Heart Rate Zones

In my reading about heart rates and training, I have found that different authors use different heart rate zones. The zone names are similar zone 1, zone 2, ect....but what they mean are different.

In Sally Edwards books she uses:
zone 1: 50-60% of max HR also called Healthy Heart
zone 2: 60-70% of max HR temperate
zone 3: 70-80% of max HR aerobic
zone 4: 80-90% of max HR threshold (changing to anaerobic)
zone 5: 90-100% of max HR redline

Other books like ones by Gale Bernhardt go off your lactate threshold.

So when you hear or read people saying they are training in ## zone, how do you know which system of zones they use. Because Edwards zone 2 is not Bernhardt's zone 2.

For me Edwards zone 2 is 111-130 and Bernhardt's zone 2 is 142-152.

Also peoples Lactate threshold can change as your fitness levels go up, right? But your max heart rate doesn't really change except as you age, right?

When reading about training to build your base...what zone do you train in and by who's definitiion.

I have been trying to train in my aerobic zone...with a max running heart rate of 185, I figure using Edwards method my aerobic zone is 130-148. I try and stay around 140.

Thanks for any insight...and helping me clarify zones.
Kathy