My MAX HR is naturally very high or very low. Do I need to adjust my MAF HR?
The max HR (like resting HR), is not a factor in determining one’s training MAF HR.
Maximum heart rate (MAX HR) and MAF heart rate measure two entirely different things. MAX heart rate is a cardiovascular measure—meaning that it tests how hard your heart can pump. The MAF HR, on the other hand, is a metabolic measure: it tells you how high your heart rate can go while still performing completely aerobically (burning fat for fuel).
Whether your workout is aerobic or anaerobic has to do with stress levels: greater stress means that the heart rate rises, which makes exercise more anaerobic. The MAF heart rate is the highest heart rate where exercise has a virtually nonexistent anaerobic component.
In other words, your MAX HR doesn’t necessarily change your MAF HR; If your MAX HR is very low, it just means that you can’t go as far beyond your MAF HR as someone else could. If, on the other hand, your MAX HR is very high, it means that your heart can tolerate far more stress than it takes for your body to switch to the anaerobic system.