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 boundary that matters
In your power profile there is an output you can hold steady for a very long time, and one that is only possible for a limited time. The transition between them is physiologically decisive. Below that boundary the body stays largely in balance: oxygen uptake, energy production and work output can all be sustained. Above it a metabolic imbalance sets in, and a limited energy reserve is progressively used up.
Critical Power describes that boundary. W′ is the size of the reserve available above it. Together they say more than any single wattage, because they explain which power your body can stabilise and from where an effort is only possible for a limited time. That is why someone can hold 250 W for a long while and be finished at 270 W after a few minutes. The difference does not feel twice as hard, but physiologically a decisive threshold has been crossed.
Adactiv Diagnostics · Sample report
Critical Power & W′
- Critical Power
- 267 W
- W′
- 17.6 kJ
- Two maximal efforts, 5 and 20 minutes
- The curve through both: the power you can hold for each duration
- Critical Power: metabolic steady state
- Above it you cannot hold the power indefinitely: 59 W above CP last 5 minutes, 15 W last 20 minutes
Two maximal efforts of different length are enough to place the line: which is why the test is one ride, not the three-to-five separate days a classical protocol asks for.
Why we build on Critical Power, not FTP
The best-known number in cycling is FTP, Functional Threshold Power: the highest power you can hold for about an hour. Because hardly anyone rides a full hour flat out on a regular basis, it is usually estimated instead: 20 minutes all-out after a hard pre-fatiguing effort, and 95% of that.
The problem is in the definition. FTP is set by a duration and a correction factor, not by anything that happens in your body. In 2019 Jones and colleagues called the idea of a 60-minute threshold “a convenient but entirely arbitrary definition that is devoid of physiological meaning”: there is nothing more special about 60 minutes than about 65, 44 or 23 [6]. On top of that comes the 95% factor, which is an average. Across a group it often lands close enough; for you as an individual rider it can miss by a wide margin.
Critical Power, by contrast, is defined by your body. Put simply, it is the highest power at which your metabolism can still settle into a balance: below it, oxygen uptake and blood lactate level off at a stable value. Just above it that no longer happens. Oxygen uptake climbs to VO₂max, W′ drains, and the effort ends after a predictable time [7]. CP is the line between a pace you can hold for a long time and one where the clock is running. Because it comes from your own power–duration curve, from how long you can actually hold which power, it needs no correction factor and no fixed duration.
VO₂max: why we need it at all
Critical Power shows which output you can stabilise over a longer period. It does not explain why your limit sits exactly there, and so it does not directly tell you which area to train. Two riders can share a Critical Power to the watt and still reach their limit for completely different reasons.
VO₂max is therefore the second number, the one that makes Critical Power properly interpretable: it describes your body's maximum capacity to take up oxygen and turn it into mechanical work. We estimate it from your five-minute power rather than measuring it in a lab with a breathing mask. That estimate is practical but not exact: in the study the formula comes from, the typical error is around ±5 ml·kg⁻¹·min⁻¹ [2]. It says the most when you compare your own tests with each other.
Understanding your fuel before it runs out
At low intensity a large share of your energy comes from fat oxidation; that reserve lasts a very long time even in lean riders. As the effort rises, energy supply shifts increasingly towards carbohydrate, and those stores are far more limited. Almost everyone knows the principle. Far fewer know where their own crossover actually sits.
That is the difference between controlled pacing and guesswork. If you know that your maximum fat oxidation sits at, say, 68% of your Critical Power, you know the pace that spares your glycogen stores best. At the same time it becomes possible to estimate how many grams of carbohydrate per hour are needed to hold a higher intensity for longer. Bonking is then no longer simply bad luck, but a foreseeable error in the energy balance.
The report shows you the ratio of fat to carbohydrate use at every intensity, and the carbohydrate intake in grams per hour that follows from it. The models behind that are documented in the provenance table above and in the references at the foot of the page. What matters is how consistently you translate the information into training, pacing and nutrition.
The aerobic potential that sets your training
Here is the central question the report is built to answer: you know your Critical Power. You know your VO₂max. What decides things is the distance between them, because neither number on its own determines which training will actually move you forward.
That distance is described by fractional utilization. It is not another word for VO₂max but a ratio: the percentage of your VO₂max you are already using at Critical Power. A low value means your engine is bigger than your ability to convert its output over time. In that case what limits you is not your VO₂max but the ability to hold a high share of it steadily. Training that only raises VO₂max then achieves little, because you are not yet using the VO₂max you already have.
A high value means the opposite: at Critical Power you are already using a large part of your VO₂max. The room below it is then small, and additional threshold work runs into a limit sooner, because there is barely any unused capacity left.
From fractional utilization the report derives one of four training directions. The boundaries are defined, not judged: below 78% the priority is building the aerobic base. Between 78 and 82% it is about improving efficiency at threshold. Between 82 and 86% the profile is balanced; here the target event decides the emphasis. From 86% the focus is on raising VO₂max. That is why the last page names a clear direction rather than an arbitrary checklist: the arithmetic only supports one sensible emphasis.
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.
Exactly the same VO₂max. At Critical Power one rider uses 72 % of it, the other 88 %. Opposite training, because their fractional utilization is not the same.
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% | Active recovery. A session in this zone costs you almost nothing metabolically. |
| Z1 | Endurance | 55–65% | The aerobic base. Energy supply is entirely oxidative, in complete physiological balance. This is where you accumulate sustainable volume for capillarisation. |
| Z2 | FatMax | 65–72% | Peak fat oxidation rate. Not merely ticking over, but a specific stimulus for mitochondrial density. You are teaching your body to burn fat. |
| Z3 | Tempo | 72–93% | Training fatigue resistance and fractional utilization. The system stays in aerobic balance, but glycolysis rises sharply. Highly productive, and metabolically expensive. |
| Z4 | Threshold | 93–103% | Training exactly at Critical Power. You train the ability to hold the maximum metabolic steady state just barely stable, without drawing on your W′ battery. |
| Z5 | VO2max | 103–130% | Beyond Critical Power the balance tips and the W′ reserve empties. The goal is to drive oxygen uptake to its limit in order to raise the physiological ceiling. |
| Z6 | Anaerobic | > 130% | Short, supramaximal efforts. The energy comes almost entirely from the limited W′ capacity, far outside any balance. |
Where our report stops being reliable
This report is built on a mathematical model fed with the data from your power meter – it is not a laboratory measurement. Three factors bound its accuracy, and none of them are secret:
Your power meterIt sets the limit on accuracy. If your device permanently reads 3% high, every calculated value in the report is shifted accordingly. For steering your training that is secondary: what matters is not whether the absolute wattages match objective truth perfectly, only that any error is always exactly the same. As long as the error stays constant, your progress remains reliably measurable (20 watts more is still 20 watts more). That is why consistency beats absolute truth: for retests use the same power meter and calibrate it identically (zero-offset before the ride).
Your pacingThe test is only as good as your pacing strategy. Both efforts have to be genuinely maximal for their respective duration. Go out too hard on the first interval and fade, and the model computes too large a W′ and too low a Critical Power; hold back on the second and the opposite happens. The mathematics does not correct mistakes, it computes exactly what you delivered on the pedals.
Your healthThis is not a medical screening. The report detects neither cardiac risk nor anaemia, overtraining or infection. If your body feels ill or completely exhausted, the model cannot see it – and the test is the wrong tool at that moment.
What remains despite these limitations is the decisive advantage: you get a consistent, repeatable measurement of your physiology that you can afford often enough to track real development. A single lab value may be more accurate on the day it is taken. For steering a whole season, this tool is more useful.
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. (2022). Five-minute power-based test to predict maximal oxygen consumption in road cycling. International Journal of Sports Physiology and Performance, 17(1), 9–15.: 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.
- Jones, A. M., Burnley, M., Black, M. I., Poole, D. C. & Vanhatalo, A. (2019). The maximal metabolic steady state: redefining the ‘gold standard’. Physiological Reports, 7(10), e14098.: why Critical Power, not the maximal lactate steady state, is the gold standard for the maximal metabolic steady state, and why the 60 minutes behind FTP are arbitrary.
- Poole, D. C., Burnley, M., Vanhatalo, A., Rossiter, H. B. & Jones, A. M. (2016). Critical power: an important fatigue threshold in exercise physiology. Medicine & Science in Sports & Exercise, 48(11), 2320–2334.: CP as the boundary between intensity domains.
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.