Why Calculating FTP is Hard: Finding Your Functional Threshold Power

What is FTP?

Functional Threshold Power (FTP) is the highest average power you can sustain for approximately one hour. It's a critical metric for cyclists because it serves as the foundation for training zones, pacing strategies, and performance tracking. Your FTP represents the boundary between sustainable aerobic effort and unsustainable anaerobic effort—the point where your body can no longer clear lactate as fast as it's being produced.

In theory, FTP is straightforward: ride as hard as you can for one hour and measure your average power. In practice, this is incredibly difficult to do accurately, and most cyclists never perform a true one-hour FTP test.

Why FTP Calculation is Hard

The One-Hour Test Problem

A true FTP test requires maintaining maximum sustainable effort for a full hour—a physically and mentally grueling task. Most riders can't or won't do this regularly, and even when they do, factors like pacing errors, motivation, and environmental conditions can skew the results. This is why FTP is often estimated from shorter efforts, typically 20 minutes, with the assumption that FTP is approximately 95% of your best 20-minute power.

Finding the Right 20 Minutes

Not every 20-minute segment in your ride data represents a valid FTP effort. A good FTP window needs to meet several criteria:

• Consistent effort: The power output should be relatively steady, not surging and recovering
• Minimal coasting: Too much time spent coasting (especially on descents) invalidates the effort
• Even pacing: The second half shouldn't fade significantly compared to the first half
• Sufficient intensity: It needs to be a hard, sustained effort, not a casual ride

Real-world rides rarely contain perfect 20-minute segments. You might have traffic stops, descents, group dynamics, or tactical decisions that interrupt or invalidate what could have been a good FTP effort.

Modality Matters

The same rider can produce different power outputs depending on the type of ride:

• Indoor trainer rides: Often produce slightly higher and more consistent power due to controlled conditions and no coasting
• Road rides: More variable due to terrain, traffic, and group dynamics
• Mountain bike rides: Can be misleading because microbursts, technical sections, and coasting on descents inflate short-term averages

A 20-minute average power from a mountain bike ride might look impressive, but it's often not representative of true FTP because it includes high-power bursts that can't be sustained.

Variability and Pacing

Power variability—how much your power fluctuates during an effort—matters significantly. A steady 300-watt effort is more FTP-representative than a ride that averages 300 watts but surges to 400 watts and recovers to 200 watts. The normalized power (which accounts for variability) can be quite different from average power, and this difference tells us something about the quality of the effort.

Rides with high variability (lots of surging) tend to overestimate FTP because the body can't sustain those peak efforts for an hour. We need to account for this when estimating FTP from variable efforts.

How Lasso Calculates FTP

At Lasso, we've developed a multi-step approach to estimate FTP that addresses these challenges:

Step 1: Finding Quality 20-Minute Windows

We scan through all your power data to find the best 20-minute windows that meet strict quality criteria. For each potential window, we check:

• Coasting ratio: How much time is spent below a power threshold (typically 25 watts). Too much coasting invalidates the effort
• Coefficient of variation (CV): A measure of power consistency. Lower CV means more steady effort, which is better for FTP estimation
• Pacing consistency: The second half of the window should be at least 92-95% of the first half, depending on ride type. Significant fade suggests the effort wasn't sustainable
• Minimum power: The average power must be meaningful (not just coasting or very low effort)

These criteria are adjusted based on ride type—mountain bike rides are allowed more variability and coasting than road rides, which in turn are allowed more than indoor trainer rides.

Step 2: Finding Your Best Recent Effort

Once we've identified quality windows from your activities, we look for your best 20-minute average power from recent activities (typically the last 5 months, extending to 2 years if needed). This ensures we're using a current estimate of your fitness rather than something from years ago.

We prioritize more recent efforts because FTP can change with training, fitness, and life circumstances. An FTP estimate from six months ago might not reflect your current fitness level.

Step 3: Applying Adjustments

Starting from the 95% rule (FTP ≈ 95% of best 20-minute power), we apply several adjustments:

How FTP is Used

Once we've calculated your FTP, it becomes the foundation for several important features:

Power Zone Analysis

FTP is used to define your power training zones, which help you understand the intensity distribution of your rides:

• Zone 1: Recovery (< 55% FTP)
• Zone 2: Endurance (55-75% FTP)
• Zone 3: Tempo (75-90% FTP)
• Zone 4: Threshold (90-105% FTP)
• Zone 5: VO2 Max (105-120% FTP)
• Zone 6: Anaerobic (120-150% FTP)
• Zone 7: Neuromuscular (> 150% FTP)

By analyzing how much time you spend in each zone during a ride, we can provide insights into your training intensity and help you understand whether you're doing recovery rides, endurance work, or high-intensity intervals.

Relative Power Analysis

FTP allows us to express power output as a percentage of your threshold, making it easier to compare efforts across different rides and understand intensity. A 250-watt effort might be easy for one rider but threshold for another—expressing it as 85% of FTP provides meaningful context.

Training Load and Intensity

FTP is also used in calculating training load and intensity scores for the weekly leaderboard, helping to quantify how hard your rides were relative to your fitness level.

Limitations and Considerations

It's important to understand that FTP estimation has inherent limitations:

• FTP changes over time: Your FTP isn't a fixed number—it improves with training and can decrease with detraining, illness, or age
• Requires power data: FTP can only be calculated for activities with power meter data
• Quality of effort matters: The estimate is only as good as the efforts we're analyzing. If you haven't done any hard 20-minute efforts recently, the estimate may be less accurate
• Individual variation: The 95% rule is a general guideline, but individual riders may have FTP that's slightly higher or lower than 95% of their 20-minute power
• Not a substitute for testing: A formal FTP test (whether 20 minutes or a full hour) will always be more accurate than estimates from training rides

Conclusion

Calculating FTP is hard because real-world ride data is messy, inconsistent, and rarely contains perfect test conditions. By carefully filtering for quality efforts, applying appropriate adjustments, and using your best recent data, we can provide a reasonable estimate of your functional threshold power that's useful for training zone analysis and performance tracking.

While our FTP estimate may not be as precise as a formal test, it provides valuable context for understanding your rides and tracking your fitness over time—all without requiring you to perform grueling hour-long tests on a regular basis.