Critical power is the highest power output you can sustain indefinitely without accumulating fatigue that forces you to stop. In practice, “indefinitely” means roughly 30 to 60 minutes of hard effort, not literally forever. It marks a physiological boundary: below it, your body can reach a stable state and keep going; above it, fatigue builds relentlessly until you’re forced to quit. For cyclists and other endurance athletes, critical power (CP) is one of the most useful numbers you can know because it defines the ceiling of your sustainable performance.
The Boundary Between Sustainable and Unsustainable
Your body responds to exercise in fundamentally different ways depending on whether you’re riding above or below critical power. Below CP, in what physiologists call the “heavy” intensity domain, your oxygen consumption and blood lactate levels rise at the start of exercise but then flatten out. You reach a steady state. You’re working hard, but your aerobic system can handle the demand, and you can keep going for a long time.
The moment you push above critical power, everything changes. You enter the “severe” domain, where oxygen consumption climbs continuously toward your maximum and blood lactate rises without leveling off. There is no steady state. Your body is drawing on a finite reserve of energy that cannot be replenished at the rate you’re spending it. Exhaustion isn’t a question of if, but when. Research published in Medicine and Science in Sports and Exercise confirmed this distinction clearly: subjects riding at CP completed 24 minutes of exercise without difficulty and maintained stable oxygen consumption and lactate levels. Those riding just above CP saw both metrics rise continuously until they had to stop before the 24-minute mark.
W Prime: Your Finite Battery Above CP
The critical power model has two components. CP itself is your sustainable ceiling. The second component, called W prime (written as W’), represents the total amount of work you can do above that ceiling before you’re exhausted. Think of it as a battery with a fixed charge. As long as you stay at or below CP, the battery isn’t draining. The instant you exceed CP, the battery starts depleting, and when it hits zero, you’re done.
W’ is measured in joules. One watt sustained for one second equals one joule, so the math is straightforward. If your CP is 200 watts and your W’ is 10,000 joules, riding at 210 watts (10 watts above CP) drains the battery at 10 joules per second. You could hold that pace for 1,000 seconds, a little over 16 minutes. Push to 250 watts, 50 watts above CP, and you’d last 200 seconds. The harder you go above CP, the faster the battery empties.
This is what makes CP so powerful for pacing. If you know both numbers, you can predict how long you can sustain any effort above your threshold and plan surges, climbs, and finishing kicks accordingly. Athletes with a large W’ tend to excel at short, explosive efforts like sprints and attacks, while those with a high CP relative to their maximum power are better suited to sustained efforts like time trials and climbing.
How Critical Power Is Measured
The classic method requires multiple time trials on separate days. You perform all-out efforts at different durations, typically ranging from about 3 to 12 minutes. The relationship between the total work done (power multiplied by time) and the duration of each effort forms a straight line when plotted. The slope of that line is your critical power. The y-intercept is your W’.
A faster alternative is the 3-minute all-out test. You ride as hard as you possibly can for three minutes against a fixed resistance. During the first couple of minutes, your power drops sharply as your W’ depletes. By about the 135-second mark, your power levels off to a steady value for the final 45 seconds or so. That end-test power closely matches CP derived from the traditional multi-trial method. In validation studies, the two values agreed to within about 5 watts in 8 out of 10 subjects, with a correlation of 0.99.
Field testing also works. Researchers found that cyclists performing maximal efforts of 3, 7, and 12 minutes with recovery between efforts produced reliable CP values, with a day-to-day coefficient of variation around 2.4% and intraclass correlation coefficients of 0.99. Even extracting best efforts from regular training and racing data produced valid results, though with slightly more variability.
Critical Power vs. FTP
Functional Threshold Power, the more familiar metric in cycling, is typically estimated from a 20-minute all-out effort (taking 95% of the average power). Many athletes assume FTP and CP are the same thing. They’re not.
In a study of highly trained cyclists and triathletes, CP was significantly higher than FTP: 282 watts versus 266 watts on average, a difference of about 16 watts. The two metrics were strongly correlated (r² = 0.97), meaning you can reasonably predict one from the other, but they are not interchangeable. The limits of agreement ranged from +4 to -36 watts, which means for some individual athletes the gap could be substantial.
The reason for the difference comes down to what each test measures. CP is derived from the mathematical relationship between power and duration across multiple efforts, capturing a true physiological threshold. FTP is a single-effort estimate that tends to underestimate that threshold, partly because pacing a 20-minute effort perfectly is difficult and partly because the 95% correction factor is an approximation. If you’ve been training with FTP-based zones, your actual sustainable power may be somewhat higher than you think.
CP and Maximal Lactate Steady State
Another common comparison is between CP and the maximal lactate steady state (MLSS), the highest intensity at which blood lactate stabilizes during prolonged exercise. Earlier research found consistent discrepancies between the two, with CP often coming in higher. More recent analysis suggests that when both are measured carefully with strict protocols, the difference narrows to just a few watts. In practice, both metrics estimate the same underlying boundary: the highest metabolic rate your body can sustain in a true steady state.
Using CP for Training and Racing
Knowing your CP and W’ gives you a two-number model of your performance capacity. CP tells you the intensity you can hold for long efforts. W’ tells you how much room you have for surges above that intensity.
During a race, real-time W’ balance tracking lets you monitor how much of your anaerobic battery remains. Every surge above CP depletes it. Riding below CP allows partial recovery. This creates a strategic framework: you can plan exactly how many hard accelerations you can afford on a hilly course, or how long you can sustain a breakaway effort before you’ll be forced to ease off. Athletes who deplete their W’ too early in a race have nothing left for the finish. Those who manage it well can time their final effort precisely.
CP also helps classify what type of rider you are. Athletes with high CP relative to their peak power tend to be endurance specialists, suited to time trials and general classification racing. Those with relatively lower CP but large W’ values lean toward explosive roles like sprinting. Training can shift both numbers: sustained endurance work raises CP, while high-intensity interval training tends to increase W’.