One of the most powerful features of Stryd Adaptive Training is its ability to automatically detect when your Critical Power (CP) may no longer be accurate and prompt you to update it before your workouts drift out of calibration.
This article explains how the system detects CP inaccuracy, what each RPE range means for the check, and what you can do when the system flags an issue.
Note: The CP Accuracy Flow is currently only available to users who are on an Adaptive Training plan.
What is Critical Power and Why Does Accuracy Matter?
Critical Power is Stryd’s measure of your sustainable aerobic power threshold; roughly the highest power output you can sustain for a prolonged effort without accumulating fatigue that forces you to slow down. It’s the foundation for all of your training zones.
If your CP is set too low, your “easy” zone will feel too easy, and your “hard” and “tempo” zones will feel less challenging than they should. You’ll be undertrained. If your CP is set too high, even moderate runs will feel harder than intended, and the system may assign workouts you can’t realistically complete. Getting CP right is essential to getting accurate workout prescriptions.
How the System Detects CP Inaccuracy
After every run, if you submit a Post-Run Report with your RPE (Rate of Perceived Effort), the system compares what you reported with what your power data actually shows. Think of it like a running coach cross-checking your post-run feedback with your run data; if the two don’t match, something is off.
The system runs two independent checks:
Power-based check: How much time did you spend running at or above your CP during this run? Based on your reported RPE, the system has expectations about how much time at or above CP is normal. If the actual time deviates significantly from those expectations, CP may be off.
Fatigue-based check: Stryd tracks fatigue using your Power Duration Curve (PDC), a model of how your energy systems deplete during a run. Fatigue is expressed as a percentage of your capacity. If you report an easy run but your fatigue curve shows you depleted heavily, or you report a maximal effort but your fatigue barely moved, something doesn’t add up.
If either check returns a mismatch, your CP is flagged as potentially inaccurate. Both checks run on every qualifying run (runs with valid RPE, sufficient duration, and usable power data), so the system is always monitoring.
What the System Checks for Each RPE Range
The evaluation criteria depend on how hard you reported the run to be. Here’s a summary:
RPE Range | What the system expects | Flags CP too low if… | Flags CP too high if… |
RPE 1–4 (Easy) | Low fatigue, minimal time at or near CP | Portions of the run are done at CP | — |
RPE 5–6 (Steady / Tempo) | Moderate effort, some time near CP | Portions of the run are above CP | — |
RPE 7–8 (Hard) | Hard but not maximal | — | Portions of the run are done below CP |
RPE 9–10 (Maximal / Race) | Operating at or above CP | — | Portions of the run are done below CP |
Below is a more detailed explanation of what each RPE range means for the accuracy checks.
RPE 1–4: Easy
The system expects low fatigue and very little time spent at or above your CP. An easy run should not push you anywhere near your threshold.
Your CP may be flagged as too low if:
Power-based: You spent significant time running at or above 90% of CP during a run you called “easy.” If your easy effort is producing that kind of output, your CP may be set below where it should be.
Fatigue-based: Your fatigue climbed to sustained high levels or finished very high after what you reported as an easy run. High fatigue on an easy run is inconsistent with easy effort and suggests your zones are off.
RPE 5–6: Steady / Tempo
The system expects a moderate challenge; some time near CP, but not extreme fatigue or power output far above your threshold.
Your CP may be flagged as too low if:
Power-based: You spent a significant amount of time above CP, or your average power was well above CP during what you reported as a steady or tempo effort. That level of output should feel harder than steady/tempo.
Fatigue-based: Fatigue peaked or finished at very high levels after a run you rated as moderate.
Your CP may also be flagged as too high if:
Fatigue-based: Your fatigue peak was very low during what you called a steady effort, suggesting the run wasn’t actually challenging enough to match the zone. This would indicate CP is set higher than it should be.
RPE 7–8: Hard
Hard efforts can be ambiguous in terms of power output. A controlled hard effort and an all-out effort can look similar in power data. For this reason, the system only uses fatigue-based signals for this RPE range.
Your CP may be flagged as too high if:
Fatigue-based: Your fatigue peak was very low during what you rated as a hard effort. If you felt like you worked hard but your fatigue curve barely moved, CP may be set too high relative to your current fitness.
RPE 9–10: Maximal / Race Effort
At maximal effort, the system expects strong evidence that you were operating at or above your CP for a meaningful portion of the run.
Your CP may be flagged as too high if:
Power-based: You spent very little time at or above CP despite reporting a maximal effort. If you were truly going all-out and barely crossed your threshold, CP is likely set too high.
Fatigue-based: Fatigue signals suggest the run was not as demanding as a maximal effort should be; little sustained high fatigue and a low peak fatigue level.
What Happens When CP is Flagged
If the system detects a mismatch, you will receive a notification and be given three options:
Update your RPE: If you think you mis-reported your effort level after the run, you can go back and correct your RPE. The system will re-evaluate with the updated value.
Schedule a CP Estimate run: If you believe your CP really is inaccurate, you can schedule a structured CP Estimate workout. The system will guide you through a short test run designed to get an updated CP reading.
Ignore the notification: If you’re confident your CP and RPE are both accurate, you can dismiss the alert and keep things as they are.
In some cases, if the system detects a CP inaccuracy with high confidence across multiple data points, it may be able to suggest a specific updated CP value automatically. When this happens, you’ll be given the option to accept the update or review it first.
How to Submit Your RPE After a Run
Your RPE reports are what drive this system. The more consistently you report after runs, the more accurate the CP monitoring will be.
Open the Stryd mobile app.
Select the Calendar icon.
Select the Run Report icon on the activity you want to add RPE to.
Fill out the Post-Run report and be sure to include your RPE.
For more information about Post-Run Reporting and Run Tagging, see the support article: Post-Run Reporting and Run Tagging.