Critical Power

Critical power (CP) is the highest power output that can theoretically be sustained without progressive fatigue, sitting close to the anaerobic threshold.

Definition

“Critical power” (CP) in the physiological model is the highest power output that can theoretically be sustained without progressive fatigue. It is the asymptote of the power-duration curve: above CP the available work is limited and is described by the capacity W’.

P(t) = CP + W' / t

In everyday training the term is used less strictly. “Critical power” often simply means the average power of a maximal test over a fixed duration, usually 20 minutes. In sports with high energy cost such as swimming, it is tied to distances rather than times (critical swim speed).

Why it matters

Critical power is a model with clear physiological meaning: it separates the range in which a metabolic steady state can be reached from the range in which fatigue rises inexorably. It therefore lies close to the second ventilatory threshold or the maximal lactate steady state.

The problem is the determination. The model needs at least two to three maximal tests of different durations, but in practice this is reduced to a single 20-minute test from which FTP is estimated at 95 %. This shortcut rarely delivers an objective value, because pacing, daily form, motivation and individual fatigue characteristics strongly influence the result.

Method	What is measured	Objectivity	Typical error
Metabolic / lactate test	Ventilatory or lactate thresholds directly	High	± a few watts
CP model (2–3 maximal tests)	Asymptote of the power-duration curve	Moderate to high	Depends on test quality
20-minute test × 0.95	Field estimate of FTP	Low	Easily 5–10 % off
Device estimate (automatic)	Algorithm from training data	Low	Unknown, not individual

Critical power is often used where no objective performance diagnostic is available in a discipline. As a substitute for a diagnostic test it is usable; as a substitute for measured thresholds it is not.

How obseed measures it

obseed uses the thresholds stored in the performance parameters as the reference for zones, Intensity Factor and load models. This value should come from a metabolic diagnostic test. A field-test value can be entered when no diagnostic is available, but should be understood as an estimate.

From recorded sessions, best efforts over various durations can be derived, giving a view of the power-duration curve.

Example

An athlete completes two maximal tests: 3 minutes at 400 W and 12 minutes at 320 W. From these:

work 1 = 400 W × 180 s  =  72,000 J
work 2 = 320 W × 720 s  = 230,400 J

CP = (230,400 − 72,000) / (720 − 180) = 158,400 / 540 = 293 W
W' = 72,000 − 293 × 180 = 72,000 − 52,740 = 19,260 J ≈ 19.3 kJ

A 20-minute test by the same athlete yields 310 W, so an estimated FTP of 295 W. The metabolic test two weeks later, however, shows the second ventilatory threshold at 278 W. Both field methods overestimate the physiological threshold by 15 to 17 W, which in zone-based training makes every threshold block too hard.

References

Monod, H., Scherrer, J. (1965). The work capacity of a synergic muscular group. Ergonomics.
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 and Science in Sports and Exercise.