Interval Training Test Week: How to Retest FTP Every 4–6 Weeks

Introduction

Every intensity setting in power-based training is built on one core number: FTP (Functional Threshold Power). FTP is the maximum average power you can sustain for approximately one hour; it is the power expression of your lactate threshold and the basis for calculating all training zones (Z1–Z7). However, FTP is not fixed—it rises with training adaptations and falls after extended rest or illness. Re-testing FTP every 4–6 weeks to ensure training intensity remains precisely calibrated to your current fitness level is an indispensable step for serious athletes.

Why FTP Needs Regular Updates

Take a rider with an initial FTP of 220W as an example. If FTP rises to 240W after 6 weeks of sweet-spot training, but workout intensities are still calculated based on 220W:

• Sweet spot (88–93% of old FTP): 193–205W → should actually be 211–223W
• VO2max intervals (106–115% of old FTP): 233–253W → should actually be 254–276W

Training intensity is systematically underestimated by 9%, significantly reducing the training benefit of each weekly workout. Regular re-testing ensures that training stress and current fitness levels always maintain the correct proportional relationship.

Three Mainstream FTP Test Methods

Method 1: 20-Minute Test

Procedure:

1. Warm up thoroughly (20 minutes)
2. 5-minute all-out sprint (a “flush” before the test to ensure residual lactate does not affect the test)
3. 5 minutes of easy recovery
4. 20-minute all-out effort (goal: sustain the highest steady power you can hold for 20 minutes)
5. FTP = 20-minute average power × 0.95 (correction factor, since the 1-hour threshold is lower than the 20-minute maximum)

Advantages: Test results closest to true FTP; highly accurate for those with power-meter training experience
Disadvantages: High psychological pressure, requires a complete pacing strategy; beginners often go out too hard in the first 5 minutes, causing a late-race fade

Method 2: 8-Minute Test

Procedure:

1. Warm up thoroughly (20 minutes)
2. Two 8-minute all-out efforts, with 10 minutes of rest between efforts
3. FTP = average of the two 8-minute average powers × 0.90

Advantages: Shorter test duration, relatively easier pacing; suitable for riders who struggle to maintain a steady 20-minute output
Disadvantages: Error after the correction factor may be slightly larger than the 20-minute test

Method 3: Ramp Test

Procedure:

1. Start at a low power output (around 100W), increasing by a fixed amount each minute (e.g., 20W)
2. Continue until exhaustion (typically 8–12 minutes)
3. FTP = average power of the final full minute completed before exhaustion × 0.75

Advantages: Simplest to execute, no pacing skill required; suitable for beginners; TrainerRoad, Zwift, and other platforms have built-in Ramp Tests
Disadvantages: Results favor anaerobic capacity (explosive riders) and may overestimate FTP for pure endurance riders; although the test is short, intensity is very high

Comparison of the Three Test Methods

Test Method	Test Duration	Suitable For	Main Source of Error
20-Minute Test	About 45–50 minutes (including warm-up)	Those with a power-training foundation	Starting too hard, uneven pacing
8-Minute Test	About 40–45 minutes	Beginners to intermediate	Correction factor estimation error
Ramp Test	About 25–30 minutes	Beginners, indoor trainers	FTP overestimation for explosive riders

Test Week Scheduling

Testing itself is high-intensity training and must be performed in the right condition:

• 2–3 days before the test: Reduce training volume (by 30–40%), avoid high-intensity workouts, and allow the body to fully recover
• Test day: Perform the test after at least 24 hours of rest; do not test at midday in high heat
• 1–2 days after the test: Easy recovery rides to let the fatigue from the high-intensity test dissipate

Recommended testing cadence:

Training Phase	Recommended Test Frequency
Base period	Every 6 weeks
Build period	Every 4–5 weeks
Peak period	Every 3–4 weeks
Return after off-season rest	First week after returning

Details for Improving Test Accuracy

1. Use the same equipment and environment: If you tested on an indoor trainer last time, test indoors again; outdoor tests are heavily affected by wind
2. Standardize your warm-up: Use the exact same warm-up protocol for every test to minimize differences in readiness
3. Do not know your target number in advance: Produce output “without preconceptions” to avoid the anchoring effect limiting your actual capability
4. Record your daily condition: Sleep quality, diet, and the previous day’s training—this information helps interpret fluctuations in test results

Practical Advice

1. For riders in Taiwan, the Ramp Test combined with an indoor trainer is the most beginner-friendly entry point into power training, with a low barrier to entry and high consistency of execution
2. After testing 3 or more times consecutively, you will begin to sense your own “test profile”—whether you suit the steady 20-minute type or the progressive Ramp type. Choose the method that fits you best and use it consistently
3. When your FTP number rises, celebrate moderately, but do not over-interpret the ups and downs of a single test; the 3-month trend is the truly meaningful indicator

Conclusion

Regular FTP re-testing is not just about updating a number—it is an objective review of your training progress and a guarantee that every minute of high-intensity intervals in the coming weeks is precisely applied at the correct intensity. Choose a test method you can sustain, establish a fixed testing cadence, and let your power data continuously reflect your true fitness level.

Related Reading

• The Complete FTP Test Guide: Comparing the 20-Minute Test, 8-Minute Test, and Ramp Test
• Full Comparison of FTP Test Methods: Accuracy and Applicability of the 20-Minute, 8-Minute, and Ramp Tests
• Comprehensive Comparison of FTP Test Methods: Accuracy and Use Cases of the 20-Minute Test vs. the Ramp Test
• How to Effectively Improve FTP: From Testing to Practical Training Plans