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Originally Posted by squat_this  The aim is to get the body into a state where it is burning fat for fuel, not muscle. Cross-trainer, walking on an incline at a steady pace, and cycling are best IMO. |
Exactly, although running may burn more calories it will ultimately burn muscle and fat.
I think that the optimum fat burning heart rate is about 65% to 70% of max heart rate (220-age). i.e. if you're 20 you'd multiple 200 *0.65 = 130 bpm. So as long as you are hitting that for the duration of the workout it doesn't really matter what exercise you do and you shouldn't burn muscle (as long as your pre/post workout meals are sound).
Personally I like the cross trainer or stepper.