How hard does it feel, out of ten?

We rate things from one to ten all our lives. A film, a meal, a pain in the knee, a day at work. Nobody has to explain the scale, because one is next to nothing and ten is the most there is, and everything else falls somewhere in between. That makes it one of the most natural measuring tools a person has, and it works for effort just as well as it works for a film. In the middle of a run, a ride or a swim you can ask yourself how hard this feels out of ten, and an answer comes back before you have finished the question.

The answer is also more accurate than it seems. In Norway, Stephen Seiler and Øystein Kjerland followed eleven junior cross-country skiers through 32 days of training. Before they started, each skier was tested on a treadmill to find two turning points in their breathing, the first where it begins to quicken and the second where it starts to run away from them. Those two points split every session into one of three bands, below, between or above. The researchers then sorted more than three hundred sessions into those bands twice, once by the skiers' heart rate and once by how hard the skiers said the whole session had felt, and the two ways of sorting gave almost the same picture. For a smaller set of sessions they also measured the acid in the skiers' blood, and that matched too. A number the skiers simply felt and said out loud told the researchers as much as the monitor and the blood test did.

So the scale is easy and it is honest. What most people do not know is which number to aim for on a given day, or which numbers to stay away from. A six on its own says how the swim feels, but it cannot say whether six is right. For that you need a target, and there are four of them, one for each effort you train.

Base is about 3. It is a steady effort that feels comfortable, the kind you could keep going at for hours, and it is where most of your time goes. Threshold is about 7. It is hard work, and you could hold it for about twenty minutes and not much more. VO₂max, the most oxygen you can take in and use, is about 8. It is very hard, and you could keep it up for a few minutes at most. A sprint is 10, everything you have, and it lasts until you feel yourself slowing.

Knowing these four numbers turns the scale into something you can steer by. At the wall, the six you said out loud now means something. If you set out to swim at base, six is twice what you were after, and you ease off until it feels like a three. If you set out to swim at threshold, six is not quite enough, and you lean in until it feels like a seven. In the water that matters more than anywhere, because heart rate is a weak guide there and your face is down, so a number you can feel is one of the few readings you have. The feeling was the same either way, and only the target told you which way to go. It is time spent at the right number that trains you, whatever the speed turns out to be that day.

The number most worth knowing is the one you do not want, and that is 5. It sits in the grey zone, an effort that feels like proper training because it is a little hard, and that is why it is so easy to drift into. But it is too hard to be base, so it tires you more than base does and you cannot do as much of it, and it is too easy to be threshold, so it does not push your body to the point where it has to adapt. You come home tired without having trained anything in particular.

It is also where most people put base. Ask someone what an easy run feels like and many will say about a five, halfway up the scale, because a run is supposed to feel like work. That is a big mistake. If you think base is a 5, you will run every base session too fast, and the hours that should build you will leave you tired instead.

The best endurance athletes know this. In Seiler and Kjerland's study, the skiers spent most of their sessions clearly below the first turning point and a good share clearly above the second, and very little in the band between them. The researchers noted that the same pattern had been seen in elite athletes across several sports.

A group of runners in Spain showed what the middle costs. Jonathan Esteve-Lanao and his colleagues split twelve well-trained distance runners into two groups for five months. Both groups did the same amount of the hardest training. The difference was in the rest of their time. One group spent more of it below the first turning point, and the other spent more of it between the two. Both groups ran a 10.4-kilometre cross-country race before and after, and the group that stayed below the middle improved by about two and a half minutes, half a minute more than the group that trained in it.

The scale also tells you about more than the training. Some days the session that should be a 3 feels like a 5 from the first minutes, however gently you go. Nothing about the effort changed. What changed is you: a short night, a hard week at work, a cold on its way. When a 3 feels like a 5, you are carrying too much stress, and it is a good day to keep everything at 3 or to rest.

Intervals tell you the same thing. A threshold block that should be a 7 feels like a 9, or the first VO₂max effort already feels like a 10. The number you feel is higher than the effort you are in, and the gap is the stress you brought with you. On those days the harder sessions take more than they give, so let the session be gentler, or leave it for another day.

Put the numbers side by side and they line up with everything else you can feel. A 3 is an effort you could hold for hours, a 7 for about twenty minutes, an 8 for a few minutes, and a 10 until you slow. The rating and the time you could last are two ways of reading the same effort, and when they agree you can be sure which effort you are in.

You can use the scale anywhere and you need nothing to read it, which makes it the simplest of all the ways to judge an effort. It works on a hill, in the wind, in the water and on a day when your watch says something strange. All it asks of you is that you know the four numbers to aim for and the one in the middle to stay away from. Then, whenever you ask yourself how hard it feels, the answer tells you not only where you are but where you meant to be.

• Seiler and Kjerland 2006. Scandinavian Journal of Medicine and Science in Sports 16:49-56. Quantifying training intensity distribution in elite endurance athletes.
• Esteve-Lanao et al. 2007. Journal of Strength and Conditioning Research 21:943-949. Impact of training intensity distribution on performance in endurance athletes.