) you never achieve this (to the max, as the next stage is probable death), however if you do this on a bike after 20 min warm up, whatever your max heart rate achieved, add on 10 beats for your assumed mrh. If you do this running, add on 5 beats. The difference between the two is because you can't use as many muscles on a bike. Your max heart rate is pre-coded in your body, it will not change, however your ability to get near to it will. So, once you have determined whatr you mrh is, see if it changes with time, and fine tune your zones( as indicated in previous replies). Doing it this way with measured MRH means you will be less likely to damage yourself. Hope this helps