target heart rate question HELP

bmw4deb

Hi everyone thanks for your time

I was reading some post and a few people mention
keeping your heart rate at target for 30 minutes to achieve
calorie burn.
I work out everyday with HRM my heart rate usually ranges
between 120 and 140, but i dont know if it is that the whole time
should it be higher? how do I know what it should be?
am I just wasteing my time if it is not higher? :sick:
also do you stop the monitor as soon as you stop your exercise?
sorry for so many questions thanks in advance