Heart rate zones?

DrG3n3

So I checked out webmd and this is what it gave me:

0 - 99 bpm* Warm up
118 - 138 bpm Weight Management & Endurance Building
138 - 158 bpm Weight Loss & Improving Cardio Fitness
158 - 394 bpm Anaerobic. Don't stay here too long.
*bpm = beats per minute

and this is supposed to be personal to me. Now, when I work out on the elliptical, my heart rate (with an HRM) is somewhere around 160-170. Should I really try and keep it around 138-158? And if so, how come? I don't feel like I am doing much work unless I'm hitting the machine hard, and at 138 I'm not moving too fast.

DrG3n3

Ah. Found this :

http://exercise.about.com/od/weightloss/a/The-Truth-About-The-Fat-Burning-Zone.htm

In case anyone else is interested.