App for training based on cardiac rate

Dear all,
I think that the best way to improve in our training if by monitoring our cardiac rate. On that way I would like to know if there is any app to guide me in this kind of training (recommendations and monitoring). I am trying to find it everywhere with no success.
Thanks in advance.
 
I have been thinking about fitness trackers with heart rate monitors, still haven't decided on one.
 
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