critical power · the basics
What is Critical Power, and what determines it?
Based on the review article Goulding RP, Marwood S. Interaction of Factors Determining Critical Power. Sports Medicine 2023;53:595–613 (open access, CC BY 4.0). Each block gives the page reference in the article.
Rob Bohte, physician and coach · updated
1
Definition
Critical power (CP) is the highest power output at which your body can still stay in a metabolic steady state: the boundary between heavy and severe exercise. W′ is the amount of work you can still do above CP. Once W′ = 0, you can no longer produce any power above CP.
Three passages combined, pp. 595, 596, 608
T = W′ / (P − CP)
T = time you can sustain · P = power
Worked example (values from the chart)
| CP | 270 W |
| W′ | 15 kJ = 15,000 J |
| P | 353 W |
| P − CP | 83 W |
| T | 15,000 / 83 = 180 s = 3 min |
After 3 minutes at 353 W, W′ is used up.
p. 596
2
The evidence
Poole's findings (late 1980s) have since been confirmed repeatedly by other research. Above CP, oxygen uptake rises to VO2max; in the blood, lactate keeps rising while bicarbonate and pH keep falling. So the metabolic balance is disrupted. Measurements in the muscle itself show the same for phosphocreatine, inorganic phosphate, pH and lactate.
Two passages combined, pp. 596–597
3
How CP is measured
A good way to determine CP is with three maximal tests at constant power, on separate days, each lasting between 2 and 15 minutes to exhaustion (the article says three to five). The exact duration and power of each test are used to build the curve (see the figure in section 1). That duration is needed because you have to reach VO2max by the end: only then does each test fall within the severe domain.
p. 596
Later literature shows that in trained, experienced cyclists, two maximal tests are also enough to determine CP and W′ reliably [10]. Three or more tests give a more precise picture. As standard I offer a 3-minute and a 12-minute test, with an optional 7-minute test the next day.
[10] Parker Simpson & Kordi 2017
4
What determines CP
CP is a measure of aerobic function. Anything involved in transporting oxygen, from the outside air all the way into the mitochondria, can affect CP. That transport has four steps: (1) from the air via the lungs into the blood; (2) via the blood to the muscle (convection); (3) from the capillaries in the muscle to the mitochondria (diffusion); (4) the use of oxygen by the mitochondria.
§2, p. 597
Oxygen delivery via the blood and capillaries, together with the oxygen the muscle consumes, determines how large the (metabolic) disturbance in the muscle cell is at the start of an effort, and therefore whether that effort can be done in a steady state. Whether that works for the “whole body” at a given power output depends on your muscle fibre type, on which muscle fibres you recruit, and on how much disturbance and fatigue builds up in those fibres.
Caption fig. 4, p. 606
4a. O₂ delivery by convection
CP is sensitive to the amount of oxygen in the air you breathe in: more oxygen gives a higher CP, less oxygen a lower one.
p. 601
4b. O₂ delivery by diffusion
In trained endurance athletes, CP is strongly related to the number of capillaries per muscle fibre [8]. More capillaries means oxygen gets into the muscle cell more easily, and a steady state is possible at a higher power. So: a higher CP.
p. 602
4c. O₂ use: the kinetics of oxygen uptake (τV̇O₂)
τVO2 (tau-VO2) is the time constant of oxygen uptake: the time VO2 needs to reach 63% of its final rise after a change in workload. As long as oxygen uptake lags behind, the energy shortfall is covered from the oxygen stores and from energy supply without oxygen. There is a strong inverse relationship between τVO2 and CP: the faster the VO2 kinetics, the higher the CP. This is independent of delivery via the blood and capillaries: each of the three can change CP without the others changing.
Three passages combined, pp. 603–604
4d. Muscle fibre type and recruitment
CP also depends on the type of muscle fibres. Type I fibres have fast VO2 kinetics, better blood flow and more oxygen. That lets them produce a lot of ATP without the muscle cell being thrown off balance. So at the same training status, someone with more type I fibres has a higher CP.
p. 608
The number of muscle fibres being used counts too. When the work is spread over more fibres, each fibre is loaded less, and the metabolic balance stays undisturbed and below the critical threshold for longer. In short: more recruitment, higher CP.
p. 608
4e. The authors' conclusion
In summary: oxygen delivery via the blood and capillaries affects CP, and so does the speed at which the muscle starts using oxygen at the start of an effort. These factors interact and determine how far the metabolic balance in the muscle cell is disturbed at a given power, and therefore whether that power can still be sustained in a steady state. How that plays out for the “whole body” depends on your muscle fibre type and on which fibres you recruit.
Conclusions, p. 608
5
What changes CP in trained cyclists
Adapted from table 1 in Goulding & Marwood (2023): only the studies with endurance-trained (ET) or anaerobically trained (AT) participants are shown here. Percentages calculated from the absolute values in the original, rounded to whole percentages.
| Study | Population | Intervention | Effect on CP | For the rider | Protocol |
|---|---|---|---|---|---|
| Gaesser & Wilson [1] | ETM (2) HM (3) | Endurance training (6 weeks) | +13% | better | 4CWR |
| Gaesser & Wilson [1] | ETM (3) HM (3) | HIIT (6 weeks) | +15% | better | 4CWR |
| Barker et al. [2]* | ETM (5) ATM (6), cross-country runners and sprinters | Cadence 100 rpm vs 60 rpm | −9% | worse at high cadence | 4CWR |
| Mueller et al. [3] | ETM (11) | Strength + vibration training (8 weeks) | +3% | better | 4CWR |
| Broxterman et al. [4]* | ETM (5) ATM (5), cross-country runners and sprinters | Cadence 100 rpm vs 60 rpm | −8% | worse at high cadence | 4CWR |
| Deb et al. [5] | ETM (11) | Hypoxia (FiO₂ = 0.145); also with sodium bicarbonate | −12% / −13% | worse; bicarbonate doesn't help | 3MT |
| Townsend et al. [6] | ETM (9) | Hypoxia, FiO₂ 0.18 / 0.159 / 0.14 / 0.123 | −5% / −13% / −19% / −27% | worse, more so with less oxygen | 3TT |
| Clark et al. [7] | ETM (6) | 2 hours of heavy exercise beforehand | −8% | worse | 3MT |
| Mitchell et al. [8] | ETM (21) | SIT; SIT + blood flow restriction (4 weeks) | +3% / +3% | better; restriction adds nothing | 3–5CWR |
| Clark et al. [9] | ETM (16) | 2 hours of heavy exercise beforehand | −9% | worse | 3MT |
Population (original legend): ET = endurance trained, AT = anaerobically trained, H = healthy, M = male, n = number of participants in brackets. In Barker [2] and Broxterman [4] the participants were collegiate cross-country runners and sprinters, tested on the cycle ergometer (source: abstract Barker et al. 2006).
Intervention: HIIT = high-intensity interval training, SIT = sprint interval training, FiO₂ = fraction of oxygen in the inhaled air (normally 0.209), rpm = revolutions per minute.
Protocol: nCWR = number of constant-power tests, 3MT = 3-minute all-out test, nTT = number of time trials.
* The later publication uses a subset of the data from the earlier one.
Correction: for Barker [2] the review prints “189 vs 297 W”; the original abstract gives 189 W (100 rpm) vs 207 W (60 rpm). The percentage is calculated on 207 W.
Intervention: HIIT = high-intensity interval training, SIT = sprint interval training, FiO₂ = fraction of oxygen in the inhaled air (normally 0.209), rpm = revolutions per minute.
Protocol: nCWR = number of constant-power tests, 3MT = 3-minute all-out test, nTT = number of time trials.
* The later publication uses a subset of the data from the earlier one.
Correction: for Barker [2] the review prints “189 vs 297 W”; the original abstract gives 189 W (100 rpm) vs 207 W (60 rpm). The percentage is calculated on 207 W.
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What this means for your training
In practical terms, CP and W′ are the best basis for building training and assessing training load, of particular value to amateurs and pros alike. FTP is an arbitrary point (60 minutes) on the power-duration curve, with no physiological basis. Training in the severe domain is a fixed part of training in endurance sports; how hard and how long you can sustain it is determined by W′ and CP. Building such training as a percentage of FTP or CP alone doesn’t work, because neither is proportional to W′. Intervals should be based on the share of W′ being used; that way sessions can be tailored far more precisely. With a model for W′ recovery (Skiba) you can also plan the work and rest phases of an interval session individually. A caveat: W′ recovery varies widely from person to person and slows down after repeated efforts; the models are still being researched.
What it gets you: intervals tailored in length, power and rest, and a race plan based on how much W′ you use and how quickly it comes back [11].
What it gets you: intervals tailored in length, power and rest, and a race plan based on how much W′ you use and how quickly it comes back [11].
[11] Chorley & Lamb 2020, conclusion
Sources
Main source: Goulding RP, Marwood S. Interaction of Factors Determining Critical Power. Sports Med. 2023;53:595–613. doi:10.1007/s40279-022-01805-w (open access, CC BY 4.0). Figures and table adapted from this source.
- Gaesser GA, Wilson LA. Effects of continuous and interval training on the parameters of the power-endurance time relationship for high-intensity exercise. Int J Sports Med. 1988;9:417–21.
- Barker T, Poole DC, Noble ML, Barstow TJ. Human critical power–oxygen uptake relationship at different pedalling frequencies. Exp Physiol. 2006;91:621–32.
- Mueller SM, Aguayo D, Lunardi F, Ruoss S, Boutellier U, Frese S, et al. High-load resistance exercise with superimposed vibration and vascular occlusion increases critical power, capillaries and lean mass in endurance-trained men. Eur J Appl Physiol. 2014;114:123–33.
- Broxterman RM, Ade CJ, Barker T, Barstow TJ. Influence of pedal cadence on the respiratory compensation point and its relation to critical power. Respir Physiol Neurobiol. 2015;208:1–7.
- Deb SK, Gough LA, Sparks SA, McNaughton LR. Determinants of curvature constant (W′) of the power duration relationship under normoxia and hypoxia: the effect of pre-exercise alkalosis. Eur J Appl Physiol. 2017;117:901–12.
- Townsend NE, Nichols DS, Skiba PF, Racinais S, Périard JD. Prediction of critical power and W′ in hypoxia: application to work-balance modelling. Front Physiol. 2017;8:180.
- Clark IE, Vanhatalo A, Bailey SJ, Wylie LJ, Kirby BS, Wilkins BW, et al. Effects of two hours of heavy-intensity exercise on the power-duration relationship. Med Sci Sports Exerc. 2018;50:1658–68.
- Mitchell EA, Martin NRW, Bailey SJ, Ferguson RA. Critical power is positively related to skeletal muscle capillarity and type I muscle fibers in endurance trained individuals. J Appl Physiol. 2018;125(3):737–45.
- Clark IE, Vanhatalo A, Thompson C, Wylie LJ, Bailey SJ, Kirby BS, et al. Changes in the power-duration relationship following prolonged exercise: estimation using conventional and all-out protocols and relationship with muscle glycogen. Am J Physiol Regul Integr Comp Physiol. 2019;317:R59–67.
- Parker Simpson L, Kordi M. Comparison of Critical Power and W′ Derived From 2 or 3 Maximal Tests. Int J Sports Physiol Perform. 2017;12(6):825–30. doi:10.1123/ijspp.2016-0371.
- Chorley A, Lamb KL. The Application of Critical Power, the Work Capacity above Critical Power (W′), and Its Reconstitution: A Narrative Review of Current Evidence and Implications for Cycling Training Prescription. Sports. 2020;8(9):123. doi:10.3390/sports8090123.
your turn
Want to know your CP and W′, or are you sticking with FTP?
It's your call.