Training Readiness or Fatigue: What Your Data Really Says
Your watch says you're ready, but your legs say otherwise. Who's right? Here's what each metric can prove, what it can't, and how to check it against how you feel.
The KILYO team
The sports coach that reads your workouts
The short answer
No single metric tells you your training readiness on its own. If your watch and your legs disagree, trust how you feel and lighten the session: in 22 of 54 studies in one review (British Journal of Sports Medicine, 2016), mood and perceived recovery were more sensitive than objective measures.
Key takeaways
- No single metric settles training readiness on its own: HRV, resting heart rate and watch scores are clues, not verdicts.
- Consumer watches measure nighttime HRV with an average error ranging from about 6% to about 16%, depending on the device tested.
- In a review of 54 studies, mood, stress and perceived recovery were more sensitive than objective measures in 22 studies.
- Fatigue that doesn't improve after several weeks of rest needs a doctor, not a watch.
Your watch now shows a lot of numbers: resting heart rate, HRV, training load, recovery score. Each one looks precise. But none of them alone can tell you whether you're fresh or fatigued.
Here's the breakdown: what the research says these numbers can tell you, what they can't prove, and how to read them alongside how you feel.
Short-term fatigue, overreaching, overtraining: not the same thing
After a hard week, feeling tired is normal. The real question is what happens next: does your performance come back with rest, or not?
The joint consensus from the European College of Sport Science (ECSS) and the American College of Sports Medicine (ACSM), published in 2013, describes two situations in particular:
- Non-functional overreaching: performance stalls or drops for weeks or months, but rest lets you recover fully.
- Overtraining syndrome: performance drops in your sport, your mood gets worse, and this lasts despite several weeks or months of recovery. Full recovery can take months, or even years.
This consensus treats overtraining as a clinical diagnosis, made by ruling out other causes. That's a call for a doctor, not for a watch.
What your watch actually measures
A 2025 study in Physiological Reports compared consumer watches and rings with an electrocardiogram over 536 nights in 13 healthy adults. The error is the average gap, in percent, between the device's nighttime HRV and the electrocardiogram's HRV. It varies a lot from one device to another:
| Device tested | Average error on nighttime HRV |
|---|---|
| Oura Gen 4 | about 6% |
| Whoop 4.0 | about 8% |
| Garmin Fenix 6 | about 10.5% |
| Polar Grit X Pro | about 16% |
These results have limits: 13 healthy adults, measured during sleep only. They say nothing about other watches, or about HRV during exercise. The less accurate the device, the more a small change from one night to the next may just be measurement noise. So keep wearing the same watch and follow the trend.
HRV and readiness score: read the trend
A single HRV value is ambiguous. In elite athletes, both a drop and a rise in HRV have been linked to negative adaptation, and an unusual drop has even come with signs of positive adaptation, such as higher cardiorespiratory fitness. HRV is very individual: no single number applies to everyone. Look at the trend over several days, not at one morning reading.
Using HRV to guide your training is not settled. Studies disagree: a 2020 meta-analysis found a small gain in VO2max, while a 2021 one found no clear gain in endurance performance. In that second review, the difference was small and "not significant," which means it could be due to chance.
Some readiness and fatigue scores are based on fitness-fatigue models, including the Banister model. A 2025 study pointed to major statistical flaws in this type of model: parameters that are hard to identify, overfitting to past data, and the added fatigue term didn't significantly improve predictions. You also can't see how each watch calculates its score, so treat it as one more clue.
How you feel counts as much as your numbers
Your own sense of how you feel is real data. Here's how to decide when your watch and your feelings disagree:
- Green score, heavy legs: take your legs seriously. Swap the hard session for an easy run or a rest day, then see how you feel the next day.
- Red score, but you want to go: do an easy or shortened session, not the planned one. If you feel good partway through, you can always add a bit more.
- Numbers and feelings agree: you can decide quickly. Both say "go," or both say "ease off."
- Signs of illness (fever, body aches, trouble breathing): rest instead of training, even if the score looks good.
When to see a doctor
What you can do before your next workout
- Rate your mood, stress and recovery (1 to 5) before you check your watch.
- Look at your resting heart rate and HRV over a week, not from a single night.
- Compare yourself to your own past data, never to a "normal" range you found online.
- After the session, note your pace and heart rate, and whether the effort felt harder than usual.
KILYO reads your watch sessions and spots, week after week, a heart rate that climbs at the same pace or a volume that rises too fast, then adjusts your plan. How you feel is for you to log.
To build a week that leaves room for recovery, read improve your running with 3 workouts a week.
Frequently asked questions
My HRV dropped: should I cancel my workout?
One value isn't enough to decide. In elite endurance athletes, a drop in HRV has sometimes come with good adaptation. Look at the trend over several days and at how you feel. If your HRV is low and your legs feel heavy, lighten the session.
Why does my watch say I'm "ready" when I feel exhausted?
Nobody can see how each watch calculates its score, and the fitness-fatigue models some of them are based on have major statistical flaws (2025 study). Your own sense of fatigue tracks training load at least as well as objective measures. If the two disagree, trust how you feel.
Which watch is the most accurate for HRV?
In the 2025 study, the average error ranged from about 6% (Oura Gen 4) to about 16% (Polar Grit X Pro), in 13 healthy adults during sleep. Keep wearing the same watch and follow the trend.
What's the difference between overreaching and overtraining?
With non-functional overreaching, rest lets you recover fully. Overtraining syndrome persists despite several weeks or months of recovery and can take months, or even years, to resolve.
Sources
- Meeusen R., Duclos M., Foster C., et al. (2013). Prevention, diagnosis and treatment of the overtraining syndrome: Joint consensus statement of the ECSS and the ACSM. European Journal of Sport Science, 13(1):1-24. DOI
- Saw A. E., Main L. C., Gastin P. B. (2016). Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures: a systematic review. British Journal of Sports Medicine, 50(5):281-291. DOI
- Plews D. J., Laursen P. B., Stanley J., Kilding A. E., Buchheit M. (2013). Training Adaptation and Heart Rate Variability in Elite Endurance Athletes: Opening the Door to Effective Monitoring. Sports Medicine, 43:773-781. DOI
- Manresa-Rocamora A., et al. (2021). Heart Rate Variability-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance: A Methodological Systematic Review with Meta-Analysis. International Journal of Environmental Research and Public Health, 18(19):10299. DOI
- Marchal A., Benazieb O., Weldegebriel Y., Méline T., Imbach F. (2025). Statistical flaws of the fitness-fatigue sports performance prediction model. Scientific Reports, 15:3706. DOI
- Validation study of resting heart rate and nocturnal HRV from consumer wearables compared with an electrocardiogram (2025). Physiological Reports. DOI
Written with the help of AI.
