Target Heart Rate?

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I take a jogging class and today we talked about target heart rate during exercise.

How do I find that out?

My HRM says 130-170 is the "zone" I should stay in but when I was at the gym today I EASILY got over 170bpm. It stayed around 195 when I was running or on the elliptical. It was 186 when I was just walking..... I've been googling and searching the internet but I'm still at a loss at what I should aim for.

Any suggestions?

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