Testing instead of guessing – Knowledge

1. Why this article?

Your watch tells you how fast your heart can beat at most. It tells you how good your endurance is. It draws five zones on the screen and informs you every morning how recovered you are. It measured none of that. It calculated it.

That is not an accusation. Calculating is cheap, measuring is expensive, and for most purposes calculating is enough. The catch lies elsewhere: a formula was developed on many people and describes their average well. About you it says considerably less – and nobody tells you how much less.

This article explains what can actually be measured in endurance training, how reliable each of those quantities is, why the widespread formulas are right on average and far off for individual people – and which tests are worth it for people who train for pleasure or for their health and have no laboratory in the cellar.

Scope. This is about endurance – about the heart, the circulation and metabolism under load. Tests of strength, power, balance and flexibility do not appear here; for those see the articles Strength training and Assessing fall risk. And what lactate is, what LT1 and LT2 mean, how the zone models relate to each other and how to steer without a device – that is in the article Understanding endurance training. This text starts where measuring begins. Where the two touch, we point across rather than repeat.

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

1.1 What a test should do – and what it should not

Before you measure anything, one minute is well spent on the question of what you want to do with the result. For training there are three good reasons:

• Steering. You want to know how fast your easy days are allowed to be and how hard your hard ones. For that you need boundaries, not best values.
• Following progress. You want to know whether anything has changed over the past three months. For that you need a test you can repeat under the same conditions – the exact absolute value is secondary.
• Placing yourself. You want to know where you stand compared with others, usually for health reasons. For that you need a value for which comparison data exist.

Anything that serves none of these purposes is entertainment. That is allowed – but you should not base training decisions on it.

In medicine there are further reasons that go beyond training: investigating an unclear symptom, assessing risk before a procedure, judging the course of a condition under treatment. Those tests are ordered by a doctor and are not the subject of this article.

And a fourth reason, often present and rarely stated: a test motivates. A number that moves keeps people going. That too is legitimate, as long as the number is real and not merely a picture of the day's form.

1.2 Ten words you need

• Maximum heart rate (HRmax). The highest pulse rate your heart reaches at full exertion. It declines with age, cannot be raised by training, and is not a performance measure: a high value does not mean "fit".
• Resting heart rate. The pulse at complete rest, most informative lying down immediately after waking.
• Heart rate reserve. The span between resting and maximum heart rate. Someone with 55 at rest and 180 at maximum has a reserve of 125 beats.
• VO2max (maximal oxygen uptake). How much oxygen your body can use at most at full exertion, given in millilitres per kilogram of body weight per minute.
• LT1 and LT2. The two points at which metabolism noticeably changes: LT1 where it stops being genuinely easy, LT2 where the balance tips. In detail in the article Endurance training.
• MLSS (maximal lactate steady state). The highest sustained intensity at which blood lactate still remains stable. It is measured with several long constant-load trials on different days and serves as the reference against which simpler methods are judged [49].
• FTP (functional threshold power). The power you can hold for roughly an hour. In practice usually estimated from a 20-minute test.
• Critical power. A mathematically estimated boundary between the range in which oxygen uptake and lactate can still settle at a level, and the range in which both keep rising until exhaustion. It is calculated from several maximal efforts of different duration [48][49]. Not to be confused with "sustainable indefinitely": you fatigue at critical power too, just more slowly.
• Typical error. How much a test result varies when nothing has in fact changed. The most important and least often quoted number of any test [5][63].
• Limits of agreement. The range within which the difference between two measurement methods lies for around 95 out of 100 people. A method can be perfect on average and still have very wide limits of agreement – then it is usable for groups and not for individuals.

2. What can be measured at all

The list is shorter than the market suggests. At its core there are five families of measures, and everything else your watch displays is derived from them.

2.1 Heart rate

Heart rate is at once the most accessible and the hardest to interpret. Accessible because any chest strap delivers it. Hard to interpret because it responds to a great many things at once: to the load, but also to heat, fluid loss, sleep, coffee, excitement, a starting infection and altitude.

Three values are useful:

• The resting heart rate – as a trend over weeks, not as a snapshot.
• The heart rate at a fixed, repeated workload – the underrated classic, see section 5.4.
• The maximum heart rate – only if it was actually measured rather than calculated.

How reliable is the measurement itself? A good chest strap measures the electrical activity of the heart and, with a regular heart rhythm and correct fit, agrees very closely with an ECG measurement. A watch on the wrist uses light to measure the rhythmic changes in blood flow under the skin – and that works with varying success. In a study of 50 adults the chest strap agreed almost completely with the ECG, while the watches ranged from moderate to good depending on device and activity; on an elliptical trainer with moving arm levers none of the devices tested reached an acceptable range [25]. A review covering many devices found the same picture: adequate at rest and at steady loads, unreliable at changing intensity and with arm movement [26].

Practical consequence: if you use your heart rate to steer or to compare, use a chest strap. If you are only watching it roughly, the watch will do.

2.2 Maximal oxygen uptake

VO2max is the best-known number in the endurance world. It describes how much oxygen the body can use at full exertion, and it is one of the strongest known predictors of illness and mortality – in many analyses clearer than blood pressure, blood lipids or smoking [1][2]. Extensive comparison tables by age and sex exist for placing a value [3].

It is measured with breath-by-breath gas analysis: you wear a mask, the device measures how much oxygen you inhale and how much carbon dioxide you exhale while the load is increased in steps to exhaustion.

Two limitations matter. First, VO2max says little about how fast you can run for an hour – for that the two thresholds and movement economy are at least as important [4]. Second, it often changes more slowly under training than the values that count in daily life; someone whose VO2max is unchanged after eight weeks of training has not necessarily stayed the same.

2.3 Lactate and the two thresholds

Lactate is measured from a drop of blood from the earlobe or fingertip, usually at the end of each stage of an incremental test. From the resulting curve LT1 and LT2 are calculated – with very different methods that give different values [19][20][21]. That is described in detail in the article Endurance training.

What counts for this text is measurement accuracy: portable lactate analysers have a typical error of roughly 0.4 to 1.0 millimoles per litre depending on the model [51]. A comparison of six devices arrived at similar magnitudes [52]. That sounds small, but it is exactly the range in which the classic fixed threshold values lie. Anyone who defines LT1 as "2 millimoles" is working with a boundary that may lie within their device's own measurement error.

2.4 Power and pace

Watts on the bike and pace when running are the only quantities in this list that measure what you actually produce – everything else measures what your body is doing about it.

Watts are unimpressed by how you feel: a power meter shows the same number in the heat, when tired and on a climb, because it records the external work and not what it costs you. That is exactly why they are so useful – provided it is always the same, correctly calibrated device. And exactly why they are not enough on their own. The same 200 watts cost you little on a good day and a great deal at the end of a hard week. Only power together with heart rate shows that.

When running, pace is the natural substitute. Its drawback is its dependence on surface, gradient and wind – on a flat, always identical loop it is nevertheless a very good measure.

2.5 Your own perception

The effort you feel is not a stopgap for people without devices. It is a measure in its own right with its own literature [33]. Two forms are well studied:

• The rating of perceived exertion after Borg, usually from 6 to 20 or from 0 to 10.
• The talk test, which sits surprisingly close to the first threshold [34][35].

The great advantage: perception incorporates everything a device does not know – sleep, heat, stress, the infection that is announcing itself.

2.6 Recovery measures

This is where the market is loudest and the evidence thinnest. Two quantities have a scientific basis:

• Heart rate variability – the variation in the intervals between heartbeats. Certain measures of it mainly reflect the influence of the calming nervous system on the heartbeat; they also depend on breathing, body position, time of day, age and measurement conditions [68], and typically fall under load and stress [27].
• Heart rate recovery – how far the pulse drops in the first minute after the end of exercise [38][39].

Everything displayed beyond that as "readiness", "body battery" or "recovery score" is a proprietary combination of these and other quantities. The underlying formulas are not disclosed and cannot be checked independently.

2.7 What belongs together – and what does not

A common error of reasoning is to treat these quantities as a ranking with VO2max at the top. But they answer different questions:

3. Why formulas are right on average and can be wrong for you

3.1 The average is nobody

This is the most important section of this article, and it is not about sport but about statistics.

Imagine someone measures the maximum heart rate of a thousand people and plots it against age. A cloud of points appears, and a line is drawn through it. That line is the formula. It describes the average of the cloud very precisely – and says nothing in itself about how wide the cloud is.

That is exactly the part almost every presentation of such formulas leaves out. A formula can fit the group excellently and be 15 or 20 beats off for many individuals without anything being wrong with it. It simply answers a different question: not "how high is your maximum heart rate" but "how high is the maximum heart rate of people your age on average".

That statements about groups and statements about individuals are not the same thing is well studied – far beyond sports science. A widely noted paper compared group results with repeated measurements in the same individuals and regularly found the variation within a person to be two to four times as large as the group result suggested [9]. What was examined there were psychological and medical measures, not heart rates; the paper therefore supports the principle below, not the numbers. What holds for the group holds for you only approximately – and how approximately is known only once someone states the spread.

The same applies to the effect of training. That a group improves by 12 per cent on average does not mean every member gains 12 per cent – and the apparent differences between people always contain measurement error and day-to-day form as well [7][8]. Anyone concluding from a single measurement that they do not respond to training has usually confused the two [61].

3.2 "220 minus age" – where the number comes from

The best-known formula in the sporting world is also the weakest. It does not come from a study designed for the purpose but from a survey of older data, and it was never tested as an individual prediction.

How good it is can be quantified. In a study of 762 people the maximum heart rate was measured and compared with the formula values. The average prediction error was 12.4 beats per minute [13]. That number describes the typical deviation, not the largest: about a third of participants were further out than 12 beats, and covering the usual 95 per cent means allowing around 25 beats in each direction.

For a 50-year-old that means: the formula says 170. The actual value quite plausibly lies between 145 and 195. Drawing a zone boundary at "75 per cent" on that basis gives 128 beats on paper – while the right boundary for that person may lie anywhere between 109 and 146. That is a span of almost 40 beats for a single zone boundary.

One more point is notable: the error was larger in people with lower starting fitness and higher body weight [13]. The formula is therefore least sharp precisely where it is used most often – with people who are just beginning.

3.3 The better formulas – and why they do not solve the problem

There are better formulas, and they are properly done.

A survey of 351 studies with 18,712 people produced the relationship 208 − 0.7 × age, cross-validated in a further 514 people in the laboratory. The result was independent of sex and of the amount of habitual physical activity, and it showed that "220 minus age" systematically underestimates the maximum heart rate in older people [10]. A study following the same people for 25 years reached a very similar result [11].

The largest single measurement comes from Norway: 3,320 healthy adults, taken to exhaustion in the laboratory. Formula: 211 − 0.64 × age. And – this is the decisive sentence of the paper – the prediction error was 10.8 beats per minute [12]. The authors write explicitly that the formula is practically usable provided this error is taken into account.

Here is the point: the better formula shifts the line, but it does not make the cloud narrower. From 12.4 beats to 10.8 beats – that is the entire gain [13][12]. The individual difference between people of the same age remains, because it is real and does not come from an error in arithmetic.

The ranges in the last column cover around 95 out of 100 people. They are not an expression of poor formulas but of the actual differences between people of the same age.

3.4 Karvonen: what the formula actually does

The Karvonen formula does not work with maximum heart rate but with the reserve in between:

Target heart rate = resting heart rate + percentage × (maximum heart rate − resting heart rate)

That is a genuine improvement, for a precise reason: a percentage of the heart rate reserve corresponds roughly to the same percentage of the oxygen uptake reserve, whereas a percentage of maximum heart rate does not correspond to the same percentage of maximal oxygen uptake [15]. The relationship also holds in very well trained people [16]. So anyone already working with percentages does better working with the reserve.

Three qualifications belong with it.

First, the formula needs two values, and both are error-prone. If the maximum contains a formula estimate, the target heart rate inherits its entire error. And the resting heart rate varies in itself: measured after getting up, after the first coffee or after a bad night, 10 beats of difference are easily possible.

Second, the origin of the formula is more modest than its spread suggests. It goes back to a Finnish paper from 1957 that examined, in a very small group of young healthy men, how high the pulse has to be during training for anything to change at all [14]. Over decades that observation became a universal zone formula – it was never meant that way.

Third, and this is the real problem: the reserve method also divides a span into percentages. Whether the percentage coincides in your case with the actual transition in metabolism is something it does not know. That is precisely the subject of the next section.

3.5 Percentages as zone boundaries: the actual error

So far this has been about how precisely a maximum value is estimated. Now comes the more serious point: even if your maximum heart rate were known exactly, a percentage of it does not tell you which metabolic state you are in.

A study of 100 people – 46 women, 54 men – measured this cleanly. In all of them the first threshold was determined in the laboratory, and in addition the maximal lactate steady state was measured with several long constant-load trials. Then it was checked at which percentage each of these two points lay [17]:

Read the first row again. In some people the genuinely easy range ends at 60 per cent of maximum heart rate, in others only at 90 per cent. The ranges also overlap: 80 per cent of maximum heart rate can still be below the first threshold in one person and already above the lactate steady state in the next. The authors conclude that prescriptions in fixed percentages do not adequately control the metabolic stimulus [17]. A more recent paper comparing different definitions of the much-discussed "zone 2" reached the same result [18].

One qualification: the figures 60 to 90 per cent describe how far apart the values lay in that study, using the threshold methods applied there. They are not a biological upper and lower limit. The message is not in the end points but in the width: it is too great for any single percentage to fit everyone.

This is the core of the whole formula question. Not: "your watch calculates your maximum heart rate wrongly." Rather: even with the correct maximum heart rate, a percentage is not a metabolic boundary. It is a share of a span – and where in that span your body switches over differs from person to person.

3.6 What your watch knows about your oxygen uptake

The VO2max display on modern watches usually rests on the device combining your heart rate with pace and gradient: a low pulse at a given speed produces a high estimate. Older or simpler methods work only with the ratio of maximum to resting heart rate [24].

How well that works has been quantified by an international expert network in a survey of 14 studies [22]:

• Devices estimating from resting values were on average 2.2 millilitres too high – with a spread from about 13 millilitres too low to 17 millilitres too high.
• Devices using exercise data were on average practically correct – but with a spread of about 10 millilitres downwards to 10 millilitres upwards.

These spreads are pooled across all the studies included; depending on device, person and measurement situation they come out smaller or larger [22]. As an order of magnitude for an individual they still hold.

For context: 10 millilitres per kilogram per minute is, for a 55-year-old man, roughly the difference between the bottom quarter and the top quarter of his age group [3]. The paper's conclusion is correspondingly clear: for statements about population groups these estimates are adequate, for the individual the error is large [22]. A single study of several wrist devices reached the same conclusion [23].

What you can do with it: do not take the absolute value seriously. As a rough observation over months – under comparable conditions and with the same device – the display can be useful. If it rises from 38 to 42 over three months, that is a hint and no more: it does not establish that your real VO2max is 42, and how reliably such watches track change is far less well studied than how accurate their single values are. So do not read anything into small movements.

3.7 Thresholds, recovery, readiness: the other displays

The estimated lactate threshold. Some watches give a heart rate or pace for the threshold, derived from the course of your heart rate during a rising run. In principle that is the right idea – it is a field test, not a calculation. How accurate it is cannot be judged from outside, because the methods are not disclosed. Treat the value like a test of its own: usable as a trend, not as truth.

Recovery and readiness scores. The heart rate variability behind them is a real quantity with a solid literature [27]. What is made of it is a company matter. Two things are well established: single daily values vary strongly, which is why specialists work with weekly averages [27]. And: training guided by variability is not clearly superior to conventional planning. A summary of 8 studies with 198 participants found a moderate advantage in submaximal measures but no established advantage in performance or oxygen uptake – notably with fewer hard sessions [30]. Other papers found advantages in individual groups [28][29].

Race predictions. These rest on empirical values from large user databases. For rough orientation they are usable, for pacing a race they are not: they know neither your experience of the course nor your nutrition nor the heat on race day.

3.8 When a formula is nevertheless enough

After all this the impression might arise that formulas are worthless. They are not. They are simply something other than many people think.

A formula is usable:

• as a rough upper limit. If you are not to exceed a certain range in rehabilitation, a conservatively calculated limit is better than none.
• for groups. Anyone setting a guideline intensity for a cardiac exercise group necessarily works with averages – and supplements them with observation and the talk test [59].
• as a starting point. You have to begin somewhere. But after the first weeks the formula should be replaced by observation, not confirmed by it.

A formula is not usable where it is used most often: as an exact boundary between "easy" and "too fast" for a particular person.

4. What makes a test usable

4.1 Repeatable beats accurate

Two properties of a test are readily confused.

Accuracy means: the test hits the true value. Repeatability means: the test delivers the same result when nothing has changed.

For the question "has anything changed?" the second property is the more important. If your field test overestimates your VO2max by 4 millilitres and is wrong consistently, you will still see changes reliably. If it varies randomly up and down from measurement to measurement, you will see very little – even if it is right on average.

That does not hold for every question, though. As soon as it is about placing yourself against others, or about a zone boundary, the absolute value counts – and with it accuracy. And a consistent error stays harmless only for as long as it really is consistent: it can change with intensity, with training status or with the device [63].

The decisive question to put to any test is therefore: how much does the result vary if I do it twice? That number is called the typical error [5]. For maximal oxygen uptake and maximum heart rate it is a few per cent in the laboratory; in a large study across four centres the variability of repeated maximal tests was below 10 per cent, and considerably lower for heart rate and oxygen uptake [6]. For field tests it is larger, and for submaximal reference tests under good conditions surprisingly small: for a standardised submaximal cycling test, typical errors between 1.3 and 4.4 per cent have been reported [36].

4.2 The smallest change that counts

From the typical error follows directly how large a change has to be before it is more than noise. For an individual the usual figure is about 2.8 times the typical error; the technical term for the result is the smallest detectable change [62][63]. As a rule of thumb to carry in your head, two to three times over will do.

A distinction that often gets lost: "detectable" and "meaningful" are two different things. The smallest detectable change says when a change is larger than the measurement error. The smallest meaningful change says when it matters noticeably to the person concerned [62]. The two can lie far apart – a change can be measurable and yet irrelevant, or noticeably important and still within the measurement error.

The values are orders of magnitude for practice, not cut-offs. On the last column: for the first five rows it is derived from the measurement error – when, in other words, a change is larger than the test's own noise. For the 6-minute walk test the 14 to 30 metres describe something else: the order of magnitude at which a change is meaningful to the person concerned, established in various disease groups [55]. Whether it also exceeds the measurement error in a given case depends on the population and the test procedure [62].

The practical use of this table lies less in celebrating small gains than in the opposite: it protects against false conclusions downwards. A time trial four watts worse is not a loss of form, it is a Tuesday.

4.3 Standardising: the checklist

A test measures your form only if everything else stays the same. What you can control:

• Time of day. Always the same, ideally within two hours.
• Preceding load. Easy or free the day before. Not the day after the long session.
• Food and drink. The same meal at the same interval. Coffee: either always or never.
• Warm-up. Always the same length and intensity – write it down.
• Route or equipment. The same loop, the same treadmill, the same bike, the same position.
• Weather. Note temperature and wind. Above 25 degrees a comparison with a cool test day makes no sense – heat drives the pulse up and performance down [41].
• Measuring device. Always the same one – and with power meters, mind the calibration. Changing power meter or ergometer can invalidate a series; changing chest strap is usually less consequential, whereas moving from a chest strap to wrist measurement certainly is.
• Sleep and load outside sport. Note it, do not control it. It explains some outliers afterwards.

A test protocol on a sheet of paper is worth more than the more expensive device. Anyone who no longer remembers six months later whether they had breakfast that day can no longer compare their numbers.

4.4 A test measures one day – and a fresh one at that

Every test measures you in the state in which you turn up for it. That sounds banal but has a little-noticed consequence: almost all laboratory tests take place in a rested state and therefore say little about how your body behaves after two and a half hours.

For endurance sports that is often exactly the decisive question. The literature calls this property durability – the ability to hold your own threshold values even late in a long effort. Studies show that thresholds shift downwards in the course of a long session, and to different degrees in different people [42]. Two people with the same threshold when fresh can differ markedly after three hours.

In practice this means: a result from Sunday morning is not a statement about the last third of your marathon. Anyone training for long efforts should therefore also measure at the end of long sessions – see section 5.5.

4.5 Before the test: when to have things checked

Maximal tests are safe for the vast majority of people. In supervised training of almost 5,000 people with heart disease, more than 175,000 training hours produced three serious events in total [32]. The absolute risk is therefore very low – but not zero, and ruling out problems is a medical task, not a question of courage.

What matters under today's recommendations is not age but four things: your current activity level, known diseases of the heart, circulation, metabolism or kidneys, current symptoms – and the intensity you are planning [31].

Have things checked beforehand if:

• you experience chest pain, chest tightness, breathlessness, dizziness or palpitations on exertion – then before any test.
• you have a known heart, vascular, lung, kidney or metabolic disease and have not been training intensively on a regular basis.
• you have not trained for years and want to start straight away with a maximal test. Begin submaximally instead.
• you take medicines that affect the pulse – see section 9.1.

A maximal test is in any case unnecessary for most questions. The most interesting tests in this article are submaximal.

5. The tests one by one

5.1 Resting heart rate

What it measures. How often your heart beats under resting conditions – no more and no less. What influences that includes the autonomic nervous system, the amount of blood moved per beat, training status, medication, temperature and illness. Viewed over weeks it is nevertheless a useful mirror of your training state and your load.

How to measure it. Lying down, immediately after waking, before getting up, over one minute. If your watch gives the nightly low value, that is usable too – but not comparable with the one-minute measurement.

What it is worth. As a trend, quite a lot: a morning pulse raised by 5 to 10 beats over several days is a usable sign that something is wrong – a starting infection, too little sleep, too much training. As a snapshot, little.

As a health measure. A high resting heart rate is associated with increased mortality, independently of physical fitness. In a study of 2,798 men over 16 years, the risk of death rose by around 16 per cent per 10 beats of higher resting heart rate [40]. That is an association and not proof of cause and effect – but a reason to raise a persistently high resting pulse with your doctor.

Do not overrate it: a low resting heart rate is not a certificate of fitness. There are untrained people at 48 and well-trained people at 68.

5.2 The talk and sing test

What it measures. Whether you are below or above the first threshold. Breathing couples to metabolism, and speaking couples to breathing.

How to do it. Speak a longer sentence in one go – two to three lines. If you can say it fluently and without snatching a breath, the test is positive. If you have to take a breath mid-sentence, you are above it.

What it is worth. More than its price suggests. The last point at which speaking is still effortless lies close to the ventilatory threshold, and it does so even when that threshold shifts through training or through blood loss [34]. Tested against lactate thresholds, the relationship also holds [35]. A review in people with heart and lung disease confirms its usefulness for guiding intensity, but points out that the "last positive", "equivocal" and "negative" stages of the test are not simply interchangeable with the first threshold [66].

The common trap. "Talking yes, singing no" does not describe the middle of the easy range but already its upper limit. Anyone who really wants to be easy should be able to sing effortlessly – in detail in the endurance article. In fairness: effortless speech is well studied, whereas the singing rule is a practical, deliberately cautious guide rather than an equally well tested threshold measure.

5.3 The exertion scale

What it measures. Your overall effort, everything included.

How to use it. Give a number at the end of a session or a segment, on a fixed scale. What matters is that you always use the same scale and the same question – "how hard was that overall" is something other than "how hard was the breathing".

What it is worth. Well studied as a steering measure, with a notable property: it automatically takes account of what devices miss [33]. If a familiar session suddenly feels much harder, that is a real signal – even when pulse and watts look normal.

Limitation. Beginners often underestimate their effort in the easy range; the scale has to be practised for a few weeks.

5.4 The reference session – the most important test in this article

If you take only one test from this article, take this one. It costs nothing, is submaximal, is done in twenty minutes and answers the question most people actually have: am I getting better?

The set-up. You define a fixed session that you repeat unchanged every three to four weeks:

• Always the same warm-up, for example 10 minutes very easy.
• Then a fixed load of fixed duration at fixed pace or fixed wattage – for example 12 minutes at a pace that clearly feels easy.
• Record: the average heart rate of the last few minutes, your effort on the scale, plus temperature and time of day.

How to read it. The pace stays the same. What changes is the pulse. If it falls over the months at the same pace, your endurance has improved. If it rises repeatedly under comparable conditions, that can point to incomplete recovery, a starting infection or a changed training load – but before assuming any of that, check temperature, sleep, coffee and fluids.

Why this works. Because this test avoids the problem of maximal tests: it requires no full exertion, hardly depends on motivation and is therefore very repeatable. In a study of trained cyclists the typical errors of a standardised submaximal protocol lay between 1.3 and 4.4 per cent, and the values predicted 40-kilometre time trial performance well – the authors point out explicitly that this allows changes to be detected more precisely than through VO2max [36]. A review of heart-rate-based monitoring reaches the same conclusion: heart rate at a fixed submaximal load is one of the most practicable measures there is [37].

Two notes. First, it takes patience: expect nothing before six to eight weeks. Second, the five-beat rule is a guide – differences below that are usually weather, sleep or coffee.

5.5 Heart rate drift and decoupling

What it measures. Whether your easy pace really is easy for you – and whether your endurance holds up late in a long session.

How to do it. Run or ride at least 60 minutes at a constant, easy pace. Compare the average heart rate of the first half with that of the second.

How to read it. Some rise is normal and not a failure: during long efforts the heart rate rises even at constant power, because core temperature and blood volume change [41]. As a practical grading – identical to the one in the endurance article:

• Under 5 per cent over 60 minutes: you were genuinely in the easy range.
• 5 to 10 per cent: normal. Usually heat or too little to drink. Hold the pace, drink.
• Over 10 per cent: ease off. For that duration the effort was no longer genuinely easy.

These boundaries are an established training heuristic, not a validated diagnostic cut-off – how much someone drifts depends on duration, heat, fluid intake and training status.

The further point. How large this shift turns out to be is a feature of endurance in its own right and does not simply follow from VO2max [42]. Someone who drifts hardly at all over 60 minutes in spring but markedly in autumn has gained real information – even though no maximal value was measured.

Careful with the interpretation: heat is the most common cause. Compare only sessions at similar temperatures.

5.6 Heart rate recovery

What it measures. How quickly the pulse falls after the end of exercise – a measure of how rapidly the calming nervous system takes hold again.

How to measure it. After a defined effort, stop immediately, stand or sit quietly, and note the difference between the peak value and the value after exactly one minute.

What it is worth. As a health measure it is well established. In a study of 2,428 adults followed over six years, a fall of 12 beats or fewer in the first minute counted as abnormal; those people had roughly twice the mortality even after accounting for age, medication and exercise capacity [39]. That is a finding from a clinical testing situation and no reason for alarm if you measure 11 beats once – but a reason to mention it at your next appointment if it is regularly so.

As a training measure it is weaker than its reputation suggests. A systematic review found relationships with training state but no simple direction: heart rate recovery can both rise and fall under very high training loads [38]. As the sole basis for "am I recovered?" it does not serve.

5.7 The 12-minute Cooper test

What it measures. How far you can run in 12 minutes – and from that, an estimate of your endurance capacity. The test dates from 1968 and was checked against laboratory measurements at the time [43].

How to do it. On a track or a flat, measured route: warm up for 10 to 15 minutes, then cover as much ground as possible in 12 minutes. Pace it evenly – the most common source of error is starting too fast.

What it is worth. Usable as a field test, not as a VO2max measurement. A survey of 123 studies found a relationship between the 12-minute test and measured maximal oxygen uptake of about 0.78 – among field tests one of the best values, but still implying considerable deviation in individuals [44]. The authors state explicitly that the result remains an estimate and not a measurement.

As a progress test, by contrast, it is good: the same track, the same time of day, the same preparation – and the distance compares cleanly with last time.

Who it is not for: for people who have not run for years the test is too hard and the injury risk unnecessarily high. Start with a walking variant or with section 5.4.

5.8 The time trial and FTP

What it measures. The highest power you can hold over a given time. On the bike in watts, in running as pace.

How to do it. The most widespread form is a 20-minute time trial, of whose average power 95 per cent is taken – that gives the FTP. Alternatively a 30-minute time trial, whose average over the last 20 minutes is used directly.

What it is worth – honestly. Here a closer look is worthwhile, because the studies differ and the reason for that is instructive.

• In 15 trained and well-trained cyclists the FTP agreed well on average with the maximal lactate steady state – a deviation of 1.4 per cent. Individual scatter was considerable, however: ±7.4 per cent in the well-trained and ±11.8 per cent in the less well trained [46].
• In 23 trained cyclists the 20-minute test, the 60-minute test and the laboratory-determined individual threshold lay only a few watts apart on average – but the limits of agreement ranged from 40 watts too low to 60 watts too high [45]. Time to exhaustion at the FTP thus determined averaged 51 minutes with a spread of ±16 minutes – for some people the "hour power" is a three-quarter-hour power.
• Clearest of all is a study of 18 people, 10 of whom were retested after seven months of training: the maximal lactate steady state rose by an average of 12 watts – the 20-minute test did not register this improvement [47].

What follows from that? The time trial is a good performance test and a mediocre threshold test. As the question "how much power can I produce over 20 minutes" it is clean and repeatable. As the question "where is my second threshold" it is right on average and uncertain in the individual – most uncertain, of all people, in the less well trained [46].

Practical advice: use the result as a quantity in its own right, not as a substitute for a threshold. "My 20-minute power has risen from 235 to 248 watts" is a good statement. "My threshold is 236 watts" is one not to take too literally.

5.9 Two time trials: critical power

What it measures. The transition between two ranges of intensity: below it, oxygen uptake and lactate settle at a level – high, but a level; above it, both keep rising until you have to stop. This transition lies closer to the actual physiology than FTP does [48][49].

One widespread misunderstanding right away: critical power is not an intensity that can be held indefinitely. You fatigue there too – time to exhaustion is typically in the order of a few tens of minutes. What happens above it is something else: there the point of failure is predictable and moves closer with every additional watt [48].

How to determine it. On different days you ride or run two to three maximal efforts of different duration – typically about 3, 7 and 12 minutes. Critical power is calculated from the relationship between power and duration.

What it is worth. Scientifically better founded than FTP and determinable without taking blood. The price is substantial: you need several maximal efforts on different days, and the result depends noticeably on how long the individual efforts were, how many there were and which calculation model was used [64] – plus on whether you really went all out each time. For ambitious recreational athletes with a power meter it is a genuine option, for health-oriented exercisers unnecessary effort.

5.10 The incremental lactate test

What it measures. The relationship between load and blood lactate, and from it LT1 and LT2 with their associated heart rates.

How it works. Stages of 3 to 5 minutes with rising load, a drop of blood from the earlobe at the end of each stage. Duration including preparation and follow-up, about an hour.

What it is worth. It answers the question no formula can answer: where your two transitions lie. That is exactly why it is more valuable for training guidance than any maximal test.

What you should know about its limits. Three things:

• The analysis method matters. From the same curve, different recognised methods produce noticeably different wattages [19][20]. Have them tell you which method was used, and stay with it next time.
• The measurement error is relevant. Portable devices have typical errors of 0.4 to 1.0 millimoles per litre [51][52] – in the order of magnitude of the fixed threshold values themselves.
• LT2 is not the maximal lactate steady state. The second threshold estimated from an incremental test deviates in individuals from the steady state actually measured [50].

Who it is worth it for. For people who train regularly and in a structured way and want to keep their easy sessions genuinely easy. For someone who walks for half an hour three times a week, the talk test is the more sensible choice.

If you have had a test done and cannot make anything of the numbers: our lactate threshold analysis computes the common methods side by side, so that it becomes visible how far apart they are.

5.11 Cardiopulmonary exercise testing

What it measures. Directly: oxygen uptake, carbon dioxide output, ventilation. From these: VO2max, the two ventilatory thresholds, breathing efficiency – and, where the question calls for it, indications of heart and lung disease.

What it is worth. It is the most comprehensive method: capacity, breathing and cardiovascular response are assessed at the same time, and where the question calls for it the result yields important medical pointers [67]. Repeated maximal tests are well reproducible [6]. Two qualifications belong with that: other exercise investigations – ergometry with ECG, blood pressure response, oxygen saturation – also yield medical information. And a cardiopulmonary exercise test does not make a diagnosis; it narrows down the causes of a limitation, which then have to be interpreted clinically [67].

Who it is worth it for. Where there is a medical question – unexplained breathlessness, a drop in performance, known heart or lung disease, assessment before a bigger undertaking. For training guidance alone it is almost always excessive: an incremental lactate test or a well-run reference session answers the training questions at a fraction of the cost.

5.12 The 6-minute walk test and everyday tests

The 6-minute walk test is a standardised exercise test at home above all in heart and lung disease – and it fits wherever the tests named so far are too hard: after an illness, in older age, with reduced exercise capacity.

How it works. In a straight 30-metre corridor: walk for six minutes at a self-chosen, brisk pace; pauses are allowed, and the distance covered is measured [54]. The length is part of the procedure and not a minimum – a shorter course adds turns, and that changes the result. The encouragement given during the test is prescribed word for word, because otherwise it co-determines the outcome, and a learning effect is to be expected on the first attempt; current standards therefore provide for two runs under some circumstances [65].

What it is worth. As a progress test a great deal, as a fitness measurement little. Walking distance estimates oxygen uptake well on average but scatters considerably in individuals [57]. What counts is the change: an improvement of 14 to 30 metres is regarded as meaningful to the person across various disease groups [55]. That is not the same as "larger than the measurement error" – how large that is depends on the condition and the test procedure [62][65]. Where both are known, the larger of the two figures applies.

For context. In 444 healthy people between 40 and 80 years from seven countries the mean walking distance was 571 metres, with a range from 380 to 782 metres [56]. That range is another example of this article's basic message: the average alone tells you little.

In addition, with low exercise capacity a strength test of the legs is worthwhile, because that is often where the real limitation lies: how often can you stand up from a chair and sit down again in 30 seconds without using your hands [58]? More on this in the article Muscle weakness in older age.

5.13 What you can measure but do not need

• Body fat scales. Measurement via body currents responds strongly to fluid balance. Useless as a day-to-day comparison.
• Step count as a training measure. Sensible for everyday activity, not for intensity control – 10,000 slow steps and one hard hour are not physiologically the same.
• Calorie figures. Device estimates deviate considerably [26].
• Running economy metrics, ground contact time, vertical oscillation. Interesting, but with no established benefit for your training decisions.
• Daily readiness scores as a basis for decisions. Look at them, but do not let them determine your training when your body tells you something else.

6. Which test for whom

7. How often to test – and when not to

A test costs recovery and delivers information only if enough time has passed since the last one for a real change to occur.

• Reference session: every 3 to 4 weeks. It is submaximal and does not disturb the plan.
• Time trial or 12-minute test: two to three times a year. More often gains nothing, because the changes are then smaller than the variation.
• Incremental lactate test or cardiopulmonary exercise test: once a year, or when training form or health status has clearly changed.

Do not test if: symptoms of illness are still present or your usual capacity has not returned – after a feverish or pronounced infection the return should be staged and guided by symptoms, not by a fixed number of days [69]; you trained hard in the last two days; you slept badly; it is markedly warmer than last time. A test under poor conditions is not merely imprecise – it is worse than no test, because it writes a number into your series that does not belong there.

And a common mistake: not testing when you are just finishing a hard phase. Wait for a quieter week. Otherwise you measure your fatigue and call it your form.

8. What to do with the result

A test from which no decision follows was a hobby. Three routes lead from the result back into training:

First: anchor the zones. If you know your two thresholds from a lactate test or a well-conducted field test, hang your zones there – not on percentages of an estimated maximum. How the common zone models relate to each other is in the endurance article; our planning tool ZoneForge takes the values directly.

Second: adjust the dose. If nothing moves over three months, the most common cause is not a lack of talent but too little stimulus – too little total volume, too few genuinely hard sessions, or both. In one study of 78 people the apparent non-responders disappeared as soon as the amount of training was increased [61]. It does not follow that every person responds on every outcome – the capacity to adapt does differ from person to person [60]. What follows is something more modest but more useful: an absence of progress at this dose is no proof of biological non-responsiveness. Measurement error, day-to-day form, too small a dose or an unsuitable choice of outcome are often behind it, and a higher or differently built dose then does produce something after all.

Third: change nothing. The underrated option. If the test confirms that the direction is right, the correct consequence is to carry on with the plan. Training plans fail more often through constant rearrangement than through wrong direction.

9. Special situations

9.1 Beta blockers and other medication

Beta blockers lower the heart rate at rest and under load, to different degrees depending on the substance and dose. That makes calculated target zones useless: a formula based on "220 minus age" knows nothing of your medication.

The heart rate itself, however, remains usable. What you need is a reference value measured under the same medication – an exercise test on therapy delivers exactly that [59]. Important: after any change of dose the old values no longer apply.

Other medicines also act on the heart rate, for instance asthma treatments or thyroid hormones. For heart-rate-based estimation methods this is a fundamental problem [53]. Anyone taking medicines that affect the pulse is often better served by the exertion scale and the talk test than by numbers.

9.2 Atrial fibrillation and pacemakers

In atrial fibrillation the heart rate varies independently of the load; a watch's average values cannot then be meaningfully interpreted. Heart rate variability also loses its meaning. With a pacemaker everything depends on the programming.

In both cases: steer by perceived exertion, the talk test and – where available – power in watts. The instructions of your cardiologist take precedence over any recommendation in this article.

9.3 Heat, altitude, illness

• Heat. At the same power the heart rate is higher, and it rises more steeply in the course of the session [41]. Tests above 25 degrees are not comparable with cool days.
• Altitude. Above about 1,500 metres performance falls at the same heart rate. In the Valais mountains that is no minor matter for comparing tests: the same test at the Simplon and in Glis produces different numbers.
• After an infection. The pulse often stays elevated for one to two weeks after you feel well again. Do not test during this time – and do not train at all with a fever.

9.4 Older people and those returning to training

For older people the formulas are least sharp – partly because the spread of maximum heart rate does not shrink with age, partly because "220 minus age" systematically falls short here [10][12]. Someone training at 70 according to this formula may well be training far too easily to produce any stimulus at all.

That is not an argument for hard tests but one against formulas: the talk test, the exertion scale and the reference session work at any age and need no maximal effort. The capacity to adapt to endurance training is preserved lifelong – those with the lowest starting fitness gain the most in percentage terms.

10. Ten misunderstandings

1. "My watch measured my maximum heart rate." Only if you actually went to exhaustion once and the watch adopted that value. Otherwise it calculated – with a typical error of around 11 to 12 beats [12][13].
2. "A high maximum heart rate means good endurance." No. Maximum heart rate is largely a property of age and was independent of fitness level in the large Norwegian study [12].
3. "70 per cent of maximum heart rate is base training." For some people yes, for others that is already too hard or far too easy. In a study of 100 people the first threshold lay between 60 and 90 per cent of maximum heart rate [17].
4. "The Karvonen formula is the accurate version." It is the better one, because heart rate reserve relates more closely to oxygen uptake reserve [15]. Accurate it is not – it inherits the error of both input values.
5. "My VO2max is 47." If the number comes from the watch, it is an estimate whose deviation can be ten units or more depending on device and situation [22]. A displayed 47 does not establish an actual 47.
6. "The value has fallen, I have lost form." First check whether the change is larger than the test's normal variation [5]. Usually it is not.
7. "The FTP test shows my threshold." Roughly, on average; in individuals with considerable scatter – and in one study it failed to register a real improvement of the threshold after seven months of training [47].
8. "A laboratory test is objective, a field test is imprecise." The laboratory test also has measurement error, and its analysis contains decisions [19]. A cleanly repeated field test can be the better choice for questions of progress [36].
9. "My heart rate variability was low today, I should rest." Single daily values vary strongly; specialists work with weekly averages [27]. And steering by them is not clearly superior to conventional planning [30].
10. "I need a test before I can start." The most common and most expensive error. Start, go easy, use the talk test – and test in six weeks, when there is something to measure.

11. When to contact us

• If you have had a test done and cannot place the numbers – from a laboratory, from a sports shop or from your watch.
• If you have been training for months and nothing is moving. Usually it is the distribution of intensities or the total amount.
• If you are returning after an illness or an operation and want to know where you stand and what to start with.
• If you take medicines that affect the pulse and are looking for a way of steering that works nonetheless.
• If you have symptoms on exertion – breathlessness, chest tightness, dizziness, palpitations. Then see a doctor first, not us.

What we offer in this area is described under Performance tests & training design.

12. In summary

Formulas describe groups, not persons. "220 minus age" is typically 12 beats off in an individual, the better formulas just under 11 [13][12]. That is not a calculation error but the real spread between people of the same age.

The more serious point is not the maximum heart rate but the percentage. Even with an exactly known maximum, a percentage does not tell you which metabolic state you are in: in 100 people examined, the first threshold lay between 60 and 90 per cent of maximum heart rate [17].

The VO2max display on your watch is a population estimate. Good on average, off by ten units or more for an individual depending on device and situation [22] – usable as a rough observation over months, not as a value.

For the question of change, repeatability matters more than accuracy – for placing yourself and for zone boundaries it is accuracy that counts. Ask of every test how much it varies, and believe a change only when it is clearly larger than that variation [5][63]. And keep apart whether a change is detectable or whether it is meaningful [62].

The most useful test for most people costs nothing: the same easy session, every three to four weeks, under the same conditions, watching what the heart rate does [36][37].

And for steering during training the best question remains the simplest: can you still speak effortlessly [34]?

The one sentence to take away: a formula tells you what applies to people of your age on average – a repeated test tells you what applies to you.

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Transparency

• Authorship: Roger Hilfiker
• AI support: the literature search and the draft were produced with Claude (Anthropic). Roger Hilfiker checked all statements, figures and sources and revised the text.
• Created: 22 August 2026
• Last updated: 22 August 2026
• Sources: the 69 works in the reference list.
• Competing interests: the practice offers performance testing, lactate analysis and training support as services, and provides two of its own tools for this. This article recommends some of these tests – and explicitly advises against others. We disclose this so that you can take it into account while reading.
• Funding: Physiotherapie Tschopp & Hilfiker, 3902 Glis. The article was produced from the practice's own resources.
• Next review: planned for August 2028