why is it so important to only run at 65% heartrate to burn fat?

[kaptainkeezy04]

Yeah you watch all the pro's do cardio and they do it on the treadmill at about, 3.5mph at about a 4-5 incline

i also see a lot of pros get on the step gauntlet.

[helium3]

staying around 70% means you(usually) dont go over your areobic threashold
which is important.sometimes it is difficult to know your threshold limit so i guage it like this:if you can feel some intensity but you can still hold a conversation down with out getting too out of shape i think thats somewhere within the "zone".