What exactly IS the difference between high intensity cardio and low intensity? In other words, if I run for 45 minutes to an hour, do I run at a high intensity, thus running the risk of burning muscle? If so, is fast paced walking a better idea to keep my heart rate at about 135 and burn fat only? (I am 27, 5'8, 170lbs.) -Chris

If you're truly doing some high intensity cardio, you shouldn't be *able* to keep it up for an hour. Maybe moderate intensity... but high intensity is typically defined as working in the 75-95% range of your theoretical maximum. This is like sprinting. So when you do HIIT, you walk for 1 minute and sprint for 1 minute or something to that effect. If you're really sprinting, your legs will give out after a couple minutes, tops. But you don't need to do it very long to get the benefits.

Dropping to around 135 bpm is fine for low intensity, so long as you keep it up for 45-60 minutes.

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Keep your body lean, your blood clean and your mind sharp. -Rollins

Naxis,

What if I am running for 45-60 minutes? My heart rate averages 160-175 when I do that. Is this too much for straight fat burning (no muscle)?...

I wouldn't go that high. No matter what you do, you have no guarantees that you will be burning *only* fat, but your much much less likely to lose muscle using low intensity. (Intensity has nothing to do with duration, btw. It is all about heart rate.)

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Keep your body lean, your blood clean and your mind sharp. -Rollins