Moderate or vigorous intensity: how to tell without a device

Two ways to measure intensity

The WHO and US guidelines describe intensity in two ways.¹² Absolute intensity measures the energy an activity requires, in METs: one MET is what the body uses sitting at rest. Moderate is 3 to less than 6 times that, and vigorous is 6 times or more.¹ Walking at about 4.8 km/h (3 mph), for example, uses about 3.5 METs; running at 9.7 km/h (6 mph), about 10.²

Relative intensity measures how much effort it takes for you, on a scale from 0 (sitting) to 10 (the greatest possible effort): moderate is usually a 5 or 6, and vigorous a 7 or 8.¹² For many adults with reasonable fitness, the two scales line up fairly well.² Spain's Ministerio de Sanidad translates this into sensations: warmth, sweating, breathing and whether you can talk.³

The talk test: simple and backed by evidence

The test consists of trying to talk while you move: if you can hold a conversation but not sing, you're at moderate intensity; if you can only get out a few words at a time, you're at vigorous.² The US guidelines present it as a rule of thumb, not an exact measure.²

Studies have compared it with the ventilatory threshold: the point beyond which, if you keep increasing the effort, your breathing speeds up faster than it had been, measured in a laboratory by analyzing the air you breathe. In 16 healthy volunteers, on a treadmill and on a bike, the moment when talking began to get difficult coincided almost exactly with that threshold, and once talking was no longer comfortable, the intensity was above it.⁴ In another series of experiments with young adults, the test tracked the threshold when it went down (after giving blood) or up (after training).⁵

A 2014 review concluded that it is a valid, reliable, practical and inexpensive tool for competitive athletes, healthy active adults and people with cardiovascular disease; that it gives similar results when walking, running, cycling or using an elliptical or a stair machine; and that it may not be practical for high-intensity interval training.⁶ Another review, from 2022, concluded that, together with the effort scale, it appears to be a valid measure of the breathing thresholds.⁷

The effort scale, and why we tend to fall short

The other device-free tool is to ask yourself how hard it feels, from 0 to 10.² In the lab, the Borg scale, which runs from 6 to 20, is often used.⁹ A meta-analysis found that perceived effort is related to heart rate, oxygen uptake and breathing, but only moderately, less closely than had been thought.⁸ It works as a guide, not as an exact measure.

What's more, descriptions on their own are misleading. In a Canadian study, 129 adults were asked to walk or run on a treadmill at a “light,” “moderate” and “vigorous” effort, described in words about warmth and breathing. They got light right, but fell below the target for the other two, and when asked to walk at the minimum pace they thought was healthy, just over half (52%) did so at light intensity.⁹

What counts as moderate depends on your fitness

A less fit person needs more effort to do the same thing as a fitter one.² That's why, according to the US guidelines, some older or less fit adults find the activities the table calls moderate to be vigorous, and are better off starting with light activity and building up; by contrast, a young or fit person may be able to sing at that same intensity, and may need vigorous activity to get some of the benefits.²

For many older people with low fitness, an activity of 3 to 6 METs is relatively vigorous, or simply impossible; that's why the guidelines recommend that they go by relative effort rather than METs.²¹ That's the advantage of the talk test and the 0-to-10 scale: they adjust on their own to your fitness today, and as it improves you'll be able to do more demanding activities.²

If you walk, your step rate also gives a clue: in laboratory studies with adults aged 21 to 85, about 100 steps per minute was associated with 3 METs, the start of absolute moderate intensity, and about 130 with 6 METs in people under 60.¹²¹³ These are rounded figures for groups and they measure absolute intensity, not your effort; the authors themselves warn that their accuracy for any one person is limited.¹³ What's known about walking pace and health is covered in the article on how many steps to take a day.

What about heart rate? Why age-based zones are only an approximation

Relative intensity can also be expressed as a percentage of your maximum heart rate or of your reserve, the margin between resting and maximum. The US guidelines, for example, define moderate as 40% to 59% of aerobic capacity reserve, and vigorous as 60% to 84%.² Nor is there a single criterion: as the authors of a study on walking pace point out, moderate intensity has been defined as 64% to 76% of maximum heart rate, as 55% to 69%, or as around 60%.¹²

The bigger problem is the starting point. Measuring your maximum heart rate requires a maximal exercise test, so it's almost always estimated from age.⁹ The best-known formula, “220 minus age,” underestimates the maximum of older people, according to an analysis of 351 studies with 18,712 people that proposed another one: 208 minus 0.7 times age.¹⁰ A study of 3,320 healthy adults in Norway proposed 211 minus 0.64 times age and found that the earlier formulas fell short from age 30 onward; even with its own formula, the typical error was 10.8 beats per minute.¹¹ In a mostly young sample the opposite can happen: in the 35 people in the Canadian study with an acceptable exercise test, “220 minus age” gave an average of 190 beats, and the peak reached in the test was 178.⁹

That's why the heart rate zones a watch calculates from your age are an approximation, with the sensor's own error added on top; which watch data deserve more trust is covered in the article on wearables. The same goes for so-called “zone 2,” which has its own article: what it means and why it isn't a universal number.

How to use it in practice

What follows is a practical way to apply the above, deduced from the sources; it isn't a protocol that has been tested as such.

• Start easy and hold your pace. After a few minutes at the same pace, say a couple of sentences out loud or talk to whoever is with you.
• Read the answer. If you can talk without trouble but couldn't sing, you're at moderate intensity; if you can only get out a few words, at vigorous; if you could sing, you're going easy.²
• Check it against the 0-to-10 scale. Moderate usually feels like a 5 or 6, and vigorous like a 7 or 8.² If in doubt, remember that it's easy to fall below what you think you're doing.⁹
• Add up the minutes. For the guidelines, one vigorous minute counts as two moderate ones: 75 vigorous minutes a week are roughly equivalent to 150 moderate minutes.² An equivalent combination of the two also works,¹ and short bouts count too.

To start moving gradually, with light or moderate activity, people without contraindications generally don't need a medical checkup first; and those who already do moderate activity regularly can gradually move up to vigorous.¹ The US guidelines add that, if you want professional advice, it's especially appropriate before starting vigorous activity, because its risks are greater than those of moderate activity.² If new symptoms appear as you increase your activity, see a health professional.¹²