Understanding Exercise Intensity in Everyday Language

Activity guidelines are written in a small technical vocabulary: light, moderate, vigorous. The words sound vague, but they have fairly precise meanings, and you can tell which one applies to you at any moment without a watch, an app or a heart-rate strap. You only need to notice your breathing and try to speak.

Why intensity is in the guidelines at all

The weekly recommendations for adults are expressed in minutes at a given intensity. In England, the NHS advises at least 150 minutes of moderate-intensity activity a week or 75 minutes of vigorous-intensity activity, and says 75 vigorous minutes give similar benefits to 150 moderate ones. US guidance from the Centers for Disease Control and Prevention (CDC) uses the same pair of figures and allows an equivalent mix of the two.

So intensity is a kind of exchange rate. It explains why a 20-minute run and a 40-minute brisk walk can be counted as roughly the same contribution, and why a slow stroll, pleasant and worthwhile as it is, is counted differently.

The three levels in plain terms

The descriptions and examples in the moderate and vigorous rows are drawn from the NHS and the CDC, and the light row from the NHS description of light activity as moving rather than sitting or lying down. The “talk and sing” entry for light activity is our shorthand for normal breathing; the official talk test is defined for the other two levels.

The NHS also describes a fourth category, very vigorous activity: short bursts of maximum effort broken up with rest, as in interval training or sprinting up hills. It is one option among several for people who are already active, not a requirement.

The talk test, step by step

The CDC puts it as a rule of thumb. During moderate-intensity activity a person can talk but not sing. During vigorous-intensity activity they cannot say more than a few words without pausing for a breath. The UK Chief Medical Officers’ guidelines use the same test.

To try it:

1. A few minutes into an activity, say a full sentence out loud, such as your address.
2. If that was easy, try singing a line of a song.
3. Easy to sing: light. Can talk but singing breaks up: moderate. Sentence breaks up after a few words: vigorous.

If you are alone and feel self-conscious, counting aloud is another way to hear how your breathing is coping.

Intensity is personal

This is the point most often missed. The CDC distinguishes two ways of measuring. Absolute intensity is about the energy an activity typically demands: brisk walking is classed as moderate and jogging as vigorous, whoever is doing it. Relative intensity is about how hard an activity is for a particular person.

For relative intensity the CDC uses a scale of 0 to 10, where 0 is the effort of sitting and 10 is the most you could possibly do. On that scale moderate activity is a 5 or 6, and vigorous activity begins at 7 or 8.

Two practical consequences follow.

• Lists of activities are only a guide. If a “moderate” activity leaves you unable to speak, it is vigorous for you at present, and it is sensible to treat it that way.
• Your levels move. As fitness changes, the same walk may shift from moderate to light. The NHS notes that most moderate activities can become vigorous with more effort, so the route does not have to change; the pace or the gradient can.

Because it is relative, effort can also feel different from one day to the next, for instance when you are tired or unwell. A pace that was comfortable last week may not be today, and the talk test will tell you so.

What about strength activity?

Muscle-strengthening is judged differently, because breathlessness is not the point. The NHS describes the right level as working the muscles until you need a short rest before repeating the movement. It is counted in days per week, not in minutes at an intensity. See getting started with strength activity.

Using the levels in a week

Once you can tell the levels apart, the guideline arithmetic becomes simpler.

• Minutes at moderate intensity count once.
• Minutes at vigorous intensity count roughly double, since guidance treats 75 vigorous minutes as similar to 150 moderate ones.
• Light activity is not counted towards those totals, but it is not nothing: the UK guidelines say light activity brings some health benefits compared with being sedentary, and it is what breaks up long periods of sitting.

A week can mix all three. Our worked example in planning a walking routine around your day shows how brisk minutes add up. And the figures are guidance for adults in general, with the usual qualifier from the CDC that some activity is better than none; setting activity goals without an all-or-nothing mindset explains how to hold them loosely.

A note on gadgets

Watches and phones estimate intensity from heart rate and movement and label minutes as “active” or “zone” minutes. Manufacturers define these labels in their own ways, and they do not always match the guideline categories. If a device and the talk test disagree, the talk test is the method the public health bodies themselves describe. Our piece on what sleep trackers can and cannot tell you looks at the same question of how far to trust consumer devices.