Zone 2 is considered the target range for aerobic and base fitness work and should be about 60-70% of your HRmax. Zone 3 is the range for aerobic endurance work and should put you at about 70-80% of your HRmax.
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Beyond this point, your body will begin to utilize more and more carbohydrates and then begin to decline in metabolic efficiency. Zone 2, therefore, is where you should spend a lot of your time training if you want to improve your fat-burning capability.