How long could you hold this?

In France, 25 competitive cyclists rated how long they thought they could hold a series of efforts. From their answers, the researchers predicted how long each would last at one hard effort. The prediction was about ten minutes.

Then the cyclists rode that effort until they could not go on. They lasted about 11 minutes. The best riders, the ones most tuned in to their own bodies, guessed closest.

The same question also works the other way. Decide how long you want to hold an effort, and you will find the effort that fits. So the question is a gauge and a throttle. Ask it, and you know where you are. Answer it, and you can set where you want to be.

How long you have been going is not the question. Ask how long you could keep going, from here, at exactly this effort.

You already judge this without trying. In a short race, the effort starts to feel hard sooner than in a long one, because you are always pacing toward the finish. You set your effort with the end point in mind, often without noticing. The question just makes that judgement something you can use.

The answer is honest because at every effort you run out of something real: sugar at base, the muscle's steady chemistry above threshold, a small quick store in a sprint.

At a base effort, the honest answer is hours. As far as you can tell, you could keep going all day.

That is because base work runs mostly on fat, a fuel you carry plenty of. The harder you work, the more your muscles burn their small store of sugar instead.

What ends a long day is that store of sugar. Seven trained cyclists rode at a solid, steady effort until they could not go on, drinking only flavoured water. After about three hours, their blood sugar had fallen and they stopped. On another day they drank sugar as they rode, and lasted about an hour longer. The harder the steady effort, the faster that store runs down. At a true base effort, it lasts for hours.

So an effort you could hold for anything less than hours is not base.

Threshold is hard work. It is an effort you could hold for about twenty minutes, and not much more.

Above a certain effort, a different thing runs out. In England, six men kicked one leg against a load inside a scanner that could read the chemistry of their thigh muscle as it worked. The researchers found a line. Just below it, the muscle settled and held steady for 20 minutes. Just above it, the muscle's quick energy store kept falling and acid kept building, all the way to the point where they had to stop. Threshold sits close to that line, which is why it has a time limit and base does not.

Twenty minutes is the answer for one block, not the whole session. Threshold work comes in blocks of twenty minutes or less, with short breaks between them, and the session can last an hour.

The answer tells you which way to adjust. If you think you could hold it for an hour, you are going too slow. If you think only five minutes, you are going too fast.

The question works in the water too. Face down, your breathing follows the stroke, so it tells you less than it does on land. How long you could hold the effort still tells you everything. On a swim, it is the gauge.

VO₂max is the most oxygen your body can take in and use. An effort near it is hard, hard work, and you could hold it for a few minutes at most. It sits well above that line, where the muscle never settles, so its quick energy store falls much faster. The higher the effort above that line, the sooner you have to stop.

It is easy to go too hard. On a short, steep hill you want to attack it. But if you could only keep that pace for thirty seconds, you are sprinting. Your legs give out before your heart and lungs reach their limit, and reaching that limit is what this effort is for.

A sprint is everything you have. You do not think about how fast or how far. You just go.

It runs on a small store already inside the muscle, a fuel called phosphocreatine that releases energy in an instant. Half a minute all out empties most of it, and it takes minutes of rest to fill again. Your power comes back in step with it, so the sooner you sprint again, the weaker the next sprint is.

So you watch for one thing. When you feel yourself slowing, stop. That is the store running low. From that moment you are no longer making the power, and no longer training the nerves that make it. Rest until it fills, and the next sprint can be everything again. That is power, on a road, a bike or in the water.

Put the four answers side by side and they make a ladder. Hours is base. About twenty minutes is threshold. A few minutes is VO₂max. And a sprint lasts until you feel yourself slowing. Each step down is something real running out sooner: the sugar store, the muscle's steady chemistry, the quick store in a sprint.

You can ask it anywhere. In the water, where your breath follows the stroke. On a day when your heart rate reads oddly. On a hill, in the wind, in the heat. Wherever you are, feel how your body is today and ask how long you could really keep this up. The answer tells you which effort you are in, and if it is the wrong one, which way to go.

• Garcin et al. 2011. Journal of Strength and Conditioning Research 25:1393-1399. Prediction of time to exhaustion in competitive cyclists from a perceptually based scale.
• Coyle et al. 1986. Journal of Applied Physiology 61:165-172. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate.
• Jones et al. 2008. American Journal of Physiology, Regulatory, Integrative and Comparative Physiology 294:R585-R593. Muscle metabolic responses to exercise above and below the critical power assessed using 31P-MRS.