You do not need to achieve your maximum heart rate to feel the satisfaction that you have had a good workout. There are a number of factors that determine your heart rate when you train.
1. Your level of fitness.
2. The efficiency with which exercise (You may have improved your technique with pedaling,running etc and need less effort to do the same amount of work)
3. Intensity with which you exercise.
4. Your level of rest. ie if you are overtraining or have not slept well than you heart rate will be higher. The same is true if you overexerted yourself the previous day.
5. Illness. You will have a higher heart rate if you have a cold or a flu(Although you should not exercise if you have a flue)
Im not sure which heart rate monitor you use. I have a polar which is able to do a fitness test. If you have that function, than use it. Its good to chart your fittness. If you dont have this function than use the gym equiptment. Most cardio machines will have this function.
Another good indicator of your level of fitness is your average heart rate. try to compare your average heart rate whilst riding the same rout or the same level of exertion. As you get fitter, than the average heart rate will decrease.
Your resting pulse will also come down as you get fitter.
As you become fitter, that spinning class or that tough climb will become easier so you may get the impression that you are not pushing yourself hard enough. (Your max heart rate may also be lower)
I suppose if you are hard core, than you can go for a VO2 max test, or a lactacte threshold.
As per yor heart rate zones. Think of your heart rate like like a cars rev meter. Its good to open up the valves for a short duration but you will overexert yourself if you are at the max persistently. I usuall keep my Hr at 70-80% max most of the time while doing short bursts towards max.
Hope this helps.
