Understanding endurance training – Knowledge

1. Why this article?

Most people who take up endurance training make the same mistake. They train too hard for an easy day and too easy for a hard one. Everything ends up in the middle.

That sounds like a detail. It is one of the most common reasons people run, cycle or walk for months and still barely get fitter – not the only one, but one that can be changed without doing more.

This article explains why that happens – and how to do it better. It explains what lactate really is (almost everything you have heard about it is wrong), what the two points LT1 and LT2 mean, why your watch shows five zones and your bike computer seven although there are physiologically only three – and how to find out without a laboratory where your own limits lie.

Why we are writing this. Our practice offers two tools we developed ourselves: PTH ZoneForge for training planning, and the lactate threshold analysis for evaluating a lactate test. Both use technical language, because they are made for professionals. This article is the translation of that language into ordinary words – in the same order and with the same terms, so that you find your way when you open one of the tools. (One practical note: the interface of the lactate analysis exists in German and French, not in English. The scientific explanations inside it are in German.)

What you will find here:

• What endurance actually is, and why it is one of the best-studied health factors there is.
• Why lactate is not a waste product but a fuel – and where the burning in your muscles really comes from.
• What LT1 and LT2 are, how they are measured, and why sixteen different calculation methods give sixteen slightly different answers.
• All the common zone systems side by side, with a table for translating between them.
• How to tell without any device which zone you are in – the talk test, the singing test, heart rate drift, your appetite afterwards.
• The big training models – polarised, pyramidal, Norwegian – and what the studies actually support (less than the headlines promise).
• And in detail: how much you need, what applies in illness and in older age, and when to get checked out first.

Who this is written for. For people without any background in sports science. Technical terms appear, because you will meet them in every training plan, every app and both of our tools – but each one is explained the first time it comes up. The numbers in square brackets refer to the reference list at the bottom; every entry has been individually verified and linked.

Scope. This article is about endurance training – the cardiovascular system, metabolism, intensity control. Everything about muscle strength is in our articles Strength training and Building muscle. The two belong together: train only endurance and you lose strength over the years; train only strength and you cannot use it for long.

And the usual, important note: this article does not replace an examination. What your doctor decides for your situation takes precedence.

1.1 Twelve words you will need

So that the rest stays readable, here are the terms that occur throughout. They appear in the same form in our tools.

• Aerobic. Literally "with air". Energy production that uses oxygen. It is slow but practically unlimited as long as there is fuel.
• Anaerobic. Literally "without air". Fast energy production that does not depend directly on oxygen. It delivers a lot of power immediately, but only for seconds to a few minutes. The word is still misleading: even at maximal effort, a large share of the energy still comes from aerobic metabolism.
• Lactate. A substance produced when sugar is broken down in the muscle. Not waste, not poison, but a fuel – see section 3. It is measured in millimoles per litre of blood (mmol/L), usually from a drop of blood taken from the earlobe.
• LT1 (first lactate threshold, also "aerobic threshold"). The intensity at which blood lactate first rises clearly above the resting value. Below it, training can be sustained for a very long time – limited then not by the intensity but by fuel, fluid, heat and muscular fatigue.
• LT2 (second lactate threshold, also "anaerobic threshold"). The highest intensity at which lactate still stays stable. Above it, lactate keeps rising until you have to stop.
• Zone. An intensity range between two boundaries. How many zones there are depends on the model – but the boundaries that matter are always LT1 and LT2.
• VO2max (maximal oxygen uptake). How much oxygen your body can use at most under full load. The measure of endurance capacity.
• MET (metabolic equivalent). A unit of load. 1 MET is the energy expenditure of sitting quietly; leisurely walking is about 3 MET, brisk cycling about 8 MET.
• Watt. Measured power output, usually on a bicycle ergometer. In running its place is taken by pace (minutes per kilometre).
• FTP (functional threshold power). The power you can hold for roughly one hour. A practical approximation of LT2 – but not the same thing, see section 6.6.
• HIIT (high-intensity interval training). Short hard efforts with recovery in between. In our tools this is called "Zone 3".
• Training volume. The amount of training, usually in hours or minutes per week.

2. What endurance is – and why it is more than sport

2.1 The number behind it: maximal oxygen uptake

Endurance is the ability to sustain a load for a long time. Physiologically it depends on a chain: the lungs take up oxygen, the heart pumps it with the blood to the muscle, the muscle burns fat and sugar with it.

The measure of the capacity of this whole chain is called VO2max – maximal oxygen uptake. It is given in millilitres of oxygen per kilogram of body weight per minute. An untrained middle-aged person is often at 25 to 35, a well-trained recreational athlete at 45 to 55, a world-class endurance athlete above 80.

The bottleneck in this chain is usually not the lungs but the cardiovascular system: how much blood can be transported per minute, and how much oxygen that blood carries [6]. That is why VO2max rises with training mainly because the heart ejects more blood per beat and because blood volume and the total mass of oxygen-carrying haemoglobin increase. One point matters for reading a laboratory report: the haemoglobin concentration in the blood need not rise – plasma volume expands quickly alongside it, so the measured value may stay the same or even fall slightly.

In technical language the same thing is called cardiorespiratory fitness. You will meet this term constantly in studies.

2.2 What endurance training changes in the body

Endurance training works at several places at once; a detailed overview of these remodelling processes in muscle was published recently [11]. The most important:

• The heart becomes a better pump. The left ventricle becomes roomier and its wall more elastic, so more blood is ejected per beat. That is why resting heart rate falls in trained people – the heart has to beat less often to move the same amount of blood.
• There is more blood. Blood and plasma volume rise after only a few sessions; the total mass of haemoglobin – the protein that carries oxygen – also increases with longer training. This explains the largest share of the gain in endurance in controlled studies [8].
• The muscle gains capillaries. Capillaries are the finest blood vessels, where oxygen passes from blood into tissue. The denser this network, the better the supply.
• The muscle gains more and better power plants. Mitochondria are the cell components in which energy is produced using oxygen. Endurance training increases their number – and additionally makes them more capable by increasing the density of their inner surface [12].

How large are these changes? That can now be answered very precisely. A large review pooled 353 studies on mitochondria and 131 on capillaries, involving almost 6000 participants [10]. Four findings matter in practice:

• Mitochondrial content rose to a similar extent with all forms of training: around 23 per cent with easy continuous training, around 27 per cent with interval training and around 27 per cent with very short, very hard sprint interval training. There was no meaningful difference between the three.
• Per hour invested, however, hard training was far more economical: sprint intervals delivered about 2.3 times as much per hour as classic intervals and about 3.9 times as much as easy continuous training.
• The number of capillaries per muscle fibre also rose similarly with all forms (10 to 15 per cent). For the density of the vessel network, however, easy continuous training was the most effective form.
• And the sentence that matters most: the ability to adapt to endurance training is preserved lifelong – regardless of age, sex, and whether an illness is present. Those with the lowest starting fitness gain the most in percentage terms.

Note for later how these two findings fit together: hard training saves time, easy training builds the vessel network better. The training models in section 8 grow out of exactly this tension.

2.3 Why this is a health question, not a sport question

In public, endurance is seen as a concern of people who want to run marathons. In research it counts among the strongest predictors of health there are.

• The order of magnitude. A pooled analysis of 33 observational studies with almost 103,000 healthy adults found: for each 1 MET higher endurance capacity, mortality over the observation period fell by 13 per cent and the risk of cardiovascular events by 15 per cent [3]. People with low fitness had about a 1.7-fold higher risk of death than people with high fitness.
• The largest study on the subject. At an American centre, 122,007 patients were followed for a median of more than eight years after a treadmill exercise test [4]. The result: the better the fitness, the lower the mortality – with no observable upper limit. The fittest participants had an 80 per cent lower risk of death than the least fit. The benefit was particularly pronounced in older people and in people with high blood pressure.
• Why professional bodies take this seriously. In 2016 the American Heart Association issued a statement calling for endurance capacity to be treated like a vital sign – recorded regularly, like blood pressure or pulse [2]. The reason: it says more about the future than most laboratory values, and it is modifiable.
• And not only the heart. A pooled analysis of 39 randomised trials found that physical exercise improves cognitive function in people over 50 – regardless of whether participants were cognitively healthy or already showing decline [9]. The combination of endurance and strength training performed best.

Two qualifications, so that these numbers do not mislead.

First, these are observational studies. They show an association, not necessarily a cause. People with good fitness differ from people with poor fitness in other ways too – they smoke less, sleep differently, often live in different circumstances. Part of the effect belongs to those circumstances.

Second, the large randomised trial on the subject was more sobering than many expected. In Trondheim, 1567 people aged 70 to 77 were followed for five years and randomly assigned to three groups: interval training twice a week, moderate continuous training twice a week, or simply the national physical activity guidelines [5]. The result: between the exercising groups combined and the comparison group there was no difference in mortality. In the interval group 1.7 percentage points fewer died than in the comparison group – but this difference was not statistically secure. The most important reason is stated in the paper itself: the comparison group joined in enthusiastically. 80 per cent of all participants were already moderately to highly active at baseline, and the comparison group trained more intensely of their own accord than the moderate group did.

What follows from this is sober and still good news: the big gain does not lie between "a lot of training" and "very much training", but between "nothing" and "something". Those already active gain capacity and enjoyment from more structure – but no longer the same dramatic health jumps as someone starting from zero.

2.4 "It doesn't work for me" – the most stubborn error

There is a widespread idea that roughly one in five people is a "non-responder": someone whose endurance simply does not improve with training. The idea comes from real data – in many studies there are participants whose values barely moved after weeks.

One study tested this deliberately [8]. 78 healthy adults were divided into five groups that differed in one thing only: one, two, three, four or five training sessions per week, over six weeks. The result was unambiguous. The more people trained, the fewer non-responders there were. And when everyone who had not responded after the first phase completed a second phase with two additional sessions per week, the non-responders disappeared completely. Every single person improved.

From this follows one of the practically most useful insights of recent years: a lack of improvement is rarely a fixed personal characteristic. For many people it simply takes a higher dose, more time or a different stimulus.

Two qualifications belong with it. First, these are 78 participants in a single study. That everyone in this group responded at a higher dose does not mean that every person responds under all circumstances. Second, every measurement contains scatter: a VO2max measurement varies from day to day, and sleep, illness, medication, how consistently someone trains and the choice of outcome shift the result as well. Part of what appears in studies as "non-response" is measurement noise rather than biology.

3. Lactate – the biggest misunderstanding in sport

3.1 Lactate is not a waste product

If you take one thing from this article, let it be this section. Almost everything in circulation about lactate dates from the 1920s and has been disproved for decades.

The old story goes like this: when the muscle gets too little oxygen, it switches to emergency mode. Lactic acid is produced as waste. It accumulates, acidifies the muscle, causes the burning and forces you to stop. Later it is laboriously cleared away, and the next day it causes muscle soreness.

Almost every sentence of this is wrong. What holds today was assembled above all by the American physiologist George Brooks over four decades [15][16]:

• Lactate is produced constantly, even at complete rest and with plenty of oxygen. It is a normal intermediate step in the breakdown of sugar – not an emergency programme. Your body is producing lactate while you read this.
• Lactate is a fuel. It is taken up by other muscle fibres, by the heart muscle, by the liver and by the brain, and there burned or converted back into sugar. Under load the heart draws a considerable share of its energy from lactate. This transport from the site of production to the site of consumption is called the lactate shuttle – Brooks' central contribution.
• Lactate is also a signalling substance. It acts like a messenger telling the body that hard work is going on, and it triggers exactly the adaptations training is about – among them the formation of new mitochondria [17].
• Lactate is not an acid. Under body conditions it exists as a salt, not as lactic acid. Using the term "lactate" rather than "lactic acid" is therefore not word play but a factual correction.
• Lactate does not cause muscle soreness. The lactate level is back to normal an hour after exercise. Soreness comes one to two days later and follows unaccustomed loading – above all braking, so-called eccentric movements such as downhill running. Mechanical loading, a subsequent inflammatory response and a temporarily increased sensitivity of the nerve endings in the muscle are all involved. "Small tears" alone do not explain it.

3.2 Where the burning actually comes from

The burning in the muscle during hard exercise is real. It just is not caused by lactate.

Several processes are responsible at once: an accumulation of hydrogen ions (which does indeed make the muscle more acidic – but lactate is more the counterpart than the cause), a build-up of phosphate from energy metabolism, and changes in the potassium balance of the muscle cell. On top of that, nerve endings in the muscle report this metabolic state directly.

Why this matters in practice: the burning is an indicator of a particular metabolic state – not its cause. When it burns, you know you are working well above LT2. That is useful information. But you do no harm by enduring it, and you "clear away" nothing by cooling down afterwards.

There is a learning game on this site about lactate: Lactate Shuttle. It shows how lactate travels from producing to consuming tissue.

3.3 Why the lactate curve is still so useful

If lactate is not a waste product – why do we measure it?

Because it is an excellent indicator. The lactate value in blood is always the difference between what is being produced and what is being consumed. As long as the body keeps up with consumption, the value stays flat. As soon as production grows faster than consumption, it rises.

Exactly these two switching points are what we are looking for. They are called LT1 and LT2, and the next section is devoted to them.

A picture to go with it: imagine a bathtub with an open drain. The tap is lactate production, the drain is consumption. At low load the tap only drips and the drain copes easily – the water level stays low and constant. Open the tap further and the level rises a little, then settles at a new, higher level: the drain works harder and just about copes. Beyond a certain point it no longer copes – the level rises and rises until the tub overflows. The first point is LT1, the second LT2.

4. The two points that everything depends on

4.1 LT1 – where it stops being genuinely easy

LT1 is the intensity at which blood lactate first rises clearly above the resting value. The older term is aerobic threshold; in breath-gas measurement the corresponding point is called VT1 (first ventilatory threshold).

What you experience below LT1:

• You can keep going for hours, as long as fuel and fluid do not run out.
• You can speak effortlessly – and, more importantly, sing effortlessly (see section 7.2).
• Your body covers a large share of its energy needs from fat.
• At a sensible duration you are little fatigued by it the next day. A very long session is tiring even below LT1 – and anyone who is untrained or has several illnesses feels it considerably sooner.

LT1 is the most important boundary for the amount of your training. At a sensible duration, everything you do below it costs comparatively little recovery – so you can do a lot of it. Everything above it costs more, and disproportionately so.

In our tools LT1 anchors Zone 1. Explicitly the measured LT1, not a percentage of anything – why, is explained in section 6.7.

4.2 LT2 – where it tips

LT2 is the highest intensity at which you can hold a state of equilibrium. Lactate is clearly elevated but stays constant, because production and consumption balance out. Increase a little further and the balance tips: lactate rises further and further, and the time to exhaustion is foreseeable.

The older term is anaerobic threshold. It is an unfortunate term, because nothing there proceeds "without oxygen". A detailed review has traced the argument over that name across fifty years [18]; the short version is that the point is real and useful but its label is misleading.

A related term that is often equated with it – wrongly: MLSS. The maximal lactate steady state describes the same physiological boundary, but it is determined quite differently: with several long constant-load efforts on different days, each checking whether lactate stays stable over half an hour [57]. That counts as the reference method. LT2 from a step test, by contrast, is an estimate of this region from a single, shorter measurement.

The two lie in the same territory but are not interchangeable. A direct comparison of several LT2 models with measured MLSS in running and cycling found good agreement at group level, but deviations of roughly five to six per cent in either direction in individual people [58]. The same holds for the other members of this family – VT2 from breath-gas measurement, OBLA, D-max, critical power: same territory, not the same number.

What LT2 means in practice:

• A trained person can hold this intensity for roughly 30 to 60 minutes, an untrained person often considerably less.
• Speaking is possible only in short phrases, with breaths in between.
• Energy comes predominantly from carbohydrate.
• It is the intensity that decides competitive performance over 30 to 60 minutes.

LT2 is the most important boundary for the intensity of your training. What lies above it is genuine high-intensity work and requires recovery.

4.3 Why there are exactly two and not five

Your watch shows five zones. Your bike computer shows seven. Some apps show six. For the practical steering of endurance training, two transition regions have proved their worth – and therefore three intensity domains.

This is not merely an opinion but the state of the methodological literature: a much-cited review and critique of methods for determining exercise intensity concludes that any subdivision beyond three ranges is practical convenience rather than physiology [19]. Between "Zone 4" and "Zone 5" on your watch, nothing happens in the body that deserves its own name. Between below and above LT1, a great deal happens.

To be precise: this three-domain model is very well established, but it is not the only possible physiological description. It is the division that has proved the most useful for steering training – not a law of nature with exactly two lines.

That does not mean five or seven zones are useless. They are more finely divided and sometimes practical for steering individual sessions. They just must not obscure what counts: how much time really lies below LT1 and how much really above LT2.

4.4 How a lactate step test works

So that the next sections become concrete, here is how such a measurement proceeds.

You pedal on an ergometer or run on a treadmill. The load is increased in steps – for example by 20 watts every three minutes. At the end of each step three things are recorded: the lactate value from a drop of blood taken from the earlobe, the heart rate, and your own rating of effort. This continues until you cannot go on or a predefined stopping value is reached.

At the end you have a series of value pairs: at this many watts, this much lactate. Plotted out, they form the lactate performance curve – flat at first, then a bend, then a steep rise. LT1 and LT2 are hidden in exactly this curve.

Three things that can distort the test, and which you should know about if you have one done:

• Step length. Short steps (1–2 minutes) produce different values than long ones (4–5 minutes), because lactate needs time to distribute in the blood. A test can therefore only be compared with a test using the same protocol [19][20].
• Carbohydrate stores. Someone starting with full sugar stores produces more lactate at the same load than someone with empty ones. That shifts the whole curve.
• Preceding load. A hard training day beforehand lowers the measurable threshold. So an easy day belongs before a test.

5. Sixteen methods, one curve

Now comes the part that surprises most people. You have the curve – and it is still not clear where LT1 and LT2 lie. There is no little flag in the curve. There are only calculation methods that try to find the point.

Our lactate threshold analysis computes sixteen such methods from six families and displays them side by side. Here are the families, in understandable form.

5.1 Fixed lactate values – the old school

The oldest and simplest approach: define a fixed lactate value and read off the power at which it is reached. Classically 2 mmol/L for LT1 and 4 mmol/L for LT2. The technical term is OBLA (onset of blood lactate accumulation). It goes back to a 1979 paper that defined the "aerobic–anaerobic transition zone" in exactly this way [21].

The advantage: simple, the same everywhere, in use for decades.

The problem: the 4 mmol/L is a population average. In individual people the actual equilibrium point lies somewhere between about 2 and 8 mmol/L. Someone with a naturally low resting value will have their threshold overestimated by the rigid 4-rule; someone with a high one will have it underestimated [20]. Our analysis therefore computes OBLA values too, but does not rely on them.

5.2 Baseline plus x – the individual answer

The refinement is obvious: instead of a fixed value, take the personal lowest value of the curve (the baseline) and add an amount. Our analysis offers this in four gradations: baseline plus 0.3 – plus 0.5 – plus 1.0 – plus 1.5 mmol/L.

For LT1 this family is today the best supported. A controlled laboratory study in 50 cyclists compared nine different markers for the upper boundary of the easy range. Variability between people ranged from 6 to 29 per cent depending on the marker – fixed percentages of maximum heart rate and fixed lactate values performed worst. The closest agreement was between VT1 from breath-gas measurement and "baseline plus 0.5" [25]. That is why "baseline plus 0.5" is the default for LT1 in our tool.

A detail that matters: "baseline" can mean two things – the value of the first step, or the lowest value measured at all (the nadir, the low point of the curve). In many people lactate dips slightly again on the second or third step before it rises. Our analysis therefore computes both variants and shows them separately.

5.3 D-max – the greatest distance from the line

For LT2 the most widespread family is D-max. The idea is geometric and easy to picture:

Draw a straight line from the first to the last point of the lactate curve. Then find the place where the actual curve is furthest from this line – measured at a right angle. That place is called D-max, from maximum distance [23].

In everyday language D-max is often described as "the point of greatest curvature". That is only approximately right: what is sought is the greatest distance from the connecting line, and mathematically that need not coincide with the point where the curve bends most sharply. Where the line starts therefore shifts the result noticeably – which is exactly why the following variants exist.

There are several variants, differing in where the line begins:

• D-max classic: the line runs from the very first to the last measurement point.
• D-max modified: the line begins only at the point of the first clear lactate rise. This makes the method less sensitive to how many easy steps were ridden at the start.
• D-max from nadir: the line begins at the low point of the curve. This is the default in our analysis.
• Log-poly modified D-max: a combination in which the starting point comes from the log-log method (see below).

How good is D-max? One study compared D-max with critical power, an independently measured reference for the same physiological boundary [24]. At group level the two agreed almost perfectly – the mean difference was about half a watt. In individual people, however, the spread was wide: up to around 50 watts in either direction, without either measurement being "wrong".

Remember this finding. It is the reason for the most important statement of this whole section, which comes in 5.6.

5.4 Log-log – straightening out the curve

A neat mathematical trick from 1985 [22]. If lactate and power are plotted not directly but both logarithmically, the bent curve becomes approximately a kink between two straight lines. Finding a kink is considerably less ambiguous than estimating a bend.

Log-log gives good values above all for LT1 and is particularly insensitive to small measurement errors in the lower steps. It is also the method most closely related to the point of maximal fat oxidation – see section 11.

5.5 Two further methods

• LTP – segmented regression. Instead of a smooth curve, two or three straight segments are fitted through the data and the computer looks for the place where the transition fits best. The technical term "breakpoint" describes it well. Advantage: no assumption about the shape of the curve is needed.
• LTratio. Here it is not lactate itself that is considered but the ratio of lactate to power – "how much lactate does one watt cost me". This ratio has a low point, and in many people that low point lies close to LT1. In our analysis the same principle appears in the charts as lactate economy: a flat, low course shows an economical metabolism.

5.6 What to do when sixteen methods give sixteen numbers?

This is the normal case, not the exception. One and the same curve can yield an LT1 anywhere between, say, 150 and 190 watts depending on the method.

The honest answer is: there is no "correct" method. All of them describe the same physiological transition, but they draw the line in different places. And the transition is in truth not a line but a range.

How to deal with that:

• Choose one method and stay with it. This is the most important point. If you test again in six months, the comparison is only meaningful if the same method was used. That is why our analysis anchors the zones to the method chosen first and remembers that choice.
• Look at several at once. Our tool lets you display up to three methods simultaneously as coloured lines in all charts. If they lie close together, the estimate is robust. If they lie far apart, trust the zones less and pay more attention to how the training feels.
• Treat the zones as a starting point, not a verdict. They are a physiologically grounded starting value. How the training feels, how heart rate behaves over a long session, and how you feel the next day all correct that starting value.

That is precisely why our tools never give a single number with an exclamation mark, but always a selection with context.

6. The zone systems – why every device says something different

You now have LT1 and LT2. Zones can be built from them. There are several common systems, and our analysis computes four of them – all anchored to your measured thresholds rather than to population averages.

6.1 The 3-zone model (Seiler)

The plainest and scientifically best-founded model. It has exactly three zones, because there are exactly two physiological boundaries. It was developed by the physiologist Stephen Seiler at the University of Agder in Norway, from observations of how top endurance athletes actually train [26][27].

• Zone 1 – below LT1. Predominantly aerobic, fat-dominant, easy to recover from – the anaerobic pathways always run alongside, they simply hardly matter here. Lactate stays near the resting value. The bulk of training volume belongs here. In Seiler's original study, elite athletes spent around 75 per cent of their sessions here.
• Zone 2 – between LT1 and LT2. The inter-threshold zone, in the jargon the "grey zone". Training here is effective – it improves threshold power, movement economy and load tolerance – but it costs considerably more recovery than easy training. At high total volumes the polarised model therefore keeps it deliberately small.
• Zone 3 – above LT2. Genuine high intensity, sustainable only in short structured intervals. Drives maximal oxygen uptake and cardiac output.

Careful, easy to confuse: when you read online about "Zone 2 training" as the great recommendation, it is almost never this Zone 2 that is meant, but Zone 2 of the 5- or 7-zone system – that is, the upper part of what is called Zone 1 here. This terminological confusion is the source of many misunderstandings; more on it in section 11.

6.2 The 5-zone model (Coggan, anchored to LT1/LT2)

A pragmatic compromise: more resolution than three zones, but still only LT1 and LT2 as genuine boundaries.

• Z1 base – from zero to LT1. The whole sub-threshold aerobic range.
• Z2 tempo – from LT1 to LT2. The inter-threshold zone.
• Z3 threshold – from LT2 to about 106 per cent of LT2. Genuine threshold work.
• Z4 VO2max – about 106 to 120 per cent of LT2. Targets maximal oxygen uptake.
• Z5 anaerobic – above that. Very short, very hard efforts.

This model is particularly useful for non-elite athletes and for physiotherapy, because it simplifies the choice without blurring the two relevant boundaries.

6.3 The 7-zone model (Coggan / Rouvy)

The most widespread system in cycling, developed by the exercise physiologist Andrew Coggan and popularised by the book Training and Racing with a Power Meter. It is the standard system in Rouvy, TrainerRoad, TrainingPeaks and most power meters.

The zones are called Z1 active recovery, Z2 endurance, Z3 tempo, Z4 threshold, Z5 VO2max, Z6 anaerobic and Z7 neuromuscular. Normally they are given as percentages of FTP. In our analysis they are anchored instead to your measured thresholds: Z2 ends at your LT1, Z4 begins at your LT2.

Why this anchoring matters: the usual 75 per cent of FTP as the upper boundary of Zone 2 assumes that FTP and LT2 coincide – which is roughly true for elite athletes but not for recreational athletes and patients. There, FTP often underestimates LT2 considerably, and the 75-per-cent rule sets the Zone 2 boundary far too high.

The limitation, honestly stated: the subdivisions above Z4 – Z5, Z6, Z7 – correspond to no physiological boundaries. They are practical subdivisions of the high-intensity range.

6.4 The Norwegian 5-zone model

The system used in Norwegian elite sport. It uses LT1 as the boundary between Z1 and Z2 and LT2 as the boundary between Z3 and Z4 – but subdivides both the range below LT1 and the range between the thresholds into two parts each.

• Z1 easy – clearly below LT1. Lactate typically under 1.5 to 2.0 mmol/L. Norwegian coaches report that 75 to 80 per cent of all sessions take place here [35].
• Z2 moderate – also below LT1, but close to it.
• Z3 sub-threshold – from LT1 to LT2. The characteristic zone of this model and the intensity of the famous "double threshold" sessions.
• Z4 threshold – from LT2 to about 115 per cent of LT2. Used sparingly, typically one hard session per week.
• Z5 maximal – above that. Sprints, very short efforts.

The decisive difference from the polarised model: the inter-threshold zone is not avoided here but used deliberately. More on this in section 8.3.

6.5 The translation table

If your watch or power meter shows zones 1–5 or 1–7, you need this table. It appears in the same form in ZoneForge. The anchors are always LT1 and LT2 – everything else is subdivision.

The biggest trap in this table: when your watch shows "Zone 3 – tempo", that sounds like a sensible training zone. In truth it is the grey zone. Devices that give this range a friendly name tempt people to train there constantly – in a range that tolerates only a limited dose.

6.6 FTP, critical power, MLSS – three names, one territory

Three terms that circle the same physiological boundary and are still not the same thing:

• MLSS (maximal lactate steady state). The physiological reference value, measured directly through several long constant-load efforts on different days. Laborious, but the reference method. LT2 from a step test estimates this value – with deviations of around five to six per cent in individual people [58].
• Critical power (CP). Calculated from several tests to exhaustion. Typically lies somewhat above FTP – in trained cyclists by about 5 to 10 watts.
• FTP. The power that can be held for roughly one hour. In practice usually estimated as 95 per cent of the best 20-minute effort.

The approximate hierarchy is: CP ≥ FTP ≥ MLSS ≈ LT2 – where the "≈" is to be taken literally: adjacent, not identical. In well-trained people the differences are small. In recreational athletes they can be considerable – and the reason is simple: the definition of FTP assumes you can hold your LT2 power for a full hour. That is exactly what untrained people often cannot do; they may manage 25 minutes. Our analysis therefore asks explicitly how long you could hold your LT2 power and corrects the FTP accordingly.

6.7 Why percentages of maximum heart rate get it wrong

The most widespread rule of thumb in endurance sport goes: maximum heart rate is 220 minus your age, and the easy zone lies at 60 to 70 per cent of that.

Both halves are unreliable.

First, "220 minus age" is a regression line through a large cloud of points. The scatter around this value is around ten to twelve beats per minute in either direction. For a 60-year-old, the actual maximum heart rate therefore lies somewhere between about 145 and 175 – and the formula says 160.

Second – and this is more serious – LT1 does not occur at the same percentage of maximum heart rate in everyone. The study of 50 cyclists mentioned earlier quantified this [25]: variability between people amounted to up to 29 per cent of the wattage for fixed percentages of maximum heart rate. Put differently: two people with the same maximum heart rate can be in completely different metabolic states at 72 per cent of it – one comfortably below LT1, the other already well above.

That is why our tools anchor the zones to measured thresholds and not to percentages. And it is why the field signals in the next section are more valuable for most people than any number on a watch.

7. Finding your zones without a laboratory

Most people will never have a lactate test. That is not a problem – there are well-studied alternatives that are in everyday life even more reliable than an outdated laboratory number.

7.1 The talk test

The talk test is the simplest and best-studied method of intensity control without equipment. The basic principle: above a certain intensity, breathing rises so much that connected speech becomes impossible.

The pioneering study comes from a group at the University of Wisconsin [49]. Participants in a step test were repeatedly asked "can you still speak comfortably?" and the answers were compared with measured ventilatory thresholds. The result was surprisingly clean:

• As long as the answer was a clear "yes", participants were below VT1 (that is, below LT1).
• At the first uncertain answer – "yes, but…", in the literature called equivocal – they were exactly at the threshold.
• A clear "no" corresponded to VT2, that is LT2.

Three answer levels, three training zones. The most comprehensive critical review to date confirms that the talk test is a valid and repeatable method in athletes, healthy adults and cardiac patients – with limitations depending on the group [48]. A study in cyclists found the "yes, but" step at 179 watts and the measured LT2 at 170 watts – a difference that was not statistically meaningful [50]. The method also works in very well-trained competitive cyclists [51], and an adapted variant has been tested for people with overweight [52].

7.2 The singing test – and the biggest misunderstanding

Now comes the point where almost all guides get it wrong.

The widespread rule says: "you should still be able to talk, but not to sing." Almost everywhere it is presented as if it described the middle of the easy range.

That is wrong. "Can talk, cannot sing" marks the upper limit of Zone 1 – you are crossing LT1, you are not safely below it.

The evidence comes from studies that placed the talk test directly against measured thresholds. Power and heart rate at LT1 were considerably lower there than at the last step where comfortable speech was still possible. So you can already be above LT1 and still talk comfortably for quite a while. The ability to sing disappears first – it is the earliest signal.

From this follows the rule that appears in our tools:

For Zone 1 training, singing must remain effortless – not merely just about possible. As soon as it becomes an effort, slow down.

The full mapping, as stored in ZoneForge:

A second, equally good signal is breathing: below LT1, breathing should feel like walking – nose breathing is comfortably possible, breaths are somewhat deeper than at rest, but the rhythm stays calm. Anyone who has to take breaths at unnatural places in the middle of a sentence is far above Zone 1.

7.3 Heart rate drift and decoupling

A phenomenon everyone knows but hardly anyone interprets: you ride for an hour at constant pace, and your pulse still creeps upward.

This is called cardiovascular drift or heart rate drift. The cause: as core body temperature rises and fluid is lost, blood volume falls, the heart ejects less per beat – and therefore has to beat faster to move the same amount of blood. About half the effect is due to fluid loss, the other half to heat.

How to evaluate it – mentally divide the session into two halves and compare the mean heart rate:

• Under 5 per cent rise over 60 minutes: excellent. You really were in Zone 1. Carry on as planned.
• 5 to 10 per cent: normal. Probably fluid loss or heat. Hold the pace, but drink.
• Over 10 per cent, or heart rate exceeds your LT1 value: reduce the pace immediately. You have drifted into the grey zone – regardless of what your computer says.

Anyone riding with a power meter can look at the same thing more precisely: the ratio of power to heart rate in the two halves of the session. If this ratio shifts by less than 5 per cent, the aerobic base is good for that duration. The technical term is decoupling; ZoneForge computes it automatically from an uploaded training file.

Why this is useful: increasing decoupling across several sessions is one of the most reliable early signs that recovery is no longer keeping up. You see it before you feel it.

7.4 The appetite check

An underrated signal that costs nothing: how hungry are you 20 to 30 minutes after an easy session?

After genuine Zone 1 training, most people have normal to clear appetite. If appetite is noticeably suppressed, the session was not Zone 1. Suppressed appetite after exercise is a response of the sympathetic nervous system – the part of the nervous system that ramps up under stress and effort. It indicates that you worked above LT1.

This is not a measurement procedure but a plausibility check. But it is a good one: it costs nothing, needs no device, and it rarely deceives.

7.5 The effort scale

The Borg scale lets you rate your effort on a scale. It exists in two versions: the original from 6 to 20 (the numbers roughly corresponded to heart rate divided by ten) and the newer one from 0 to 10.

Rough mapping on the 6-to-20 scale: Zone 1 lies at 9 to 12 ("very light" to "somewhat hard"), the grey zone at 13 to 15, Zone 3 at 16 and above.

When the scale becomes especially important: in people taking beta blockers. These drugs clearly alter the heart rate response, at rest and under load.

What follows from that is often put too sweepingly. What becomes unreliable are above all calculated target zones: anything based on "220 minus age", or on a maximum heart rate measured at some earlier time without the drug. Heart rate itself remains usable when it was measured on the same medication – an exercise test under ongoing therapy provides individual values that zones can be anchored to. The most reliable approach is the combination: measured heart rate together with perceived effort, the talk test and, where available, power in watts. That is exactly what the European recommendations on intensity control in cardiac rehabilitation say [53].

7.6 What watches and apps can do – and what they cannot

Briefly and honestly:

• What works well: measuring heart rate with a chest strap and following it over time. Logging the duration and frequency of sessions. For cyclists: power in watts.
• What works moderately: heart rate measurement at the wrist. It is usable at steady loads but fails during rapid changes and with cold hands – that is, precisely during intervals.
• What does not work: the automatically estimated VO2max, the automatically calculated "lactate threshold" and the proposed zones. They rest on formulas built from population averages – exactly what section 6.7 advises against. As a way of following a trend within one and the same person these values are sometimes usable; as absolute numbers they are not.

The practical recommendation: use the watch to record, not to prescribe. Your breath and your voice do the steering.

8. The training models

Now to the question that is argued about most in endurance sport: how do you distribute training time across the three zones? The technical term is intensity distribution.

8.1 Polarised – the 80/20 model

The best-known model, developed by Stephen Seiler from observations of how top endurance athletes actually train [26][27]. The proposal is:

• About 80 per cent of sessions at low intensity – below LT1.
• About 20 per cent at high intensity – above LT2.
• As little as possible in between.

The name "polarised" comes from the fact that training gathers at the two poles and the middle stays empty. The rationale is plausible: easy sessions build the aerobic base without producing meaningful fatigue. Hard intervals deliver the stimulus that easy training cannot. The middle costs recovery without giving the corresponding stimulus.

A detail that is constantly overlooked: the 80/20 distribution refers to the number of sessions, not to time. Because hard sessions are shorter, an 80/20 distribution by sessions becomes roughly 90/10 by time [26]. Anyone who confuses the two trains twice as much hard work as intended. How much the picture shifts depending on whether you count sessions or minutes has been studied in its own right: the same training data look polarised under one method of analysis and pyramidal under another [37].

Evidence for it: a randomised study found greater improvements over nine weeks in maximal oxygen uptake, time to exhaustion and peak power under polarised training than under three comparison models [28]. A second study confirmed this in trained cyclists over six weeks against a threshold-focused model [29]. And an observational study in elite runners found that the time spent in the easy range correlated most strongly with competition performance [30].

8.2 Pyramidal – the underrated candidate

In the pyramidal model the proportion decreases continuously as intensity rises: much Zone 1, less Zone 2, least Zone 3. Unlike the polarised model, the middle is not avoided but merely kept small.

Interestingly, this is often what elite athletes actually do when you count their training time rather than their sessions [31]. And across a whole season the distribution shifts: in the build-up phase it is more pyramidal, towards competition more polarised.

8.3 Threshold-focused and the Norwegian model

In the threshold-focused model the emphasis lies deliberately on the inter-threshold zone. For a long time this was regarded as the classic mistake.

The Norwegian model has restored this approach's reputation. It shares with the polarised model the high volume in the easy range – 75 to 80 per cent of all sessions [35] – but uses the inter-threshold zone deliberately instead of avoiding it. Its hallmark is the double-threshold session, developed by the Norwegian runner Marius Bakken and made known worldwide through the triathletes Kristian Blummenfelt and Gustav Iden and through the Ingebrigtsen running family.

How it works:

• On two days a week, two sessions are completed daily, both with controlled intervals just below LT2.
• Lactate is measured during the session and strictly capped – usually at 3.0 to 3.5 mmol/L, at most 4.5.
• Typical sessions are long intervals: 4×10 minutes, 6×8 minutes, 3×15 minutes – ridden at a controlled pace, not as maximal efforts.

The decisive condition: this cap is not a detail but the basic prerequisite. Without it, the second session on the same day would lead to exhaustion and make the whole model impossible. That is precisely why this model needs a lactate meter – or at least very precise knowledge of one's own thresholds.

The physiological aim is to accumulate as much time as possible just below the threshold, in order to produce two adaptations: a lower rate of lactate formation and a higher capacity to clear lactate again. Both push LT2 upwards – more power at the same lactate level.

A review describes this lactate-guided threshold training within a high-volume low-intensity framework as the possible "next step" in the evolution of distance running training [36].

The honest limitation: there is almost only observational evidence for this model, no randomised comparisons. It describes well what successful athletes do. Whether it is causally better has not been shown. And it is not intended for people who train four to five hours a week in any case: twice daily on two days presupposes a volume that is rarely possible in everyday life.

8.4 What the pooled analyses say: no clear winner

And here comes the disappointment that must not be missing from this article.

The most important recent synthesis compared polarised training systematically with the other models [32]. The result:

• Polarised versus pyramidal: a standardised difference of 0.08 with a confidence interval from −0.39 to +0.55. In plain language: practically zero, and the range in which the true value plausibly lies covers both directions.
• Polarised versus threshold-focused: 0.24 with a confidence interval from −0.46 to +0.94. Likewise not secure.

A second review reaches a similar conclusion: both polarised and pyramidal training produce better adaptations than threshold-focused or purely high-intensity approaches – but the two hardly differ from each other [33]. And a much-noted rebuttal in a specialist journal argues explicitly that polarised training is not optimal for all groups of people [34].

Why the early studies were clearer: they were well controlled and carried out in trained people with high training volume. The broader syntheses include more heterogeneous studies and less trained participants. On the one hand that produces more noise – on the other it shows that the advantage of the polarised model does not hold generally. It may only appear under particular conditions: high volume, well-trained people, sufficiently long interventions.

8.5 What this means for you

If no model wins clearly, what do you do? Three points that all models share and which are therefore the actual core:

1. By far the largest part of training is easy. In all the models described, 75 to 85 per cent of sessions lie below LT1. Two qualifications belong with that: these figures come from observed training patterns of successful endurance athletes, and how they turn out depends on whether you count sessions, minutes or load points [37]. That is not a general prescription for patients and recreational athletes – but it is a usable reference point.
2. There is a small, clearly delimited hard portion. Whether it lies above LT2 (polarised) or just below it (Norwegian) is the actual point of dispute – and apparently the less important one.
3. The middle is dosed, not forbidden. No model recommends spending the bulk of the time between LT1 and LT2 – but all of them use this range in moderation. It is the dosing that most people fail at, not the zone itself.

The rest is fine-tuning for people who already train ten hours a week. If you have four hours a week available, it is not the choice of model that decides but whether you actually complete those four hours and whether the easy sessions really are easy.

9. The grey zone – effective, but expensive

This section is short, because the message is short. It is nevertheless one of the most important practical sections of the article.

The grey zone is the range between LT1 and LT2. First, the part that guides usually get wrong: this range is not ineffective. Tempo and threshold training improves threshold power, movement economy and the ability to tolerate load – section 10.3 is about nothing else. For people with little training time it is in fact distinctly useful.

The problem is a different one: this intensity costs. It produces more fatigue than easy training without delivering the stimulus of genuine high-intensity work, and it therefore tolerates only a limited dose. That is where it goes wrong – not because the zone is a mistake, but because almost every session ends up there unintentionally. Whoever plans the middle is training sensibly. Whoever gets stranded in it accumulates fatigue without the corresponding gain.

And almost everyone gets stranded. The reasons are human:

• Easy feels like too little. Anyone investing an hour wants the feeling of having achieved something. A genuinely easy session feels almost slightly embarrassing.
• Hard feels like too much. Real Zone 3 intervals are unpleasant. It is tempting to ease off a little – and there you are, in the middle.
• The devices help it along. "Zone 3 – tempo" sounds like the sensible middle – and invites people to train there every day, although this range needs to be dosed sparingly.
• In a group it is almost unavoidable. When running or cycling together, the pace settles at a level that is easy for nobody and hard for nobody.

How strong this pull to the middle is is shown with particular clarity by a clinical study. In a trial with 261 people with heart failure, two groups were supervised with clearly prescribed intensities – one hard, one moderate. The training records showed: 51 per cent of people in the hard group trained below their prescription, and 80 per cent in the moderate group above it [55]. Both groups migrated to the middle – and that under supervision, in a scientific trial, with a clear prescription.

If it happens there, it happens to you. So knowing the model is not enough. You need a signal that works during the session – which is why section 7 is so detailed.

10. Intervals – the toolbox

Interval training means hard efforts alternating with recovery. The point is simple – at an intensity you could only sustain for three minutes in one go, breaks let you accumulate twenty minutes in total.

The protocols in this section are all available in ZoneForge, each with its source. Here are the most important ones with their rationale.

10.1 Long intervals: the 4×8 protocol

The most-studied comparison of interval durations divided 35 trained recreational cyclists over seven weeks into groups that each completed, twice a week, 4×4 minutes, 4×8 minutes or 4×16 minutes – each at the maximum tolerable intensity [38].

The results were clear:

• The 4×8-minute group improved the most: 11.4 per cent gain in the measured variables.
• The 4×16-minute group reached 5.6 per cent, the 4×4-minute group 5.5 per cent, and a group that only trained easily 4.2 per cent.
• The lactate values during the intervals are instructive: 4.9 mmol/L in the 16-minute blocks, 9.6 in the 8-minute blocks, 13.2 in the 4-minute blocks.

The paper's conclusion: 32 minutes of work at 90 per cent of maximum heart rate achieve more than 16 minutes at 95 per cent – and are even perceived as less strenuous. Intensity and accumulated duration interact; the highest intensity is not automatically the best.

These numbers come from trained cyclists. Different orders of magnitude apply to people in rehabilitation – but the principle carries over: the maximum tolerable intensity is not the most effective one.

10.2 Short intervals: 30/15 and 30/30

At the other end are very short efforts with very short breaks. The idea behind them is ingenious: in 30 seconds of work you can produce far more power than in 8 minutes – and because the break lasts only 15 seconds, heart rate and oxygen uptake barely fall in between. So you accumulate a lot of time at very high oxygen uptake without having to suffer continuously.

A comparison study in cyclists over ten weeks set short intervals (three sets of thirteen repetitions of 30 seconds work and 15 seconds rest) against long intervals (4×5 minutes), matched for total effort [39]. The short-interval group improved maximal oxygen uptake by 8.7 per cent, the long-interval group by 2.6 per cent.

An older French study had examined the same principle with 30 seconds work and 30 seconds active recovery and showed that runners stay considerably longer near their maximal oxygen uptake than during continuous work [40].

How does this fit with the previous section? Both findings hold. There is no single best interval. A detailed systematic account of interval programming lists nine adjustable variables – duration and intensity of the effort, duration and type of the break, number of repetitions, number of sets and so on – and makes clear that different combinations load different systems [41]. For practice this means: variety makes sense, and the choice between "long" and "short" matters less than whether hard training happens regularly at all.

10.3 Threshold intervals

The third family lies deliberately below LT2. Typical formats: 4×10 minutes, 3×15 minutes or 6×5 minutes with short breaks. Target lactate 2.5 to 3.5 mmol/L, heart rate about 82 to 90 per cent of maximum, speaking in short sentences still possible.

These sessions feel "comfortably hard" – demanding but controlled. They are the centrepiece of the Norwegian model and suit people who are not yet ready for genuine high-intensity training particularly well: the recovery cost is considerably lower than after Zone 3 intervals, while the stimulus for threshold power is substantial.

In ZoneForge these protocols are the only intensity form above Zone 1 provided for the health-oriented categories – Zone 3 intervals are only added later there, or not at all.

10.4 How often, and how much spacing

• Frequency: one to two hard sessions per week is the sensible upper limit for the vast majority of people. More than that regularly comes at the cost of quality.
• Spacing: as a rule of thumb at least 48 hours between two hard sessions, in older people rather 72. These are cautious reference values, not established limits – how quickly someone recovers varies considerably from person to person. What counts is your state before the next hard session, not the calendar. Adaptation happens in recovery, not in the effort.
• Evenness: all repetitions should be completed at the same intensity. If the third or fourth repetition drops off noticeably, the intensity was chosen too high – not your willpower too small.
• Before and after: at least 10 to 15 minutes of easy warm-up belongs before hard intervals, at least 10 minutes of cool-down afterwards.
• When to stop: if you cannot complete the planned repetitions, stop and carry on easy. An incomplete hard session is a signal, not a failure.

11. Zone 2 – hype and reality

Hardly any term has received as much attention in recent years as "Zone 2 training". The story runs roughly: there is a magic intensity just below the first threshold at which fat burning is highest and the effect on mitochondria greatest.

What is true about it and what is not:

What is true:

• There really is an intensity at which absolute fat oxidation reaches its maximum. It is called FatMax [45]. Above it, total expenditure keeps rising but the share from fat falls – this transition is called the crossover point [13].
• Regular training in this range improves metabolic flexibility – the ability to switch between fat and sugar as fuel and to spare the sugar stores [14]. For efforts over two hours this is directly noticeable.
• Easy training is the most tolerable way to accumulate a lot of volume – and volume is the strongest predictor of the adaptation of the vessel network [10].

What is not true:

• FatMax is not the same as LT1. A pooled analysis of 14 studies with 774 participants did find an association, but agreement in individual people was too imprecise to use one in place of the other [46]. In untrained people and in people with overweight, FatMax can lie well below LT1. In the study of 50 cyclists, FatMax lay on average about 25 per cent below the first threshold [25].
• A FatMax test helps little for the training prescription. This was examined directly, with sobering results [47].
• And Zone 2 is not the optimal intensity for the general population. A critical review from 2025 explicitly contradicts the blanket recommendation [44]. Its argument: the recommendation stems from observations of elite endurance athletes who complete enormous amounts of easy training. For people with little available training time, the data support prioritising higher intensities, because they stimulate the mitochondria more per hour. This fits the finding of the large review that sprint intervals were about four times as effective per hour invested as easy continuous training [10].
• Even the experts disagree on the definition. A survey of fourteen leading experts on what "Zone 2 training" even is produced different answers regarding the boundary, the method and the expected adaptations [43].

The balanced view: easy training is valuable – as a foundation, as a tolerable way to accumulate a lot of movement, and as the precondition for being able to complete the hard sessions well at all. It is just not the one magic intensity. If your total time is limited, part of it is better invested in genuinely hard intervals than all of it in Zone 2.

12. How much do you need?

Two answers, depending on what is at stake.

For health, the 2020 World Health Organization recommendations apply [1]. For adults:

• 150 to 300 minutes of moderate endurance activity per week – or
• 75 to 150 minutes of vigorous activity – or an equivalent combination.
• In addition, on at least two days per week, strengthening of all major muscle groups.
• For people aged 65 and over, additionally on at least three days per week, a varied, multicomponent activity programme emphasising functional balance and strength training. That means one programme containing both – not two separate obligations.

Two sentences from these recommendations matter more than the numbers: every move counts, and something is better than nothing. The greatest health gain lies in the step from complete inactivity to a little activity – not in the step from 150 to 300 minutes.

For performance the answer is different. If you want to become measurably fitter, you need:

• Two sessions per week already improve endurance if they are dosed adequately. Three or more make it easier to accumulate enough volume and to distribute easy and harder stimuli sensibly across the week. The large review found clear associations between training frequency and adaptation: six sessions worked better than four, and four better than two [10].
• Some intensity helps – it is not obligatory. Stimuli above LT2 improve maximal oxygen uptake particularly time-efficiently. For beginners, for older people and for many patients, however, purely moderate training also produces clear gains. Those with little time gain most from intensity; those with time, or who have to build up cautiously, get there without it too.
• Patience. Measurable changes often show up after as little as two to six weeks. They usually become clear, noticeable in everyday life and reasonably reliable only after eight to twelve weeks – which is how long it is worth persisting before judging a programme.

How much is realistic to expect? A pooled analysis of 28 controlled studies with 723 participants found a mean increase in maximal oxygen uptake of 4.9 mL/kg/min under classic endurance training and 5.5 under interval training [42]. Starting from a value of 40, that is around 12 to 14 per cent – a clear change, noticeable in everyday life. In untrained people it is larger, in already-trained people smaller.

A second pooled analysis, which evaluated interval programmes over six to thirteen weeks exclusively, found a mean increase of 0.51 litres of oxygen uptake per minute in 334 participants [7]. In the subgroup with longer intervals it was 0.8 to 0.9 litres – and there every single participant improved. This fits the finding from section 2.4: where enough stimulus is applied, "non-response" becomes rare.

13. Endurance training in illness and in older age

13.1 Cardiovascular disease

Here the evidence is particularly good – and particularly instructive, because the simple story has not held up.

What speaks for intense training. A pooled analysis of ten studies with 273 patients with coronary heart disease, heart failure, high blood pressure, metabolic syndrome or obesity found a clear advantage of interval training over moderate continuous training: oxygen uptake rose by 3.03 mL/kg/min more, which corresponds to about 9 per cent and means roughly a doubling of the effect [54].

What speaks against it. The largest single study found no advantage. 261 people with heart failure and severely reduced pumping function were supervised for twelve weeks with either interval training at 90 to 95 per cent of maximum heart rate, moderate continuous training at 60 to 70 per cent, or only a recommendation to exercise regularly [55]. Result: for the primary outcome – the size of the left ventricle – there was no difference between the two training groups. Nor for oxygen uptake. Both were, however, better than the recommendation alone. And after a year nothing was left of the improvements.

Why the difference? The most important reason is in the study itself and was already mentioned in section 9: half the interval group trained too easily, four fifths of the moderate group too hard. In the end both groups trained similarly. The difference on paper did not exist in practice.

And how safe is intense training in heart patients? This was examined in 4846 people at three Norwegian rehabilitation centres over a total of 175,820 training hours [56]. During that time there was one fatal cardiac arrest during moderate training and two non-fatal ones during intense training. Expressed as rates: one event per 129,456 hours of moderate and one per 23,182 hours of intense training. How that is read matters: the absolute event rate was very low with both forms of training. Because there were only three events in total, however, the difference between the two forms cannot be judged reliably – three cases do not support a robust ratio.

The European society for preventive cardiology has published a detailed position statement on how intensity should be determined and prescribed in cardiac rehabilitation [53]. Two points from it for practice: on beta blockers, calculated target zones are of no use – a heart rate measured on the same medication, by contrast, is (see section 7.5); and perceived effort and the talk test are valid tools in this setting too.

13.2 Older people, limited reserves, rehabilitation

Three statements that must be placed side by side.

First: the ability to adapt to endurance training is preserved lifelong – regardless of age, sex and the presence of illness [10]. Someone starting at 78 improves. Someone with low starting fitness improves the most in percentage terms.

Second: the build-up takes more time, and recovery between hard sessions lasts longer. In ZoneForge, a minimum spacing of 72 hours between intense sessions is therefore stored for the older categories, and the week-to-week rate of increase is set smaller. That is a deliberately cautious default of the tool, not a scientifically established rule – how long someone needs to recover varies considerably.

Third: in people with limited reserves – in technical language frailty – the order is different. There, strength and balance training come first, together with easy endurance activity. Intense interval training only comes into question once a foundation exists, and then only after individual assessment. That is exactly how it is mapped in ZoneForge: for the two lowest categories no Zone 3 is provided at all.

Anyone starting again after an illness or an operation will find the relevant considerations in our articles Leaving hospital weaker and Fall prevention.

13.3 When to get checked out first

For healthy adults starting with moderate activity, no medical examination is needed. For the following situations it is – before you begin intense training:

• Known cardiovascular disease, diabetes or kidney disease.
• Chest pain, unusual breathlessness, dizziness or fainting on exertion – in this case before any increase.
• Irregular or unusual heartbeat.
• Sudden, unexplained drop in performance.
• You have been inactive for a longer period and want to start intense training – especially if one of the conditions above is also known.

Why there is no age limit here. You often read that from the age of 45 a medical examination belongs before starting to train. That is not how current screening algorithms work any more [62]. Four other things matter: whether you are currently active on a regular basis, whether cardiovascular, metabolic or kidney disease is known, whether symptoms occur on exertion – and how intensely you intend to train. A 60-year-old who has walked briskly for years and wants to continue doing so needs no assessment. A 40-year-old with known diabetes who wants to start hard intervals does.

A proven aid for this assessment is the PAR-Q+ questionnaire (physical activity readiness questionnaire). It starts with seven general yes-no questions. If every answer is "no", you can begin. A "yes" does not automatically send you to a doctor but first leads to follow-up questions about the condition concerned – only if something stands out there does an assessment belong before training starts. In ZoneForge this questionnaire is built in for the relevant categories.

14. What a training build-up over weeks looks like

Up to here it has been about individual sessions. Now to the question of how to arrange them over weeks and months. The technical term is periodisation.

The basic pattern consists of three building blocks, which are called the same in ZoneForge:

• Accumulation block. Much volume, little intensity. This is where the aerobic base is built – fat oxidation, mitochondria, the ability to tolerate high weekly volumes. The most common mistake in this phase is that the easy sessions are not easy enough.
• Transmutation block. Volume stays, threshold intervals and occasional hard sessions are added. Threshold power rises.
• Realisation block. Volume drops by 15 to 20 per cent, intensity stays or rises. Only relevant if you are working towards an event.

Between them, a deload week belongs at regular intervals: volume clearly reduced – in performance-oriented periodisation usually by 40 to 50 per cent – and no hard sessions. It sounds like going backwards and is the opposite: adaptation happens during recovery, not during the load. For general health-oriented training, however, such a fixed rhythm is not obligatory. There it is usually enough to watch for the signs in section 15 and to ease off when they appear.

Progression – and why the familiar rule is not a safety limit. That volume and intensity should be increased gradually is not in dispute. The usual figure attached to it is "at most 5 to 10 per cent more per week", with the smaller value for older and less trained people. That is a sensible precaution – but not a scientifically established limit. It has been tested, among others, in novice runners: a gently graded build-up programme there produced just as many complaints as the usual programme [59].

The same applies to the calculated measure ZoneForge displays: the ratio of last week's load to the average of the past four weeks (the acute:chronic workload ratio). It makes visible whether something has just changed, and for that it is useful. As a risk value it does not work. The method has been sharply criticised on methodological grounds: the two numbers contain the same data and are mathematically coupled, the one-week and four-week windows are chosen arbitrarily, and an observed association does not establish a cause [60]. And when such load management was actually tested in a randomised trial with 482 young footballers, it prevented no health problems [61]. A value of 1.2 or 1.3 is therefore not a safety line, but at most a reason to look more closely.

What counts instead is less spectacular: your starting level, the type of loading (running loads the legs differently from cycling or swimming), individual spikes, existing complaints – and how well you recover between sessions. The signs for that are in section 15.

A realistic example for four hours a week, health-oriented:

• Monday: rest or strength training.
• Tuesday: 60 minutes easy – singing must remain effortless throughout.
• Wednesday: strength training.
• Thursday: 45 minutes with threshold intervals, for example 4×6 minutes with 2 minutes easy between. Comfortably hard, short sentences still possible.
• Friday: rest.
• Saturday: 45 minutes easy.
• Sunday: 90 minutes easy, calmly out in the countryside.

That comes to about four hours, roughly 85 per cent of them easy. Every fourth week, everything is cut by 40 per cent and the Thursday is replaced by an easy session.

15. How to tell that it is becoming too much

Too little training achieves too little. Too much also achieves too little – only with more effort and higher risk. The signs, roughly in the order in which they appear:

• Morning resting heart rate is elevated – more than about five to seven beats above your usual value, on several consecutive days.
• Heart rate drift increases. You suddenly need less power at the same heart rate – this is the most reliable early signal there is.
• Sleep gets worse. Often first: falling asleep works, but you wake in the night.
• Mood falls and the desire to train disappears. This is not a character problem but an early marker.
• The quality of hard sessions drops. The last repetitions fail although the first were normal.
• Persistent muscle soreness and a general feeling of heavy legs.

Two or three of these signs at once mean: take a deload week. Not "push through". The technical term for the state you otherwise end up in is non-functional overreaching – and recovering from it takes weeks to months.

And the rule for days of illness: with symptoms above the neck – a cold, a mildly sore throat – easy training is fine. With fever, aching limbs or symptoms below the neck, the rule is: no training. Inflammation of the heart muscle after training too early is rare, but serious.

16. Our two tools

Both are freely accessible, run entirely in the browser and send no data anywhere.

The lactate threshold analysis evaluates a lactate step test. You enter the steps – power, lactate, heart rate, rating of effort – and the tool computes all sixteen threshold methods from section 5, displays up to three of them simultaneously as lines in all charts, and derives the four zone models from section 6. Added to that are charts for lactate economy, cardiac efficiency and heart rate course, plus a detailed background section with more than fifty sources. Everything can be downloaded as PDF, PNG, Excel or as a reference file for a bibliography manager. Note: the interface exists in German and French; the scientific explanations are in German.

PTH ZoneForge turns that into a training plan. In five steps: enter profile and thresholds, choose training phase and protocols, generate the block plan, record daily readiness, and log the completed sessions. The tool computes the target distribution across the three zones, proposes concrete interval protocols with their sources, monitors the rate of progression and reports when too much time ends up in the grey zone.

The path from one to the other: at the end of the analysis there is a button "→ transfer to ZoneForge". It passes all computed threshold values including heart rates directly across – no file, no detour. There you only choose the method the zones should hang on, and the plan is ready.

Two honest notes. First, both tools are made for professionals and presuppose a certain amount of knowledge – this article should have covered a large part of it. Second, the lactate analysis is explicitly labelled a beta version: the methods are continually being reviewed and extended.

17. Ten misunderstandings

1. "Lactate is a waste product and causes muscle soreness." No. Lactate is a fuel and a signalling substance, and the level is back to normal an hour after exercise. Soreness follows unaccustomed, above all braking, loading.
2. "No pain, no gain." For the aerobic base the opposite holds. Most training should feel easy – so easy that it is almost boring.
3. "Zone 2 is the optimal intensity." This recommendation stems from observing elite athletes with very high training volumes and does not transfer to people with little time [44].
4. "My watch knows my zones." It knows a formula. Fixed percentages of maximum heart rate vary between individuals by up to 29 per cent [25].
5. "I should still be able to talk but not to sing." That describes the upper limit of the easy zone, not its middle. For real Zone 1 training, singing must remain effortless.
6. "Fat burning requires low intensity." The percentage share from fat is higher at low intensity, but the absolute expenditure is not necessarily – and for body weight what counts is the overall balance, not the fuel [13].
7. "I don't respond to training." In one study with 78 participants the non-responders disappeared as soon as the training dose was increased [8]. So before writing yourself off: more dose, more time – and a second look at whether the measurement was accurate enough in the first place.
8. "Harder is always better." In the comparison study of interval durations, the middle variant of 4×8 minutes performed considerably better than the harder 4×4 minutes [38].
9. "Intense training is dangerous in heart disease." More than 175,000 supervised training hours in almost 5000 heart patients produced three events in total – a very low absolute risk, with moderate as well as with intense training [56]. Assessment beforehand nevertheless remains necessary.
10. "At 75 it is no longer worth it." The capacity to adapt is preserved lifelong, and those with the lowest starting fitness gain the most [10].

18. When to get in touch with us

• If you want to start again after an illness, an operation or a long break and do not know where to begin.
• If you have been training for months and nothing changes. Often it comes down to how the intensities are distributed, or simply to the total amount – rarely to diligence.
• If you have a chronic illness and would like a structured programme adapted to it.
• If you have symptoms on exertion – breathlessness, chest tightness, dizziness, palpitations. Then get medical assessment first, not us.
• If you have had a lactate test and cannot make sense of the numbers. That is exactly what the analysis is for – and we are glad to go through it with you.
• If you have a goal – a hike, a fun run, a cycling tour – and are looking for a way there that does not wear you out on the way.

19. In summary

The core of this article in a few sentences:

There are two points that matter. LT1 – where it stops being genuinely easy. LT2 – where the balance tips. Every zone system in the world is a subdivision of these two boundaries.

Lactate is not a poison but a fuel and a signalling substance. We measure it not because it is harmful but because it reveals where those two points lie.

The largest part of training belongs below LT1. All models agree on this – polarised, pyramidal, Norwegian. What they argue about is the fine-tuning of the small hard remainder, and that argument is of no consequence for most people.

The sore point is the middle. Not because it is ineffective – threshold training works – but because it costs recovery and therefore needs to be dosed. When almost every session lands there unintentionally, progress fails to appear. Even in supervised clinical trials both groups end up there.

You need no laboratory to prevent this. You need a signal that works during the session: can you still sing effortlessly? Is your pulse rising at the same pace? Are you hungry afterwards? Three questions, no cost, better steering than any formula on a watch.

And the dose is part of the answer. Anyone who believes they do not respond has often trained too little or for too short a time – and sometimes is only measuring ordinary variation. Anyone who believes they are too old is mistaken: the capacity to adapt lasts a lifetime.

The one sentence to take away: make your easy days genuinely easy – then you can make your hard days genuinely hard. Everything else is fine-tuning.

References

All entries were individually verified against Crossref – [1] to [56] on 16 August 2026, the later additions [57] to [62] on 22 August 2026. The references in the text jump to the relevant place in this list; every entry links to the original work.

[1] Bull FC, Al-Ansari SS, Biddle S, et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. British Journal of Sports Medicine. 2020;54(24):1451–1462. https://doi.org/10.1136/bjsports-2020-102955

[2] Ross R, Blair SN, Arena R, et al. Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign. A Scientific Statement From the American Heart Association. Circulation. 2016;134(24):e653–e699. https://doi.org/10.1161/CIR.0000000000000461

[3] Kodama S, Saito K, Tanaka S, et al. Cardiorespiratory Fitness as a Quantitative Predictor of All-Cause Mortality and Cardiovascular Events in Healthy Men and Women: A Meta-analysis. JAMA. 2009;301(19):2024–2035. https://doi.org/10.1001/jama.2009.681

[4] Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of Cardiorespiratory Fitness With Long-term Mortality Among Adults Undergoing Exercise Treadmill Testing. JAMA Network Open. 2018;1(6):e183605. https://doi.org/10.1001/jamanetworkopen.2018.3605

[5] Stensvold D, Viken H, Steinshamn SL, et al. Effect of exercise training for five years on all cause mortality in older adults—the Generation 100 study: randomised controlled trial. BMJ. 2020;371:m3485. https://doi.org/10.1136/bmj.m3485

[6] Lundby C, Montero D, Joyner M. Biology of VO₂max: looking under the physiology lamp. Acta Physiologica. 2017;220(2):218–228. https://doi.org/10.1111/apha.12827

[7] Bacon AP, Carter RE, Ogle EA, Joyner MJ. VO2max Trainability and High Intensity Interval Training in Humans: A Meta-Analysis. PLoS ONE. 2013;8(9):e73182. https://doi.org/10.1371/journal.pone.0073182

[8] Montero D, Lundby C. Refuting the myth of non-response to exercise training: 'non-responders' do respond to higher dose of training. The Journal of Physiology. 2017;595(11):3377–3387. https://doi.org/10.1113/JP273480

[9] Northey JM, Cherbuin N, Pumpa KL, Smee DJ, Rattray B. Exercise interventions for cognitive function in adults older than 50: a systematic review with meta-analysis. British Journal of Sports Medicine. 2018;52(3):154–160. https://doi.org/10.1136/bjsports-2016-096587

[10] Mølmen KS, Almquist NW, Skattebo Ø. Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression. Sports Medicine. 2025;55(1):115–144. https://doi.org/10.1007/s40279-024-02120-2

[11] Smith JAB, Murach KA, Dyar KA, Zierath JR. Exercise metabolism and adaptation in skeletal muscle. Nature Reviews Molecular Cell Biology. 2023;24(9):607–632. https://doi.org/10.1038/s41580-023-00606-x

[12] Nielsen J, Gejl KD, Hey-Mogensen M, et al. Plasticity in mitochondrial cristae density allows metabolic capacity modulation in human skeletal muscle. The Journal of Physiology. 2017;595(9):2839–2847. https://doi.org/10.1113/JP273040

[13] Brooks GA, Mercier J. Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept. Journal of Applied Physiology. 1994;76(6):2253–2261. https://doi.org/10.1152/jappl.1994.76.6.2253

[14] San-Millán I, Brooks GA. Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals. Sports Medicine. 2018;48(2):467–479. https://doi.org/10.1007/s40279-017-0751-x

[15] Brooks GA. The Science and Translation of Lactate Shuttle Theory. Cell Metabolism. 2018;27(4):757–785. https://doi.org/10.1016/j.cmet.2018.03.008

[16] Brooks GA. The tortuous path of lactate shuttle discovery: From cinders and boards to the lab and ICU. Journal of Sport and Health Science. 2020;9(5):446–460. https://doi.org/10.1016/j.jshs.2020.02.006

[17] Brooks GA, Osmond AD, Arevalo JA, et al. Lactate as a myokine and exerkine: drivers and signals of physiology and metabolism. Journal of Applied Physiology. 2023;134(3):529–548. https://doi.org/10.1152/japplphysiol.00497.2022

[18] Poole DC, Rossiter HB, Brooks GA, Gladden LB. The anaerobic threshold: 50+ years of controversy. The Journal of Physiology. 2021;599(3):737–767. https://doi.org/10.1113/JP279963

[19] Jamnick NA, Pettitt RW, Granata C, Pyne DB, Bishop DJ. An Examination and Critique of Current Methods to Determine Exercise Intensity. Sports Medicine. 2020;50(10):1729–1756. https://doi.org/10.1007/s40279-020-01322-8

[20] Faude O, Kindermann W, Meyer T. Lactate Threshold Concepts: How Valid are They? Sports Medicine. 2009;39(6):469–490. https://doi.org/10.2165/00007256-200939060-00003

[21] Kindermann W, Simon G, Keul J. The significance of the aerobic-anaerobic transition for the determination of work load intensities during endurance training. European Journal of Applied Physiology and Occupational Physiology. 1979;42(1):25–34. https://doi.org/10.1007/BF00421101

[22] Beaver WL, Wasserman K, Whipp BJ. Improved detection of lactate threshold during exercise using a log-log transformation. Journal of Applied Physiology. 1985;59(6):1936–1940. https://doi.org/10.1152/jappl.1985.59.6.1936

[23] Cheng B, Kuipers H, Snyder AC, Keizer HA, Jeukendrup A, Hesselink M. A New Approach for the Determination of Ventilatory and Lactate Thresholds. International Journal of Sports Medicine. 1992;13(7):518–522. https://doi.org/10.1055/s-2007-1021309

[24] Valenzuela PL, Alejo LB, Montalvo-Pérez A, et al. Relationship Between Critical Power and Different Lactate Threshold Markers in Recreational Cyclists. Frontiers in Physiology. 2021;12:676484. https://doi.org/10.3389/fphys.2021.676484

[25] Meixner B, Filipas L, Holmberg HC, Sperlich B. Zone 2 Intensity: A Critical Comparison of Individual Variability in Different Submaximal Exercise Intensity Boundaries. Translational Sports Medicine. 2025;2025:2008291. https://doi.org/10.1155/tsm2/2008291

[26] Seiler KS, Kjerland GØ. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an "optimal" distribution? Scandinavian Journal of Medicine & Science in Sports. 2006;16(1):49–56. https://doi.org/10.1111/j.1600-0838.2004.00418.x

[27] Seiler S. What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes? International Journal of Sports Physiology and Performance. 2010;5(3):276–291. https://doi.org/10.1123/ijspp.5.3.276

[28] Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology. 2014;5:33. https://doi.org/10.3389/fphys.2014.00033

[29] Neal CM, Hunter AM, Brennan L, et al. Six weeks of a polarized training-intensity distribution leads to greater physiological and performance adaptations than a threshold model in trained cyclists. Journal of Applied Physiology. 2013;114(4):461–471. https://doi.org/10.1152/japplphysiol.00652.2012

[30] Esteve-Lanao J, San Juan AF, Earnest CP, Foster C, Lucia A. How Do Endurance Runners Actually Train? Relationship with Competition Performance. Medicine & Science in Sports & Exercise. 2005;37(3):496–504. https://doi.org/10.1249/01.MSS.0000155393.78744.86

[31] Sperlich B, Matzka M, Holmberg HC. The proportional distribution of training by elite endurance athletes at different intensities during different phases of the season. Frontiers in Sports and Active Living. 2023;5:1258585. https://doi.org/10.3389/fspor.2023.1258585

[32] Silva Oliveira P, Boppre G, Fonseca H. Comparison of Polarized Versus Other Types of Endurance Training Intensity Distribution on Athletes' Endurance Performance: A Systematic Review with Meta-analysis. Sports Medicine. 2024;54(8):2071–2095. https://doi.org/10.1007/s40279-024-02034-z

[33] Rivera-Köfler T, Varela-Sanz A, Padrón-Cabo A, Giráldez-García MA, Muñoz-Pérez I. Effects of Polarized Training vs. Other Training Intensity Distribution Models on Physiological Variables and Endurance Performance in Different-Level Endurance Athletes: A Scoping Review. Journal of Strength and Conditioning Research. 2025;39(3):373–385. https://doi.org/10.1519/JSC.0000000000005033

[34] Burnley M, Bearden SE, Jones AM. Polarized Training Is Not Optimal for Endurance Athletes: Response. Medicine & Science in Sports & Exercise. 2022;54(6):1032–1034. https://doi.org/10.1249/MSS.0000000000002869

[35] Tønnessen E, Sandbakk Ø, Sandbakk SB, Seiler S, Haugen T. Training Session Models in Endurance Sports: A Norwegian Perspective on Best Practice Recommendations. Sports Medicine. 2024;54(11):2935–2953. https://doi.org/10.1007/s40279-024-02067-4

[36] Casado A, Foster C, Bakken M, Tjelta LI. Does Lactate-Guided Threshold Interval Training within a High-Volume Low-Intensity Approach Represent the "Next Step" in the Evolution of Distance Running Training? International Journal of Environmental Research and Public Health. 2023;20(5):3782. https://doi.org/10.3390/ijerph20053782

[37] Sylta Ø, Tønnessen E, Seiler S. From Heart-Rate Data to Training Quantification: A Comparison of 3 Methods of Training-Intensity Analysis. International Journal of Sports Physiology and Performance. 2014;9(1):100–107. https://doi.org/10.1123/IJSPP.2013-0298

[38] Seiler S, Jøranson K, Olesen BV, Hetlelid KJ. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine & Science in Sports. 2013;23(1):74–83. https://doi.org/10.1111/j.1600-0838.2011.01351.x

[39] Rønnestad BR, Hansen J, Vegge G, Tønnessen E, Slettaløkken G. Short intervals induce superior training adaptations compared with long intervals in cyclists – An effort-matched approach. Scandinavian Journal of Medicine & Science in Sports. 2015;25(2):143–151. https://doi.org/10.1111/sms.12165

[40] Billat V, Slawinski J, Bocquet V, et al. Intermittent runs at the velocity associated with maximal oxygen uptake enables subjects to remain at maximal oxygen uptake for a longer time than intense but submaximal runs. European Journal of Applied Physiology. 2000;81(3):188–196. https://doi.org/10.1007/s004210050029

[41] Buchheit M, Laursen PB. High-Intensity Interval Training, Solutions to the Programming Puzzle. Part I: Cardiopulmonary Emphasis. Sports Medicine. 2013;43(5):313–338. https://doi.org/10.1007/s40279-013-0029-x

[42] Milanović Z, Sporiš G, Weston M. Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials. Sports Medicine. 2015;45(10):1469–1481. https://doi.org/10.1007/s40279-015-0365-0

[43] Sitko S, Artetxe X, Bonnevie-Svendsen M, et al. What Is "Zone 2 Training"? Experts' Viewpoint on Definition, Training Methods, and Expected Adaptations. International Journal of Sports Physiology and Performance. 2025;20(11):1614–1617. https://doi.org/10.1123/ijspp.2024-0303

[44] Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population. Sports Medicine. 2025;55(7):1611–1624. https://doi.org/10.1007/s40279-025-02261-y

[45] Jeukendrup A, Achten J. Fatmax: A new concept to optimize fat oxidation during exercise? European Journal of Sport Science. 2001;1(5):1–5. https://doi.org/10.1080/17461390100071507

[46] Peric R, Nikolovski Z, Meucci M, Tadger P, Ferri Marini C, Amaro-Gahete FJ. A Systematic Review and Meta-Analysis on the Association and Differences between Aerobic Threshold and Point of Optimal Fat Oxidation. International Journal of Environmental Research and Public Health. 2022;19(11):6479. https://doi.org/10.3390/ijerph19116479

[47] Schwindling S, Scharhag-Rosenberger F, Kindermann W, Meyer T. Limited Benefit of Fatmax-Test to Derive Training Prescriptions. International Journal of Sports Medicine. 2014;35(4):280–285. https://doi.org/10.1055/s-0033-1349106

[48] Bok D, Rakovac M, Foster C. An Examination and Critique of Subjective Methods to Determine Exercise Intensity: The Talk Test, Feeling Scale, and Rating of Perceived Exertion. Sports Medicine. 2022;52(9):2085–2109. https://doi.org/10.1007/s40279-022-01690-3

[49] Foster C, Porcari JP, Anderson J, et al. The Talk Test as a Marker of Exercise Training Intensity. Journal of Cardiopulmonary Rehabilitation and Prevention. 2008;28(1):24–30. https://doi.org/10.1097/01.HCR.0000311504.41775.78

[50] De Lucca L, de Oliveira FR, Foster C, Carminatti LJ. Talk test: a simple alternative to identify lactate thresholds during progressive cycling exercise. Kinesiology. 2021;53(1):20–27. https://doi.org/10.26582/k.53.1.3

[51] Gillespie BD, McCormick JJ, Mermier CM, Gibson AL. Talk Test as a Practical Method to Estimate Exercise Intensity in Highly Trained Competitive Male Cyclists. Journal of Strength and Conditioning Research. 2015;29(4):894–898. https://doi.org/10.1519/JSC.0000000000000711

[52] Orizola-Cáceres I, Cerda-Kohler H, Burgos-Jara C, et al. Modified Talk Test: a Randomized Cross-over Trial Investigating the Comparative Utility of Two "Talk Tests" for Determining Aerobic Training Zones in Overweight and Obese Patients. Sports Medicine – Open. 2021;7(1):23. https://doi.org/10.1186/s40798-021-00315-9

[53] Hansen D, Abreu A, Ambrosetti M, et al. Exercise intensity assessment and prescription in cardiovascular rehabilitation and beyond: why and how. A position statement from the Secondary Prevention and Rehabilitation Section of the European Association of Preventive Cardiology. European Journal of Preventive Cardiology. 2022;29(1):230–245. https://doi.org/10.1093/eurjpc/zwab007

[54] Weston KS, Wisløff U, Coombes JS. High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis. British Journal of Sports Medicine. 2014;48(16):1227–1234. https://doi.org/10.1136/bjsports-2013-092576

[55] Ellingsen Ø, Halle M, Conraads V, et al. High-Intensity Interval Training in Patients With Heart Failure With Reduced Ejection Fraction (SMARTEX). Circulation. 2017;135(9):839–849. https://doi.org/10.1161/CIRCULATIONAHA.116.022924

[56] Rognmo Ø, Moholdt T, Bakken H, et al. Cardiovascular Risk of High- Versus Moderate-Intensity Aerobic Exercise in Coronary Heart Disease Patients. Circulation. 2012;126(12):1436–1440. https://doi.org/10.1161/CIRCULATIONAHA.112.123117

[57] Heck H, Wackerhage H. The origin of the maximal lactate steady state (MLSS). BMC Sports Science, Medicine and Rehabilitation. 2024;16(1):36. https://doi.org/10.1186/s13102-024-00827-3

[58] Keller S, Fischer J, Ji S, Zwingmann L, Wahl P. Comparison of modeled lactate threshold 2 with maximal lactate steady state in running and cycling. International Journal of Sports Medicine. 2025;46(10):750–758. https://doi.org/10.1055/a-2550-4988

[59] Buist I, Bredeweg SW, van Mechelen W, Lemmink KAPM, Pepping GJ, Diercks RL. No Effect of a Graded Training Program on the Number of Running-Related Injuries in Novice Runners: A Randomized Controlled Trial. The American Journal of Sports Medicine. 2008;36(1):33–39. https://doi.org/10.1177/0363546507307505

[60] Impellizzeri FM, Woodcock S, Coutts AJ, Fanchini M, McCall A, Vigotsky AD. What Role Do Chronic Workloads Play in the Acute to Chronic Workload Ratio? Time to Dismiss ACWR and Its Underlying Theory. Sports Medicine. 2021;51(3):581–592. https://doi.org/10.1007/s40279-020-01378-6

[61] Dalen-Lorentsen T, Bjørneboe J, Clarsen B, Vagle M, Fagerland MW, Andersen TE. Does load management using the acute:chronic workload ratio prevent health problems? A cluster randomised trial of 482 elite youth footballers of both sexes. British Journal of Sports Medicine. 2021;55(2):108–114. https://doi.org/10.1136/bjsports-2020-103003

[62] Riebe D, Franklin BA, Thompson PD, Garber CE, Whitfield GP, Magal M, Pescatello LS. Updating ACSM's Recommendations for Exercise Preparticipation Health Screening. Medicine & Science in Sports & Exercise. 2015;47(11):2473–2479. https://doi.org/10.1249/MSS.0000000000000664