Understanding endurance training – Knowledge

1. Why this article?

Most people who take up endurance training make the same mistake: their easy days are too hard and their hard days too easy, so almost everything ends up in the middle.

It sounds like a detail, but it is one of the most common reasons why people run, cycle or walk for months and barely get any fitter. It is not the only reason, but it is one you can change without training more.

Below we explain why this happens and how to do better. You will read what lactate really is (most of what you have probably 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, physiologically, three are enough, and how to find out where your own limits lie without a laboratory.

We are writing this because our practice offers two tools we developed ourselves: PTH ZoneForge for planning training, and the lactate threshold analysis for evaluating a lactate test. Both use technical language, because they are made for professionals. This article puts that language into ordinary words, in the same order and with the same terms, so that you will recognise them when you open one of the tools. (The lactate analysis has a German and a French interface but no English one, and the scientific explanations in it are in German.)

What you will find here:

• What endurance is, and why it is one of the best-studied health factors we know.
• Why lactate is a fuel rather than a waste product, and where the burning in your muscles 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 which zone you are in without any device: the talk test, the singing test, heart rate drift and your appetite afterwards.
• The main training models (polarised, pyramidal, Norwegian) and what the studies support, which is less than the headlines suggest.
• How much training you need, what applies in illness and in older age, and when to get checked out first.

This article is written for people without any background in sports science. We still use the technical terms, because you will meet them in every training plan, every app and both of our tools, but each one is explained when it first comes up. The numbers in square brackets refer to the reference list at the end; we checked every entry individually and linked it to the original.

The subject here is endurance training: the heart and circulation, metabolism and how to control intensity. Muscle strength is covered in our articles Strength training and Building muscle. The two belong together. If you train only endurance, you lose strength over the years; if you train only strength, you cannot use it for long.

As always, this article does not replace an examination. If your doctor has decided something for your situation, that decision takes precedence.

1.1 Twelve words you will need

These terms come up throughout the article, and 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 misleading, though: even at maximal effort, a large share of the energy comes from aerobic metabolism.
• Lactate. A substance produced when sugar is broken down in the muscle. It is a fuel, not waste or poison (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, you can keep training for a very long time; what eventually stops you is fuel, fluid, heat and muscle fatigue rather than the intensity.
• 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. It is the standard 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, pace (minutes per kilometre) takes its place.
• FTP (functional threshold power). The power you can hold for roughly one hour. It is 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, and the muscle uses it to burn fat and sugar.

The capacity of this whole chain is measured as VO2max, the 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, and a world-class endurance athlete above 80.

The bottleneck in this chain is usually not the lungs but the heart and circulation: how much blood can be transported per minute, and how much oxygen that blood carries [6]. When VO2max rises with training, it is mainly because the heart ejects more blood per beat and because blood volume and the total mass of oxygen-carrying haemoglobin increase. This matters when you read a laboratory report: the haemoglobin concentration in the blood need not rise, because plasma volume expands quickly at the same time, so the measured value may stay the same or even fall slightly.

In technical language the same thing is called cardiorespiratory fitness, a term you will meet constantly in studies.

2.2 What endurance training changes in the body

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

• 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 needs fewer beats to move the same amount of blood.
• There is more blood. Blood and plasma volume rise after only a few sessions; with longer training, the total mass of haemoglobin, the protein that carries oxygen, also increases. In controlled studies this explains the largest share of the gain in endurance [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 also makes them more capable by increasing the density of their inner surface [12].

How large these changes are is now well documented. A large review pooled 353 studies on mitochondria and 131 on capillaries, with almost 6000 participants in total [10]. Four findings matter in practice:

• Mitochondrial content rose to a similar extent with all forms of training: by 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.
• Most important of all: the ability to adapt to endurance training persists throughout life, regardless of age, sex and whether an illness is present. Those with the lowest starting fitness gain the most in percentage terms.

Two of these findings pull in different directions: hard training saves time, easy training builds the vessel network better. The training models in section 8 grow out of this tension.

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

In public, endurance is seen as something for people who want to run marathons. In research, it is one of the strongest known predictors of health.

• The size of the effect. A pooled analysis of 33 observational studies with almost 103,000 healthy adults found that for each 1 MET of 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. At an American centre, 122,007 patients were followed for a median of more than eight years after a treadmill exercise test [4]. The better their fitness, the lower their 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.
• The view of professional bodies. In 2016 the American Heart Association issued a statement calling for endurance capacity to be treated like a vital sign and recorded regularly, like blood pressure or pulse [2]. Its reasoning was that fitness says more about the future than most laboratory values, and that it can be changed.
• Beyond the heart. A pooled analysis of 39 randomised trials found that physical exercise improves cognitive function in people over 50, whether the participants were cognitively healthy or already showing decline [9]. The combination of endurance and strength training performed best.

These numbers need two qualifications.

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

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

The conclusion is sober, but it is still good news: the big gain lies between "nothing" and "something", not between "a lot of training" and "even more training". People who are already active gain capacity and enjoyment from more structure, but not the large health gains of 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: many studies include participants whose values barely moved after weeks of training.

One study was designed to test this [8]. Over six weeks, 78 healthy adults were divided into five groups that differed in one thing only: they trained one, two, three, four or five times per week. The more often people trained, the fewer non-responders there were. Everyone who had not responded after this first phase then completed a second phase with two additional sessions per week, and after that there were no non-responders left. Every single person improved.

This is one of the most useful practical findings of recent years: a lack of improvement is rarely a fixed personal characteristic. Many people simply need a higher dose, more time or a different stimulus.

The study had only 78 participants, though, and the fact that all of them responded at a higher dose does not mean that everyone responds under all circumstances. Measurements also scatter. A VO2max measurement varies from day to day, and sleep, illness, medication, how consistently someone trains and the choice of outcome all shift the result. 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

This section is the foundation for the rest of the article. Almost everything commonly said 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 an emergency mode and produces lactic acid as waste. The acid accumulates, acidifies the muscle, causes the burning and forces you to stop. Afterwards it has to be cleared away laboriously, and the next day it causes muscle soreness.

Almost every part of this story is wrong. The current understanding was built up mainly by the American physiologist George Brooks over four decades of research [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 as 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, where it is burned or converted back into sugar. Under load, the heart draws a considerable share of its energy from lactate. This transport from where lactate is produced to where it is used is called the lactate shuttle; describing it was 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 the adaptations that training aims for, among them the formation of new mitochondria [17].
• Lactate is not an acid. In the body it exists as a salt, not as lactic acid, so saying "lactate" instead of "lactic acid" is a factual correction rather than a matter of wording.
• 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, especially braking movements (so-called eccentric movements) such as running downhill. Mechanical strain, a subsequent inflammatory response and a temporary increase in the sensitivity of 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 you feel in your muscles during hard exercise is real, but lactate does not cause it.

Several processes contribute at the same time: an accumulation of hydrogen ions (these do make the muscle more acidic, but forming lactate tends to slow this acidification rather than cause it), a build-up of phosphate from energy metabolism, and changes in the potassium balance of the muscle cell. In addition, nerve endings in the muscle report this metabolic state directly.

In practice, this means the burning is an indicator of a particular metabolic state, not its cause. When your muscles burn, you are working well above LT2, which is useful to know. You do no harm by putting up with it, and cooling down afterwards does not "clear away" anything.

Our learning game Lactate Shuttle shows how lactate travels from the tissues that produce it to the tissues that use it.

3.3 Why the lactate curve is still so useful

If lactate is not a waste product, why measure it at all?

Because it is a very good indicator. The lactate level in the blood always reflects the difference between how much is produced and how much is used. As long as consumption keeps pace with production, the level stays flat. Once production grows faster than consumption, it rises.

A lactate test looks for the points at which this balance changes. There are two of them, LT1 and LT2, and they are the subject of the next section.

A picture may help. Imagine a bathtub with an open drain: the tap is lactate production, the drain is consumption. At low load the tap only drips, the drain copes easily and the water level stays low and constant. Open the tap further and the level rises a little, then settles at a new, higher level, because the drain is working harder and just about keeps up. Beyond a certain point it can no longer keep up, and the level rises until the tub overflows. The first point (where the level starts to rise) is LT1; the second (where it no longer settles) is 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 it feels like below LT1:

• You can keep going for hours, as long as fuel and fluid do not run out.
• You can talk without effort and, more tellingly, sing without effort (see section 7.2).
• Your body covers a large share of its energy needs from fat.
• If the session is of sensible length, you will feel little of it the next day. A very long session is tiring even below LT1, and people who are untrained or have several illnesses feel this considerably sooner.

LT1 is the most important boundary for the amount of your training. Provided the sessions are of sensible length, training below it needs comparatively little recovery, so you can do a lot of it. Above it, the cost in recovery rises disproportionately.

In our tools, Zone 1 is anchored to LT1, and specifically to the measured LT1 rather than to a percentage of some other value. Section 6.7 explains why.

4.2 LT2 – where it tips

LT2 is the highest intensity at which you can hold a state of equilibrium. Lactate is clearly raised but stays constant, because production and consumption balance out. Go a little harder and the balance tips: lactate keeps rising, and exhaustion is only a matter of time.

The older term is anaerobic threshold, which is unfortunate, because nothing at this point happens "without oxygen". A detailed review traced fifty years of argument over the name [18]; in short, the point is real and useful, but the label is misleading.

LT2 is often equated with a related measure, the maximal lactate steady state (MLSS), but the two are not the same. MLSS 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]. This is 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 range 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 applies to the other measures in this family, such as VT2 from breath-gas measurement, OBLA, D-max and critical power: they lie in the same range, but they do not give 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.
• You can only speak in short phrases, with breaths in between.
• Energy comes predominantly from carbohydrate.
• It largely determines performance in races lasting 30 to 60 minutes.

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

4.3 Why two thresholds are enough

Your watch shows five zones, your bike computer seven, some apps six. For steering endurance training in practice, two transition regions, and therefore three intensity domains, have proved their worth.

This reflects 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 a matter of practical convenience rather than physiology [19]. Between "Zone 4" and "Zone 5" on your watch, nothing happens in the body that deserves a name of its own. Crossing LT1, on the other hand, changes a great deal.

The three-domain model is very well established, but it is not the only possible physiological description. It is the division that has proved most useful for steering training, not a law of nature with two fixed lines.

Five or seven zones are not useless. Their finer divisions are sometimes practical for steering individual sessions. They should just not distract from what counts: how much time is really spent below LT1 and how much really above LT2.

4.4 How a lactate step test works

To make the next sections more concrete, here is how such a test is carried out.

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, your heart rate, and your own rating of the effort. This continues until you cannot go on or a predefined stopping value is reached.

At the end you have a series of pairs of values: at this many watts, this much lactate. Plotted on a graph, they form the lactate performance curve, which is flat at first, then bends and finally rises steeply. LT1 and LT2 have to be found in this curve.

If you have a test done, you should know about three things that can distort it:

• Step length. Short steps (1–2 minutes) produce different values from 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 who starts with full sugar stores produces more lactate at the same load than someone whose stores are low, and this shifts the whole curve.
• Preceding load. A hard training day beforehand lowers the measured threshold, so the day before a test should be easy.

5. Sixteen methods, one curve

Having the curve does not yet tell you where LT1 and LT2 lie. Nothing in the curve marks them; there are only calculation methods that try to locate them.

Our lactate threshold analysis calculates sixteen such methods from six families and shows them side by side. The families are described below in plain terms.

5.1 Fixed lactate values – the old school

The oldest and simplest approach is to define a fixed lactate value and read off the power at which it is reached. Classically these are 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 this way [21].

The approach is simple, the same everywhere, and has been in use for decades.

Its weakness is that 4 mmol/L is a population average. In individual people, the actual equilibrium point lies somewhere between about 2 and 8 mmol/L. In someone with a naturally low resting value, the rigid 4 mmol/L rule overestimates the threshold; in someone with a high one, it underestimates it [20]. Our analysis therefore calculates OBLA values too, but does not rely on them.

5.2 Baseline plus x – the individual answer

The obvious refinement is to take your personal lowest value on the curve (the baseline) instead of a fixed value, and add a set amount to it. Our analysis offers four steps: baseline plus 0.3, plus 0.5, plus 1.0 and plus 1.5 mmol/L.

For LT1, this family currently has the best support. A controlled laboratory study in 50 cyclists compared nine different markers for the upper boundary of the easy range. Depending on the marker, variability between people ranged from 6 to 29 per cent; 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]. For this reason, "baseline plus 0.5" is the default for LT1 in our tool.

"Baseline" can mean two things: the value at the first step, or the lowest value measured at any point (the nadir, the low point of the curve). The difference matters because in many people lactate dips slightly at the second or third step before it rises. Our analysis therefore calculates both variants and shows them separately.

5.3 D-max – the greatest distance from the line

For LT2, the most widely used 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 curve is furthest from this line, measured at a right angle. That place is called D-max, short for maximum distance [23].

D-max is often described as "the point of greatest curvature". That is only approximately right. The method looks for 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 why there are several variants.

They differ 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 rise in lactate. 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).

One study tested how well D-max performs by comparing it with critical power, an independently measured reference for the same physiological boundary [24]. At group level the two agreed almost perfectly, with a mean difference of 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".

The conclusion in 5.6 builds on this result.

5.4 Log-log – straightening out the curve

This method uses a mathematical device from 1985 [22]. If both lactate and power are plotted on logarithmic scales, the curve turns roughly into two straight lines that meet at a kink. A kink is much easier to locate unambiguously than a bend.

Log-log gives good values mainly for LT1 and is particularly robust against 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. The advantage is that no assumption about the shape of the curve is needed.
• LTratio. This method looks at the ratio of lactate to power rather than at lactate itself: "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 curve indicates an economical metabolism.

5.6 What to do when sixteen methods give sixteen numbers?

Different results are normal. Depending on the method, one and the same curve can give an LT1 anywhere between, say, 150 and 190 watts.

There is no "correct" method. All of them describe the same physiological transition, but they draw the line in different places, and the transition itself is a range rather than a line.

How to deal with this:

• Choose one method and stay with it. This matters most. If you test again in six months, the comparison only means something if the same method was used. Our analysis therefore 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 at the same time 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 first estimate, which you then adjust according to how the training feels, how your heart rate behaves over a long session and how you feel the next day.

Because no method is the correct one, our tools always show a selection of values with context rather than a single number.

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

Once LT1 and LT2 are known, zones can be built from them. There are several common systems. Our analysis calculates four of them, all anchored to your measured thresholds rather than to population averages.

6.1 The 3-zone model (Seiler)

This is the simplest model and the one with the best scientific foundation. It has three zones because it is built on the two transitions, LT1 and LT2. It was developed by the physiologist Stephen Seiler at the University of Agder in Norway, based on observations of how top endurance athletes actually train [26][27].

• Zone 1 – below LT1. Mainly aerobic, with fat as a major fuel, and easy to recover from. The anaerobic pathways always run alongside, but here they contribute very little. Lactate stays close to the resting value. Most of the training volume belongs here; in Seiler's original study, elite athletes spent around 75 per cent of their sessions in this zone.
• Zone 2 – between LT1 and LT2. The inter-threshold zone, often called the "grey zone". Training here is effective: it improves threshold power, movement economy and load tolerance. But it needs considerably more recovery than easy training, which is why the polarised model deliberately keeps it small when total volume is high.
• Zone 3 – above LT2. Genuine high intensity, sustainable only in short, structured intervals. It drives up maximal oxygen uptake and cardiac output.

The "Zone 2 training" so widely recommended online is almost never this Zone 2. It is Zone 2 on heart-rate watches, which divide intensity into five zones by percentage of maximum heart rate, or Zone 2 in Coggan's seven-zone power model (section 6.3). In both, it corresponds to the upper part of what is called Zone 1 here. In the five-zone model anchored to LT1 and LT2 (section 6.2), by contrast, Z2 is the zone between the two thresholds. The same word therefore refers to different intensities depending on the system; the figure in section 6.5 shows this. This confusion is behind many misunderstandings; more on it in section 11.

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

This model is a pragmatic compromise. It divides intensity more finely than three zones but still uses only LT1 and LT2 as genuine boundaries.

• Z1 base – from zero to LT1. The whole aerobic range below the first threshold.
• 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 in physiotherapy, because it makes the choice simpler without blurring the two boundaries that matter.

6.3 The 7-zone model (Coggan / Rouvy)

This is the most widely used system in cycling. It was developed by the exercise physiologist Andrew Coggan and popularised by the book Training and Racing with a Power Meter, and it is the standard 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. They are normally given as percentages of FTP. In our analysis they are anchored to your measured thresholds instead: Z2 ends at your LT1, and Z4 begins at your LT2.

This anchoring matters. The usual upper boundary of Zone 2, 75 per cent of FTP, assumes two things: that FTP roughly equals LT2, and that LT1 lies at about three quarters of it. That is roughly true for elite athletes. In recreational athletes and patients, however, FTP often differs considerably from LT2 (see section 6.6), and the gap between LT1 and LT2 also varies from person to person. A fixed 75 per cent boundary can then set Zone 2 far too high or too low. The measured LT1 avoids this problem.

The zones above Z4 (Z5, Z6 and Z7) do not correspond to physiological boundaries. They are practical subdivisions of the high-intensity range.

6.4 The Norwegian 5-zone model

This is the system used in Norwegian elite sport. Z1 and Z2 both lie below LT1, Z3 lies between LT1 and LT2, and Z4 and Z5 lie above LT2. LT1 is therefore the boundary between Z2 and Z3, and LT2 the boundary between Z3 and Z4. The model splits both the range below LT1 and the range above LT2 into two zones 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 well-known "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 main difference from the polarised model is that the inter-threshold zone is used deliberately here instead of being avoided. More on this in section 8.3.

6.5 The translation table

The figure places the zone systems from this chapter one below the other. All of them are built on the same two thresholds; they only divide the ranges between and beside them more or less finely.

If your watch or power meter shows zones 1–5 or 1–7, this table translates them into the three zones. It appears in the same form in ZoneForge.

One row deserves particular attention. When your watch shows "Zone 3 – tempo", it sounds like a sensible training zone, but it is the grey zone. Devices that give this range a friendly name tempt people to train there all the time, although it only tolerates a limited dose.

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

These three terms all describe the region of the same physiological boundary, yet they are not the same thing:

• MLSS (maximal lactate steady state). The physiological reference value, measured directly with several long constant-load efforts on different days. It is laborious, but it is 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. It is typically somewhat higher than FTP, by about 5 to 10 watts in trained cyclists.
• FTP. The power that can be held for roughly one hour. In practice it is usually estimated as 95 per cent of the best 20-minute effort.

Roughly, the order is CP ≥ FTP ≥ MLSS ≈ LT2, where "≈" means close to, not identical. In well-trained people the differences are small. In recreational athletes they can be considerable, because the definition of FTP assumes that you can hold your LT2 power for a full hour. Untrained people often cannot; they may manage 25 minutes. Our analysis therefore asks 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 parts of this rule are unreliable.

The first problem is the formula itself. "220 minus age" is a regression line through a large cloud of data points, and individual values scatter around it by about ten to twelve beats per minute in either direction. For a 60-year-old the formula gives 160, but the actual maximum heart rate lies somewhere between about 145 and 175.

The second problem is more serious: LT1 does not occur at the same percentage of maximum heart rate in everyone. The study of 50 cyclists mentioned earlier put a number on this [25]: with fixed percentages of maximum heart rate, variability between people was up to 29 per cent of the wattage. In other words, 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 and the other already well above it.

Our tools therefore anchor the zones to measured thresholds rather than to percentages. This variability is also why, for most people, the signals described in the next section are worth more than any number on a watch.

7. Finding your zones without a laboratory

Most people will never have a lactate test, and that is not a problem. There are well-studied alternatives that, in everyday training, are even more reliable than an outdated laboratory value.

7.1 The talk test

The talk test is the simplest and best-studied way of controlling intensity without equipment. The principle is simple: above a certain intensity you breathe so hard that you can no longer speak in connected sentences.

Key studies on it came from a group at the University of Wisconsin [49]. During a step test, participants were repeatedly asked "can you still speak comfortably?", and their answers were compared with the two ventilatory thresholds. These are the points at which breathing increases markedly as the load rises. They are called VT1 and VT2 and lie close to LT1 and LT2, but not at exactly the same place. The answers matched these ventilatory thresholds well:

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

If the answers are compared with lactate instead of breathing, the picture shifts. In a study in cyclists, the last step with comfortable speech was at about 127 watts, well above LT1 (about 88 watts). The "yes, but" step was at 179 watts, close to LT2 (170 watts); this difference was not statistically significant [50]. In practice, this means: if you can still talk comfortably, you were below the second threshold in both studies, but not necessarily below LT1. To stay safely in Zone 1, you therefore need the stricter singing test described in the next section.

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 that depend on the group [48]. The method also works in very well-trained competitive cyclists [51], and an adapted version has been tested for people with overweight [52].

7.2 The singing test – and the biggest misunderstanding

A rule found in almost every guide says: "you should still be able to talk, but not to sing."

It is usually presented as if it described the middle of the easy range.

In fact, "can talk, cannot sing" marks the upper limit of Zone 1. At that point you are crossing LT1, not staying safely below it.

The cycling study from the previous section shows this: power and heart rate at LT1 were considerably lower than at the last step at which comfortable speech was still possible [50]. You can therefore already be above LT1 and still talk comfortably for quite a while. The ability to sing disappears first, which makes it the earliest signal.

This leads to the rule used in our tools:

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

The complete scale relative to the lactate thresholds, as used in ZoneForge:

Breathing is an equally good signal. Below LT1 it should feel like walking: you can comfortably breathe through your nose, and your breaths are somewhat deeper than at rest, but the rhythm stays calm. If you have to take a breath at odd places in the middle of a sentence, you are far above Zone 1.

7.3 Heart rate drift and decoupling

Most people have noticed this, but few know what it means: you ride for an hour at a constant pace, and your pulse still creeps upwards.

This is called cardiovascular drift or heart rate drift. As your core body temperature rises and you lose fluid, blood volume falls and the heart ejects less blood per beat, so it has to beat faster to move the same amount. About half of the effect is due to fluid loss and the other half to heat.

To evaluate it, divide the session in your head into two halves and compare the average heart rate of each:

• 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 due to fluid loss or heat. Keep 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, whatever your computer says.

With a power meter you can check this more accurately by comparing the ratio of power to heart rate in the two halves of the session. If this ratio shifts by less than 5 per cent, your aerobic base is good for that duration. The technical term is decoupling; ZoneForge calculates it automatically from an uploaded training file.

Decoupling that increases over several sessions is one of the most reliable early signs that recovery is no longer keeping up, and it shows in the data before you feel it.

7.4 The appetite check

Another signal is often overlooked and costs nothing: how hungry are you 20 to 30 minutes after an easy session?

After genuine Zone 1 training, most people have a normal or a good appetite. If your 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 is activated by stress and effort, and it indicates that you worked above LT1.

This is a plausibility check rather than a measurement, but a good one. It costs nothing, needs no device and is rarely misleading.

7.5 The effort scale

On the Borg scale you rate how hard an effort feels. There are 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.

On the 6-to-20 scale, Zone 1 roughly corresponds to 9 to 12 ("very light" to "somewhat hard"), the grey zone to 13 to 15 and Zone 3 to 16 and above.

The scale is particularly important for people taking beta blockers. These drugs clearly alter the heart rate response, both at rest and under load.

The consequences are often overstated. What becomes unreliable are mainly 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 if it was measured on the same medication; an exercise test during ongoing treatment provides individual values to which zones can be anchored. The most reliable approach combines measured heart rate with perceived effort, the talk test and, where available, power in watts. This is also what the European recommendations on intensity control in cardiac rehabilitation say [53].

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

In short:

• What works well: measuring heart rate with a chest strap and following it over time, logging the duration and frequency of sessions and, for cyclists, measuring power in watts.
• What works moderately: measuring heart rate at the wrist. It is usable at steady loads but fails during rapid changes and with cold hands, which means during intervals in particular.
• What does not work: the automatically estimated VO2max, the automatically calculated "lactate threshold" and the proposed zones. They are based on formulas built from population averages, the approach that section 6.7 advises against. These values can sometimes be used to follow a trend in the same person, but not as absolute numbers.

Our advice is to use the watch to record your training rather than to dictate it, and to steer the intensity by your breathing and your voice.

7.7 The three zones at a glance

The chart summarises chapters 4, 6 and 7: what happens in the body in each zone, how to recognise it without a device, and what your watch and power meter call it. We use it in the practice to explain the zones; the button below it prints just this chart.

8. The training models

The most debated question in endurance sport is how to distribute training time across the three zones. The technical term is intensity distribution.

8.1 Polarised – the 80/20 model

This is the best-known model. Stephen Seiler developed it from observations of how top endurance athletes actually train [26][27]. It proposes:

• 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 while the middle stays empty. The reasoning is plausible. Easy sessions build the aerobic base without causing much fatigue, hard intervals provide a stimulus that easy training cannot, and the middle costs recovery without giving a corresponding stimulus.

One detail is often overlooked: the 80/20 split refers to the number of sessions, not to time. Because hard sessions are shorter, 80/20 by sessions becomes roughly 90/10 by time [26]. If you confuse the two, you do twice as much hard training as intended. How much the picture changes depending on whether you count sessions or minutes has been studied separately: the same training data look polarised with one method of analysis and pyramidal with another [37].

There is evidence in its favour. A randomised study found greater improvements over nine weeks in maximal oxygen uptake, time to exhaustion and peak power with polarised training than with 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 share of training falls steadily as intensity rises: a lot of Zone 1, less Zone 2, least Zone 3. Unlike in the polarised model, the middle is kept small rather than avoided.

When their training time rather than their sessions is counted, this is often what elite athletes actually do [31]. The distribution also shifts over a season: it is more pyramidal in the build-up phase and more polarised as competitions approach.

8.3 Threshold-focused and the Norwegian model

In the threshold-focused model, the emphasis is 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. Like the polarised model, it has a high volume in the easy range (75 to 80 per cent of all sessions [35]), but it 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 by the triathletes Kristian Blummenfelt and Gustav Iden and by the Ingebrigtsen running family.

How it works:

• On two days a week, athletes complete two sessions a day, 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 and at most at 4.5.
• Typical sessions are long intervals such as 4×10 minutes, 6×8 minutes or 3×15 minutes, done at a controlled pace rather than as maximal efforts.

Everything depends on this lactate cap. Without it, the second session of the day would end in exhaustion and the approach would fall apart. Athletes who train this way therefore need a lactate meter, or at least very accurate knowledge of their own thresholds.

Physiologically, the aim is to accumulate as much time as possible just below the threshold in order to bring about two adaptations: a lower rate of lactate formation and a greater capacity to clear lactate. Both push LT2 upwards, so that you produce 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 evidence for this model is almost entirely observational; there are no randomised comparisons. It describes well what successful athletes do, but whether it is causally better has not been shown. It is also not intended for people who train four to five hours a week: two sessions a day on two days requires a training volume that is rarely possible in everyday life.

8.4 What the pooled analyses say: no clear winner

Taken together, the studies are less clear-cut than the individual trials above suggest.

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

• 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. Here too the interval covers both directions, so the difference is not established.

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

The early studies were clearer because they were well controlled and carried out in trained people with high training volumes. The broader syntheses include more varied studies and less trained participants. That adds noise, but it also shows that the advantage of the polarised model does not hold in general. It may only appear under particular conditions: high volume, well-trained people and sufficiently long interventions.

8.5 What this means for you

If no model clearly wins, what should you do? All the models share three points, and these are the real 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. These figures, however, come from the observed training of successful endurance athletes, and they change depending on whether you count sessions, minutes or load points [37]. They are not a general prescription for patients and recreational athletes, but they are a useful reference point.
2. There is a small, clearly defined hard portion. Whether it lies above LT2 (polarised) or just below it (Norwegian) is what the argument is about, and it appears to be the less important question.
3. The middle is dosed, not forbidden. No model recommends spending most of the time between LT1 and LT2, but all of them use this range in moderation. Where most people go wrong is the dose, not the zone itself.

Everything else is fine-tuning for people who already train ten hours a week. If you have four hours a week, what matters is not the choice of model but whether you actually do those four hours and whether the easy sessions really are easy.

9. The grey zone – effective, but expensive

The grey zone is the range between LT1 and LT2. Many guides treat it as wasted effort, and that is where they go wrong.

Training in this range is effective. Tempo and threshold training improves threshold power, movement economy and the ability to tolerate load; section 10.3 is entirely about this kind of training. For people with little training time it is particularly useful.

The problem is that this intensity is costly. It causes more fatigue than easy training without delivering the stimulus of genuine high-intensity work, so it only tolerates a limited dose. Things go wrong not because the zone itself is a mistake, but because almost every session ends up there unintentionally. If you plan time in the middle, you are training sensibly. If you drift into it by default, you accumulate fatigue without the corresponding gain.

Almost everyone drifts there, for very human reasons:

• Easy feels like too little. If you invest an hour, you want to feel that you have achieved something, and a genuinely easy session can feel almost embarrassing.
• Hard feels like too much. Real Zone 3 intervals are unpleasant. It is tempting to ease off a little, and then you are in the middle.
• The devices encourage it. "Zone 3 – tempo" sounds like a sensible middle ground and invites people to train there every day, although this range needs to be dosed sparingly.
• In a group it is almost unavoidable. When people run or cycle together, the pace settles at a level that is easy for nobody and hard for nobody.

A clinical study shows particularly clearly how strong this pull towards the middle is. In a trial with 261 people with heart failure, two groups were supervised with clearly prescribed intensities, one hard and one moderate. According to the training records, 51 per cent of people in the hard group trained below their prescription, and 80 per cent in the moderate group trained above it [55]. Both groups drifted towards the middle, even though they were supervised in a scientific trial and had a clear prescription.

If this happens under supervision, it is likely to happen to you as well. Knowing the model is therefore not enough; you need a signal that works during the session. That is why section 7 goes into so much detail.

10. Intervals – the toolbox

Interval training alternates hard efforts with recovery. The idea is simple: at an intensity you could only sustain for three minutes in one go, the breaks let you accumulate twenty minutes in total.

All the protocols in this section are available in ZoneForge, each with its source. The most important ones are described below, with the reasoning behind them.

10.1 Long intervals: the 4×8 protocol

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

The results were clear:

• The 4×8-minute group improved the most, with an 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 revealing: 4.9 mmol/L in the 16-minute blocks, 9.6 in the 8-minute blocks and 13.2 in the 4-minute blocks.

The authors concluded that 32 minutes of work at 90 per cent of maximum heart rate achieve more than 16 minutes at 95 per cent, and also feel less strenuous. Intensity and accumulated duration interact, and the highest intensity is not automatically the best.

These numbers come from trained cyclists. For people in rehabilitation the orders of magnitude are different, 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 of the range are very short efforts with very short breaks. 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 drop in between. This lets 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 of work and 15 seconds of 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 of work and 30 seconds of active recovery, and showed that runners stay considerably longer near their maximal oxygen uptake than during continuous work [40].

This does not contradict the previous section. Both findings hold, and there is no single best interval. A detailed systematic account of interval programming lists nine adjustable variables (among them the duration and intensity of the effort, the duration and type of the break, the number of repetitions and the number of sets) and shows that different combinations load different systems [41]. In practice, 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 of intervals is deliberately kept below LT2. Typical formats are 4×10 minutes, 3×15 minutes or 6×5 minutes with short breaks. The target lactate is 2.5 to 3.5 mmol/L and the heart rate about 82 to 90 per cent of maximum; you can still speak in short sentences.

These sessions feel "comfortably hard": demanding, but under control. They are the centrepiece of the Norwegian model and are particularly suitable for people who are not yet ready for genuine high-intensity training, because they need considerably less recovery than Zone 3 intervals while still providing a substantial stimulus for threshold power.

In ZoneForge, these protocols are the only form of training above Zone 1 provided for the health-oriented categories; Zone 3 intervals are added only later, 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. Doing more on a regular basis comes at the cost of quality.
• Spacing: as a rule of thumb, at least 48 hours between two hard sessions, and in older people more like 72. These are cautious guide values, not established limits; how quickly people recover varies considerably. What counts is how you feel before the next hard session, not the calendar. The body adapts during recovery, not during the effort itself.
• Evenness: all repetitions should be completed at the same intensity. If the third or fourth repetition drops off noticeably, the intensity was set too high; it is not a lack of willpower.
• Before and after: hard intervals need at least 10 to 15 minutes of easy warm-up beforehand and at least 10 minutes of cool-down afterwards.
• When to stop: if you cannot complete the planned repetitions, stop the intervals and finish the session easy. An incomplete hard session is a signal, not a failure.

11. Zone 2 – hype and reality

Few training terms have received as much attention in recent years as "Zone 2 training". The idea, roughly, is that there is a magic intensity just below the first threshold at which fat burning is highest and the effect on the mitochondria greatest.

Some of this is true and some 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 energy 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]. In efforts lasting more than two hours, you notice this directly.
• Easy training is the most tolerable way to accumulate a lot of volume, and volume is the strongest predictor of how the vessel network adapts [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 in individual people the agreement 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 is of little help in prescribing training. This has been examined directly, and the results were sobering [47].
• Zone 2 is not the optimal intensity for the general population. A critical review from 2025 explicitly contradicts the blanket recommendation [44]. It argues that the recommendation comes from observing elite endurance athletes who do enormous amounts of easy training. For people with little training time, the data support giving priority to 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. When fourteen leading experts were asked what "Zone 2 training" actually is, they gave different answers about the boundary, the method and the expected adaptations [43].

Easy training is valuable as a foundation, as a tolerable way to accumulate a lot of movement, and as the precondition for doing the hard sessions well. It is not a magic intensity, though. If your total time is limited, it is better to invest part of it in genuinely hard intervals than to spend all of it in Zone 2.

12. How much do you need?

The answer depends on what you are aiming for.

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

• 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. This means one programme that contains both, not two separate obligations.

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

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

• Two sessions per week already improve endurance if they are dosed adequately. Three or more make it easier to accumulate enough volume and to spread 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, but it is not essential. Stimuli above LT2 improve maximal oxygen uptake in particularly little time. For beginners, older people and many patients, however, purely moderate training also produces clear gains. People with little time gain most from intensity; people who have more time, or who need to build up cautiously, can get there without it.
• Patience. Measurable changes often appear 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, so it is worth persisting that long before judging a programme.

What can you realistically 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 with classic endurance training and 5.5 with interval training [42]. From a starting value of 40, that is around 12 to 14 per cent, a clear change that you notice in everyday life. In untrained people the increase is larger, in people who are already trained smaller.

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

13. Endurance training in illness and in older age

13.1 Cardiovascular disease

The evidence here is particularly good, and it is instructive, because the simple story has not held up.

Some of it favours 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 roughly doubles the effect [54].

The largest single study, however, found no advantage. In it, 261 people with heart failure and severely reduced pumping function were supervised for twelve weeks in one of three groups: 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]. For the primary outcome, the size of the left ventricle, there was no difference between the two training groups, and none for oxygen uptake either. Both training groups did better than the recommendation alone. A year later, nothing was left of the improvements.

The main reason for the difference lies in the study itself and was mentioned in section 9: half of the interval group trained too easily, and four fifths of the moderate group too hard. In the end both groups trained in much the same way, so the difference that existed on paper did not exist in practice.

How safe intense training is for heart patients 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, which corresponds to one event per 129,456 hours of moderate and one per 23,182 hours of intense training. The absolute event rate was very low with both forms of training. With only three events in total, however, the difference between the two forms cannot be judged reliably; three cases are too few for a robust comparison.

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 of its points are relevant in practice. For people on beta blockers, calculated target zones are of no use, whereas a heart rate measured on the same medication 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 things need to be considered together here.

First, the ability to adapt to endurance training lasts a lifetime, regardless of age, sex and whether an illness is present [10]. Someone who starts at 78 still improves, and people with low starting fitness improve the most in percentage terms.

Second, building up takes more time, and recovery between hard sessions takes longer. ZoneForge therefore uses a minimum of 72 hours between intense sessions for the older categories and a smaller week-to-week increase. This is a deliberately cautious default in the tool, not a scientifically established rule; how long people need to recover varies considerably.

Third, in people with limited reserves (the technical term is frailty), the order is different. Strength and balance training come first, together with easy endurance activity. Intense interval training only becomes an option once a foundation has been built, and then only after an individual assessment. ZoneForge reflects this: for the two lowest categories it provides no Zone 3 at all.

If you are starting again after an illness or an operation, our articles Leaving hospital weaker and Fall prevention cover the relevant points.

13.3 When to get checked out first

Healthy adults who start with moderate activity do not need a medical examination. In the following situations you do need one, before you begin intense training:

• Known cardiovascular disease, diabetes or kidney disease.
• Chest pain, unusual breathlessness, dizziness or fainting on exertion. In this case, get checked before any increase in training.
• Irregular or unusual heartbeat.
• A 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.

There is no age limit in this list. You often read that people over 45 should have a medical examination before they start training, but current screening algorithms no longer work that way [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-or-no questions. If every answer is "no", you can begin. A "yes" does not automatically send you to a doctor. It first leads to follow-up questions about the condition concerned, and only if something stands out there should you be assessed before you start training. ZoneForge includes this questionnaire for the relevant categories.

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

So far we have looked at individual sessions. This section is about how to arrange them over weeks and months, which is called periodisation.

The basic pattern has three building blocks, which have the same names in ZoneForge:

• Accumulation block. A lot of volume, little intensity. This is where the aerobic base is built: fat oxidation, mitochondria and 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 the same, and threshold intervals and occasional hard sessions are added. Threshold power rises.
• Realisation block. Volume drops by 15 to 20 per cent, while intensity stays the same or rises. Only relevant if you are working towards an event.

In between, a deload week is built in at regular intervals, with clearly reduced volume (in performance-oriented periodisation usually by 40 to 50 per cent) and no hard sessions. It may feel like going backwards, but adaptation happens during recovery, not during the load. For general health-oriented training, such a fixed rhythm is not necessary. It is usually enough to watch for the signs described in section 15 and to ease off when they appear.

Nobody disputes that volume and intensity should be increased gradually. The figure usually given 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 in novice runners, among others, and a gently graded build-up programme produced just as many complaints as the usual programme [59].

The same applies to a calculated measure that ZoneForge displays: the ratio of last week's load to the average of the past four weeks (the acute:chronic workload ratio). It shows whether something has just changed, and for that it is useful, but it does not work as a risk score. 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]. When this kind of load management was 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; at most it is a reason to look more closely.

What counts instead is more mundane: your starting level, the type of loading (running loads the legs differently from cycling or swimming), individual spikes in load, existing complaints, and how well you recover between sessions. Section 15 describes the signs to look for.

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 in between. It should feel comfortably hard, with short sentences still possible.
• Friday: rest.
• Saturday: 45 minutes easy.
• Sunday: 90 minutes easy, at a relaxed pace out in the countryside.

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

15. How to tell that it is becoming too much

Too little training achieves too little, but so does too much, only with more effort and a higher risk. These are the warning signs, roughly in the order in which they appear:

• Morning resting heart rate is raised by more than about five to seven beats above your usual value, on several days in a row.
• Heart rate drift increases. At the same heart rate you suddenly produce less power. This is the most reliable early signal there is.
• Sleep gets worse. Often the first sign is that you fall asleep normally but wake during the night.
• Mood falls and the desire to train disappears. This is an early marker, not a weakness of character.
• The quality of hard sessions drops. You can no longer complete the last repetitions, although the first ones went normally.
• Persistent muscle soreness and a general feeling of heavy legs.

If two or three of these signs appear at the same time, take a deload week instead of pushing through. Otherwise you risk ending up in a state called non-functional overreaching, from which recovery takes weeks to months.

When you are ill, a simple rule applies. With symptoms above the neck, such as a cold or a mildly sore throat, easy training is fine. With fever, aching limbs or symptoms below the neck, do not train. Inflammation of the heart muscle from training again 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 data for each step (power, lactate, heart rate, rating of effort), and the tool calculates all sixteen threshold methods from section 5, shows up to three of them at once as lines in all charts and derives the four zone models from section 6. There are also charts for lactate economy, cardiac efficiency and heart rate over time, and a detailed background section with more than fifty sources. Everything can be downloaded as PDF, PNG or Excel, or as a reference file for a bibliography manager. The interface is available in German and French, and the scientific explanations are in German.

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

To get from one to the other, use the button "→ transfer to ZoneForge" at the end of the analysis. It passes all calculated threshold values, including heart rates, straight to ZoneForge without any file in between. There you only need to choose the method on which the zones are based, and the plan is ready.

Both tools are made for professionals and assume a certain amount of background knowledge; this article should have covered much of it. The lactate analysis is also labelled as a beta version, and its methods are continually being reviewed and extended.

17. Ten misunderstandings

1. "Lactate is a waste product and causes muscle soreness." Lactate is a fuel and a signalling substance, and its level is back to normal an hour after exercise. Muscle soreness follows unaccustomed loading, especially braking movements.
2. "No pain, no gain." For the aerobic base, the opposite is true. Most training should feel easy, so easy that it is almost boring.
3. "Zone 2 is the optimal intensity." This recommendation comes 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 applies a formula based on averages. 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 of energy from fat is higher at low intensity, but the absolute amount of fat burned is not necessarily higher. For body weight, what counts is the overall energy balance, not the fuel [13].
7. "I don't respond to training." In one study with 78 participants, the non-responders disappeared once the training dose was increased [8]. Before you write yourself off, try a higher dose and more time, and check 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." In more than 175,000 supervised training hours with almost 5000 heart patients, there were three events in total. The absolute risk was very low with both moderate and intense training [56]. A medical assessment beforehand is still necessary.
10. "At 75 it is no longer worth it." The capacity to adapt lasts a lifetime, 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 the reason is how the intensities are distributed, or simply the total amount, and rarely a lack of effort.
• If you have a chronic illness and would like a structured programme adapted to it.
• If you have symptoms on exertion, such as breathlessness, chest tightness, dizziness or palpitations. In that case, have them assessed by a doctor first, not by us.
• If you have had a lactate test and cannot make sense of the numbers. That is what the analysis is for, and we are happy to go through it with you.
• If you have a goal, such as a hike, a fun run or a cycling tour, and are looking for a way to get there without wearing yourself out.

19. In summary

The main points of this article:

Two points matter. LT1 is where training stops being genuinely easy, and LT2 is where the balance between lactate production and clearance tips. Every zone system is a subdivision of the ranges defined by these two boundaries.

Lactate is a fuel and a signalling substance, not a poison. We measure it because it shows where these two points lie, not because it is harmful.

Most training belongs below LT1. All the models agree on this: polarised, pyramidal and Norwegian. They differ in how they arrange the small hard portion, and for most people that difference has no practical consequence.

The problem is the middle. Threshold training works, but it costs recovery and therefore needs to be dosed. When almost every session ends up there unintentionally, progress stalls. Even under supervision in a clinical trial, both groups drifted towards it.

You do not need a laboratory to prevent this. What you need is a signal that works during the session. Can you still sing effortlessly? Does your pulse creep up although your pace stays the same? Are you hungry afterwards? These questions cost nothing and guide your training better than any formula on a watch.

The dose matters too. People who believe they do not respond to training have often trained too little or for too short a time, and sometimes they are only seeing ordinary measurement variation. Nobody is too old, either: the capacity to adapt lasts a lifetime.

In one sentence: make your easy days genuinely easy, so that you can make your hard days genuinely hard. Everything else is fine-tuning.

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