I'm hoping in this post to go through a few terms I've seen and how they relate to each other so it's possible to use studies in your own training. This will assume you have a power meter (or software allowing you to use your turbo as a virtual power meter, eg PerfPro or Trainerroad).
I'll be describing intensities in terms of FTP; not because I agree with FTP as a concept, but because it's the main metric used by power-geeks and their software.
FTPSimilar concepts: CP (critical power), CP60, OBLA (onset of blood lactate accumulation), MLSS (maximum lactate steady state), VT2 (second ventilatory threshold), AT (anaerobic threshold)
Relation to FTP: 100%
Running equivalent: 10k to HM pace (whichever is nearer to an hour)
Functional Threshold Power is the threshold most people are talking about when they use the word 'threshold' in terms of training. Whilst people will argue about the minutiae, all the concepts above are trying to get at the same thing - the so-called 'anaerobic threshold' where anaerobic metabolism comes into play, blood lactate starts to increase, and exercise tolerance is limited by lactate accumulation. According to some models, it's possible to sustain exercise just below your FTP almost indefinitely - though I mostly think that means the model isn't much use for exercise over an hour.
As you can probably tell, I disagree fundamentally with the standard interpretation (I'll elucidate in a later post) but it doesn't mean the concept isn't useful. Although we all have slightly different 'power profiles' (think of a sprinter versus a time triallist), if two athletes are working at the same percentage of FTP then their workout will be broadly similar. We could take any fixed point really, but Pmax (see below) is much harder to measure.
FTP itself is defined as the maximum steady-state power an athlete can produce for an hour. As this test is very hard on an athlete, many people will extrapolate from a 20-minute test (taking 95%) or even an 8-minute test (taking 90%). Extrapolation like this makes assumptions about the shape of an individual's power curve - but almost everyone uses a 20-minute test so FTP might as well be defined from it anyway.
Unfortunately FTP, whilst very popular with training cyclists, isn't so common in scientific studies where they want something more clearly defined and quick to measure. Blood lactate measurement (OBLA/MLSS) gets at something very similar, which you might see in studies - but more as an outcome measure than a training prescription. Science seems mostly concerned with VO2Max.
PmaxSimilar concepts: vVO2max, pVO2max
Relation to FTP: 110-120% (depending on fitness)
Running equivalent: 1 to 1.5-mile pace (approx 6-8 minutes)
There's something of an obsession with VO2max in sports science, and amongst athletes. I find it strange that we measure athletic prowess based on how much oxygen someone consumes whilst exercising - almost like judging cars by how much fuel they use rather than how fast they can go. There have been many notable examples of athletes becoming much better and faster despite unchanged, or even reduced, VO2max.
Pmax, however, incorporates efficiency. It is usually defined as the lowest power (or speed for vVO2max, used by runners) capable of eliciting VO2max - that is, the point at which, when the athlete works harder, there is no increase in oxygen uptake. Unfortunately, this means that measuring it requires rapid analysis of inspired and expired gases using a 'metabolic cart' which, for the amateur athlete, means an expensive trip to a lab.
Pmax is probably the true 'threshold' - above it, there's no increased oxygen use so anaerobic metabolism has to be providing the rest. However, most people can only sustain exercise here for 6-10 minutes and it's difficult to measure - so I think the real world is stuck with FTP for now.
I personally use 114-116% of FTP as my Pmax pace. It is definitely possible to improve FTP and Pmax separately; there is a school of thought that views Pmax as a 'ceiling', and fitter people tend to have their FTP closer to Pmax - explaining why less fit people can improve with less intense exercise but fit people respond more to high intensity.
Of interest the original Tabata intervals are 20 seconds at 170% Pmax, with 10 seconds of rest, over 4 minutes. Even just a simple average gives 113% of Pmax for 4 minutes - which is already somewhere between 'vomit-inducing' and 'impossible'. It winds me up when I see personal trainers advertising 'Tabata' sessions where the 'on' periods are simply 'quite hard'; they need to be absolutely soul-destroying if you want to get results in 4 minutes. For the rest of us mortals, it's probably better to do something longer and less intense.
PPOSimilar concepts: Maximal Aerobic Power (MAP). NB varying definitions
Relation to FTP: 120-130% (though some versions can be up to 150%)
Running equivalent: 1-km to 1-mile pace (approx 4 minutes)
Finding a reference to Peak Power Output when reading a study generally means it's suitable for replicating in the garage, but you have to be careful exactly what the authors mean by PPO. The most common version appears to be the peak output in an incremental test, starting at 100W, and increasing 30W every minute until failure (defined as an inability to keep cadence over 60). A slight variation involves an increase of 15W every 30 seconds. Other variants use a different starting point (eg weight based such as 2.5W/kg), or change the rate of increase (eg 20W every min).
Confusingly, some studies will refer to the power of a single all-out effort as PPO - which I've called PPO-Wingate below.
PPO-WingateSimilar concepts: All-out
Relation to FTP: Highly variable - 200-300% or more
Running equivalent: 100m to 200m pace (sprint)
This is included mostly because of potential confusion with incremental PPO above. Wingate PPO is a 30 second all-out effort. The 'result' is either the highest average 5-second power (usually from the start of the test), or the average over the 30 seconds (sometimes known as MPO). As it is almost purely reliant on the phosphocreatine system, it bears little relationship to FTP. Bizarrely, as athletes get fitter their Wingate results often get worse!
AeTSimilar concepts: VT1 (first ventilatory threshold)
Relationship to FTP: 55-75%
Running equivalent: MP to MP+2:00/mile
The Aerobic Threshold is a really weaselly concept to pin down, as can be seen by the wide power range, and the confusing fact that a lot of people use the term to refer to the anaerobic threshold. Like the anaerobic threshold, I'm not remotely convinced it exists, but again I think the concept is a useful one.
This 'threshold' should mark the upper limit of 'base' (Zone 1/2 in the 5-zone system) training. Below the AeT is easy/conversational, above it is the dreaded no-man's land of 'comfortably hard'. I think the best way to find it is either to sustain a conversation, or if you're on your own then shut your mouth and breathe entirely through your nose. If you can't do either of those you're above AeT.
If you're into polarised training, this should form the 80% (or whatever percentage you've chosen) and will help prepare you for the 20% where you explore the other thresholds I've discussed.
Some interesting links