“Zones” can be very confusing as they are not “universal” and there are therefore numerous approaches and definitions out there, including those used in TftNA. As a scientist I have preferred using a system that is based on a fairly well defined physiological exertion threshold. The one I use is lactate threshold (LT) or that transition where lactate begins to exponentially accumulate. The approximate location (in bpm) of this threshold can be determined using blood lactate testing- something that has become commonplace with accurate portable measuring tools that are readily available. There are also reliable field tests that can determine LT to an accuracy that is more than sufficient for setting up your training zones.
If you set your zones up based on LT you will have an easily measured reference to monitor and adjust as necessary since LT will move around a bit based on the specifics of one’s training. Basing your zones on max HR (as is outlined in TftNA) is another valid approach albeit one that will require monitoring your max HR from time to time (max HR will not likely move around as much as LT). Determining your max HR is somewhat more painful than determining your LT (and some say doing so is more dangerous as well due to the stress put on your CV system). But I have found you will likely get to about the same individual training zones either way.
Another issue with training zones is that the numerous approaches use different “numbers” for the same zones, so there is often a disconnect in discussions about the zones as participants have different zone number definitions. For instance TftNA has the following zone definitions:
zone 1 (recovery) 55%-75% of max HR
zone 2 (No man’s land) 75%-80% max HR
zone 3 (uppermost aeorobic training) 80%-90% max HR
zone 4 (anaerobic zone) 90%-95% max HR
zone 5 (maximum effort) 95%+ max HR
Joe Friel (one the developers of Training Peaks) uses the following protocol based off of LT (this system is the default system in TP and the one I use):
zone 1 (active recovery) <85% of LT
zone 2 (aerobic threshold) 85%-91% of LT
zone 3 (tempo) 91%-95% of LT
zone 4 (sub-lactate threshold) 95-100% of LT
zone 5a (lactate threshold) 100%-102% of LT
zone 5b (aerobic capacity) 102%-106% of LT
zone 5c (anaerobic capacity) 106%+ of LT
Using my own data as an example- max HR 170, LT 155
my TftNA zones are:
zone 1 93.5-127.5 bpm
zone 2 127.5-136 bpm
zone 3 136-153 bpm
zone 4 153-161.5 bpm
zone 5 161.5+ bpm
and my Friel zones are:
zone 1 <132 bpm
zone 2 132-141
zone 3 142-148
zone 4 149-154
zone 5a 155-158
zone 5b 159-164
zone 5c 165+
You can probably see the confusion that can develop where, for instance, the TftNA zone 4 is zone 5a-b in Friel’s system and the TftNA zone 3 includes Friel’s zone 3 and some of his zone 2 and zone 4.
These two systems agree on the approximate location of the aerobic threshold (AeT) were TftNA defines it as the top of zone 2 (136 bpm for me) and Friel indicates that it is in the low end of his zone 2 (again about 136 bpm for me (although when peaking my AeT rises to around 140 bpm)). They also have similar LT values (for me 153 bpm in the TftNA system and 155 bpm in the Freil system (and as measured via blood lactate and field tests)). These LTs differ by a couple of beats but I do not find that sort of error to make a significant difference in what I do for training.
Another difference is that Friel does not reference a “no man’s land”. He does this because for some endurance event lengths part of the TftNA zone 2 is race pace and is therefore important in training plans associated with such events and he prescribes workouts in this zone for such athletes.
Since Uphill Athlete suggests that one will be well served by using TP as a training analysis tool, it is important to understand the approach that TP is taking as it concerns zones. Of course you can put in custom zones in TP that are allied with the TftNA system and thereby avoid the confusion. But if you happen to be discussing endurance training with someone who is not on the TftNA system it will be productive to have an understanding of some of the other systems that are in use. I run into this regularly with cross country skiers who use the USSA protocol (which is more like TftNA than the Friel system I use) and we plan to do intervals together. I need to understand that when they speak to, for example, doing zone 4 intervals- they are zone 5a-b for me.
Per your question on zone 1 and zone 2 and other training plans that you see online- well the other plans may be using an entirely different set of definitions for zones. My advice is pick a system and stick with it. So long as the system in use has a physiologic basis, provides physiologic explanations of the reasons for training in each zone, provides example workouts for such zones, and provides a periodization protocol (all of which are extant and provided for by TftNA) it should yield effective training progression when applied correctly.