Every number, and where it comes from
A lab measures your physiology directly. We derive it from power data using models that have been in the literature for decades. This page explains exactly which models, exactly what we assume, and exactly where the derivation stops being reliable, so you can judge the report rather than trust it.
What the report is built from
You give us one thing: a ride file containing power. Everything in the report is derived from that file plus your body mass. Nothing is measured in a lab, and the report labels each value with how it was arrived at.
One thing is read: mean power over an effort, second by second, straight from the .FIT records your head unit wrote. It is the only true measurement in the chain, and everything else hangs off it.
Critical Power and W′ are solved algebraically from two maximal efforts of different length, using the Monod–Scherrer hyperbolic model [1]. VO₂max is estimated by regression from your modelled 5-minute power per kilogram [2], with a real error bar and stated as an estimate rather than a measurement. The fat and carbohydrate split is derived from a respiratory-quotient curve mapped onto your percentage of CP, then converted to grams per hour [3].
One number is held constant instead of derived: 21.7 kJ per litre of oxygen, the value at a respiratory exchange ratio near 1.0. Fixed so your own tests stay comparable with each other.
Critical Power: the line that matters
Somewhere in your range there is a power you can hold more or less indefinitely, and a power you cannot. The line between them is not gradual. Below it your body is in balance: oxygen in, work out, steady state. Above it you are spending a battery that does not refill while you ride, and the clock starts.
Critical Power is that line, and W′ is the size of the battery above it. This matters more than any single wattage number because it is the one boundary your body actually respects. It is why a rider can hold 250 W for an hour and blow apart at 270 W in eight minutes; nothing about the effort felt twice as hard, they simply crossed the line.
That economy has a price worth stating plainly: the solution passes exactly through both of your efforts. Pace one badly and there is no third data point to argue with it. It is the largest source of error in the report and the only one entirely in your hands, which is why the protocol is worth reading before you ride rather than after.
VO₂max: why we need it at all
Critical Power tells you what you can hold. It does not tell you why that is your limit, and therefore does not tell you what to train. Two riders can share a Critical Power to the watt and be limited by completely different things.
VO₂max is the second number that makes the first one interpretable: the ceiling on how fast your body can turn oxygen into work. We estimate it from your five-minute power rather than measuring it with a mask, and the estimate carries a real error bar: roughly ±4–5 ml·kg⁻¹·min⁻¹.
Fuel: why the crossover matters
At an easy pace most of your energy comes from fat, of which even a lean rider has days. Push harder and the mix swings towards carbohydrate, of which you have a couple of hours at best. Everyone knows this. What almost nobody knows is where their own crossover sits.
That is the difference between a long ride that is arithmetic and one that is a gamble. If you know your fat oxidation peaks at 68% of your Critical Power, you know what pace protects your glycogen and roughly how many grams an hour you have to eat to hold a harder one. Bonking stops being bad luck and becomes a budgeting error you can see coming.
The report gives you the split at every intensity and the resulting grams per hour. The models behind it are listed in the provenance table above and cited at the foot of this page; what you do with them is the part that changes your season.
The ceiling, and why it decides your training
Here is the question the whole report is built to answer. You know your Critical Power. You know your ceiling. What you need to know is how much room there is between them: because that gap, not either number alone, is what determines which kind of training will actually move you.
Fractional utilization is that gap expressed as a share: how much of your oxygen ceiling you are already using at Critical Power. A low number means your engine is bigger than your ability to hold it: the ceiling is not what is stopping you, so training that raises the ceiling is training you cannot yet use. A high number means the opposite: you are already sustaining nearly everything your aerobic system can deliver, and no amount of threshold work will find room that is not there.
Raise the floor, not the roof.
The ceiling has room this rider's sustainable power has never reached. Sustained work at and just under Critical Power is what closes the gap. VO₂max intervals would be training a system that is already ahead of them.
Raise the roof, or nothing moves.
This rider is holding almost everything their aerobic system can deliver. There is no headroom left to convert, so more threshold work returns very little. The ceiling itself has to come up first, which is what high-quality VO₂max intervals are for.
- Rider A: Critical Power 242 W, VO₂max 62.0 ml·kg⁻¹·min⁻¹, VO₂ at CP 44.7 ml·kg⁻¹·min⁻¹
- Fractional utilization 72%. FU below 78%: build the base: Raise the floor, not the roof.
- Rider B: Critical Power 295 W, VO₂max 62.0 ml·kg⁻¹·min⁻¹, VO₂ at CP 54.5 ml·kg⁻¹·min⁻¹
- Fractional utilization 88%. FU 86% and above: raise the ceiling: Raise the roof, or nothing moves.
The same aerobic ceiling, to the same value. One rider uses 72 % of it, the other 88 %. Opposite training, because the distance to the ceiling is not the same.
The report picks one of four directions from this single ratio, and the boundaries are fixed rather than judged: below 78% build the base, 78 to 82% sharpen efficiency at threshold, 82 to 86% balanced so the target event decides, 86% and above raise the ceiling. That is why the last page names one direction instead of a checklist: the arithmetic only supports one.
Zones, anchored to one number
Every training range in your report is a percentage of your Critical Power, not of a threshold test done on a different day by a different method. One anchor, one system, and ranges that move when you move.
| Zone | Name | % of CP | What it is for |
|---|---|---|---|
| Z0 | Recovery | < 55% | Genuine recovery. Easy enough that it costs you nothing. |
| Z1 | Endurance | 55–65% | The bulk of your endurance hours. Sustainable all day. |
| Z2 | FatMax | 65–72% | Peak fat oxidation. Long rides and fuelling work belong here. |
| Z3 | Tempo | 72–93% | Sustained hard work below threshold. Productive, and expensive. |
| Z4 | Threshold | 93–103% | Around Critical Power. This is what raises the floor. |
| Z5 | VO2max | 103–130% | Above CP, into the oxygen ceiling. This is what raises the roof. |
| Z6 | Anaerobic | > 130% | Short, maximal, drawing on W′. Sprints and attacks. |
Where this stops being reliable
This is a model fed by your power meter, not a laboratory measurement. Three things bound how far you should trust it, and none of them are secret.
Your power meter is the floor on accuracy. A meter reading 3% high moves every number in the report by roughly 3%, and no amount of modelling downstream can recover that. Consistency matters more than absolute truth: use the same meter, calibrated the same way, for every retest.
The test is only as good as your pacing. Both efforts must be genuinely maximal for their duration. Go out too hard on the first and the model reads a large W′ and a low CP; hold back on the second and it reads the opposite. Neither is a bug: the algebra passes exactly through what you gave it.
It is not a medical assessment. Nothing here screens for cardiac risk, anaemia, overtraining or illness. If something feels wrong in your body, this report cannot see it and is not the tool to ask.
What survives all three caveats is the part that matters most: a consistent, repeatable measurement of your own physiology that you can afford to repeat often enough to actually see change. A lab number is more accurate once. This one is more useful across a season.
References
- Monod, H. & Scherrer, J. (1965). The work capacity of a synergic muscular group. Ergonomics, 8(3), 329–338.: the linear work–time model behind CP and W′.
- Sitko, S., Cirer-Sastre, R., Corbi, F. & López-Laval, I. (2021). Power assessment in road cycling: a narrative review.: the basis for the 5-minute power regression used for VO₂max.
- Péronnet, F. & Massicotte, D. (1991). Table of nonprotein respiratory quotient: an update. Canadian Journal of Sport Sciences, 16(1), 23–29.: respiratory quotient to fat/carbohydrate conversion.
- Joyner, M. J. & Coyle, E. F. (2008). Endurance exercise performance: the physiology of champions. Journal of Physiology, 586(1), 35–44.: why VO₂max, threshold and efficiency together, not any one alone, determine performance.
- Jones, A. M., Vanhatalo, A., Burnley, M., Morton, R. H. & Poole, D. C. (2010). Critical power: implications for determination of VO₂max and exercise tolerance. Medicine & Science in Sports & Exercise, 42(10), 1876–1890.: the modern treatment of CP and W′ and their limits.
See it on your own data
The models above are worth exactly as much as what they say about you. One ride file is enough to find out.