Critical Power (CP) vs FTP: Which Is Your True Threshold?

FTP’s Dominance and Controversy

Since Dr. Andrew Coggan popularized the concept of Functional Threshold Power (FTP) in the early 2000s, it has become the universal language of cycling training. FTP is defined as “the highest steady-state power you can sustain for approximately 1 hour,” and in practice is typically estimated as 95% of a 20-minute all-out test.

However, as training science has evolved, more and more researchers have pointed out FTP’s limitations:

1. FTP is not a clear physiological marker: It does not directly correspond to lactate threshold (LT2), MLSS (maximal lactate steady state), or any specific metabolic turning point
2. The 95% of a 20-minute test varies from person to person: Riders with strong anaerobic capacity may need 92-93%, while those with weaker anaerobic capacity may need 96-97%
3. FTP is a single number: It cannot tell you how long you can sustain power above threshold, or by how much you can exceed it

The Advantages of the Critical Power (CP) Model

What Is Critical Power?

Critical Power (CP) originates from the mathematical model proposed by Monod & Scherrer in 1965. It defines two key parameters:

• CP (Critical Power): The highest power that can theoretically be sustained “indefinitely” (in reality, it cannot be maintained beyond approximately 30-60 minutes due to other factors)
• W’ (pronounced W-prime): The finite work capacity available above CP (measured in joules)

Mathematical formula:

t = W' / (P - CP)

where:
t = time to exhaustion (seconds)
P = target power (watts)
CP = critical power (watts)
W' = anaerobic work capacity (joules)

Numerical Differences Between CP and FTP

Research shows that CP is typically 3-8% higher than FTP:

Rider Type	FTP	CP	Difference	W’
Amateur Rider A	240W	252W	+5%	15,000J
Amateur Rider B	280W	295W	+5.4%	20,000J
Advanced Rider C	310W	323W	+4.2%	22,000J
Sprinter Rider D	260W	270W	+3.8%	28,000J
Climber Rider E	300W	320W	+6.7%	12,000J

Note the contrast between Sprinter Rider D and Climber Rider E: a 40W FTP gap, but a 16,000J W’ gap. FTP cannot capture this critical difference; the CP model can.

W’ (Anaerobic Work Capacity) In-Depth

What Is W’?

W’ can be understood as your “anaerobic battery.” When you ride at power above CP, W’ is gradually depleted; when you drop below CP, W’ gradually recovers.

Typical W’ Ranges:

Rider Type	W’ Range	Characteristics
Sprinter/short-distance	20,000-30,000J	Large W’, can sustain above-CP efforts for a long time
All-rounder	15,000-22,000J	Balanced
Endurance/climber	8,000-15,000J	Small W’, depletes quickly above CP

Practical Significance of W’

Assume Rider A (CP=250W, W’=20,000J) and Rider B (CP=250W, W’=12,000J):

Riding at 300W (50W above CP):

• Rider A can sustain: 20,000 / 50 = 400 seconds (6 min 40 sec)
• Rider B can sustain: 12,000 / 50 = 240 seconds (4 minutes)

Riding at 350W (100W above CP):

• Rider A can sustain: 20,000 / 100 = 200 seconds (3 min 20 sec)
• Rider B can sustain: 12,000 / 100 = 120 seconds (2 minutes)

Same CP, yet dramatically different performance in critical race moments (breakaways, final climbs).

CP Testing Methods

Method 1: Classic Three All-Out Tests (Most Accurate)

Perform three all-out tests of different durations on separate days:

Test 1: 3-minute all-out (measure average power P3)
Test 2: 7-minute all-out (measure average power P7)
Test 3: 12-minute all-out (measure average power P12)

Interval: at least 48 hours between tests
Warm-up: standard 15-minute progressive warm-up

Use linear regression to calculate CP and W’:

Work = W' + CP × Time

Plug the three data sets (time, average power × time) into linear regression
Slope = CP
Intercept = W'

Method 2: Single 3-Minute All-Out Test (Vanhatalo Method)

Warm-up: 15-minute progressive warm-up
Rest: 5 minutes easy spinning
Test: 3-minute ALL-OUT (full sprint from the first second)

Analysis:
  CP ≈ average power of the final 30 seconds
  W' ≈ total work of 3 minutes - (CP × 180 seconds)

Note: This method requires a true all-out effort and is extremely challenging mentally. Many riders unconsciously “hold back” during seconds 60-90, causing CP to be underestimated.

Method 3: Automatic Calculation via Power Meter Software

Several training software platforms currently support automatic CP/W’ calculation:

Software	CP Calculation Feature	Data Requirements
Golden Cheetah	Built-in CP model (free)	Power data from multiple rides
WKO5	iLevels (includes mFTP and CP)	Power data spanning 90+ days
intervals.icu	Automatic CP curve (free)	Power data from multiple rides
TrainingPeaks	Power Duration model	Premium account

Setting Training Zones with CP/W’

Traditional FTP Zones vs CP Zones

Coggan Zone	Based on FTP	CP-Based Alternative	Description
Zone 1 Recovery	< 55% FTP	< 55% CP	Minimal difference
Zone 2 Endurance	56-75% FTP	56-75% CP	Minimal difference
Zone 3 Tempo	76-90% FTP	76-87% CP	Slightly narrower
Zone 4 Threshold	91-105% FTP	88-100% CP	Core difference zone
Zone 5 VO2max	106-120% FTP	100-130% CP + W’ management	W’ must be considered
Zone 6 Anaerobic	> 120% FTP	Determined by W’ depletion rate	Fundamentally different

W’-Guided Interval Training Design

Traditional approach: “Do 5 × 4 minutes @ 110% FTP”

CP/W’ approach: “Do 5 × 4 minutes, expending 30-40% of W’ per set, and start the next set after W’ has recovered to 80%”

The latter is more precise because it accounts for individual differences. Riders with a large W’ can use higher power, while riders with a small W’ need to use more conservative power but may require longer recovery.

W’ Recovery Dynamics

W’ recovery is not linear but approximately exponential:

• After dropping below CP, the first 30 seconds see the fastest recovery
• Recovering W’ to 50% takes approximately 2-3 minutes (at 50% of CP power)
• Recovering W’ to 80% takes approximately 5-7 minutes
• Full W’ recovery may take 15-20 minutes

Key finding: the lower the recovery segment power, the faster W’ recovers. This explains why recovery segments in interval training should be maintained at very low power (< 50% CP) rather than “moderate intensity.”

Limitations of the CP Model

1. Inaccurate for short durations: The CP model’s accuracy declines for very short (< 2 minutes) and very long (> 30 minutes) durations
2. Fatigue factors not included: The model assumes CP remains constant throughout exercise, but it actually declines due to glycogen depletion, dehydration, and other factors
3. Environmental factors: Temperature, altitude, wind, and other environmental conditions can affect actual CP and W’
4. Day-to-day variability: W’ can vary by 10-15% between days, influenced by the previous day’s training, sleep, and nutrition

Practical Advice for Taiwanese Riders

Daily Training: Replace FTP with CP

1. Use intervals.icu (free) or Golden Cheetah to track your CP and W’ trends
2. Set training zones based on CP to avoid the 5-8% error from FTP estimates
3. Reduce recovery segment power in interval training to 40-50% of CP, not 40-50% of FTP (because CP is usually higher than FTP)

Race Pacing: Strategic Application of W’

Using the Wuling East Route as an example (approximately 2.5-3 hours of climbing):

• Maintain 85-92% of CP for most of the route (avoid depleting W’)
• On steep sections (such as the sharp turns after the Jinma Tunnel), allow brief use of W’ (no more than 20% of total W’)
• In the final 10 minutes, you can tap into the remaining W’ for a sprint

Tracking Progress

Metric	Tracking Frequency	Significance of Improvement
CP increase	Every 4-6 weeks	Enhanced aerobic/threshold capacity
W’ increase	Every 4-6 weeks	Increased anaerobic capacity
CP up + W’ unchanged	-	Pure aerobic adaptation
CP unchanged + W’ up	-	Pure anaerobic adaptation
CP and W’ both up	-	Overall improvement (ideal state)

Conclusion

FTP is a useful entry-level tool, but the CP/W’ model provides a more complete description of ability. Understanding that you don’t just have a single “threshold number,” but rather a threshold power and a supra-threshold energy reserve, can help you make better training decisions and race pacing strategies. With power meters and analysis software becoming increasingly accessible, every serious cyclist deserves to spend time understanding the CP model.

Related Reading

• How to Effectively Improve FTP: From Testing to Practical Training Plans
• A Comprehensive Comparison of FTP Testing Methods: Accuracy and Applicability of the 20-Minute Test vs. Ramp Test
• The Complete Guide to CP and W′: A Hyperbolic Model That Understands Pacing Better Than FTP, from Wuling to Criterium Racing
• The Critical Power CP Model: A Framework with More Explanatory Power Than FTP