Two hard efforts, and everything they imply
A watch gives you a VO2 max with no working shown and heart-rate zones built on a maximum it guessed from your age. This does the two models that are actually useful — the race-equivalence table every coach uses, and the critical speed that separates what you can hold from what you are borrowing — from efforts you have really done.
This is arithmetic, not medical advice. Maximal efforts carry real risk if you have a heart condition or have not trained for a while — that is a conversation for a doctor. Nothing you type is uploaded.
One honest race
A time trial or a race you finished hard. Everything below comes from it.
Riegel's rule with the usual 1.06 exponent, which is fitted rather than physical. It is good between about 1,500 m and the half marathon, and known to be optimistic for the marathon — that row is shown with the steeper exponent that fits long races better.
Critical speed
Two maximal efforts on different days: one of three to six minutes, one of twelve to twenty.
Critical speed is the pace you could in principle hold for a very long time; D′ is the fixed distance you can cover above it before you have to slow down, and it does not get any bigger within a race. It is why going out too fast costs more than it gains — you spend D′ early and then run below critical speed for the rest.
Zones from your threshold
Not from 220 minus your age, which has a standard deviation of about eleven beats.
Threshold is the average heart rate you can hold for about thirty minutes flat out — measurable in one session, unlike a true maximum. Zones anchored to it move with your fitness; zones from an age formula never do.
Is this week unlike the last month?
Four weeks of daily training load, newest last. Minutes will do if you have nothing better.
Why critical speed is worth the two efforts
A single race tells you how fast you were over one distance. Two maximal efforts of different lengths tell you something structurally different: the line between the pace you can sustain aerobically and the finite amount of work you can do above it. The slope of the line through them is critical speed; the intercept is D′, a distance in metres that behaves like a battery — you can spend it fast or slowly, but not spend more of it than you have.
That is the model behind almost every pacing decision. Going out thirty seconds a kilometre too fast does not cost you thirty seconds; it empties D′ in the first third, after which you are held below critical speed and lose several times what you gained. Two efforts and some arithmetic show you where the line is.
Why threshold and not maximum heart rate
The 220-minus-age rule is a line fitted through a very scattered cloud: the standard deviation around it is about eleven beats a minute, so for one person in three it is out by more than ten. Five zones built on it inherit that error, and a "zone 2" run can comfortably be a zone 3 run. Threshold heart rate is measurable in a single hard thirty minutes, it moves as you get fitter, and zones built on it are the same zones your coach means.
About the acute-to-chronic ratio
The ratio of this week's load to the last month's is widely used and genuinely contested: the original injury-risk findings have been criticised for how the two windows share data, and later work has not consistently reproduced them. It is shown here as what it defensibly is — a description of how unlike the recent past this week has been — and not as a probability of getting hurt. A large jump is worth noticing. A number in a band is not permission.
Common questions
What is VDOT?
Daniels and Gilbert's single number for running fitness, computed from the oxygen cost of a pace and the fraction of maximum you can hold for a given duration. Its value is that one honest race gives you every other distance and a full set of training paces — it is the standard the sport has used for decades, and the formula is published rather than proprietary.
Can I use this for cycling or swimming?
Critical speed and the load ratio carry over directly — for cycling the same model is usually expressed as critical power. VDOT and the race table are running-specific, because the oxygen cost of a pace is.
Is anything I type sent anywhere?
No. Everything is worked out in this browser and nothing is uploaded. There is no account.