Strain vs exertion vs training load
Short answer: Strain (WHOOP, 0–21), exertion (0–10) and training load all estimate how much stress training put on your body, mostly from time at higher heart rates. The scales differ and aren't interchangeable. What matters is pairing daily load with how recovered you are.
Same idea, different scales
| Metric | Scale | Built from |
|---|---|---|
| WHOOP Strain | 0–21, logarithmic | Heart rate relative to max, all day |
| Apple Training Load / Effort | 1–10 effort per workout; 7-day vs 28-day load | Workout heart rate, pace, your rating |
| Pulse Coach Exertion | 0–10 | Weighted minutes in five heart-rate zones, all day |
| Session RPE (sports science) | Effort × minutes | How hard it felt (Foster et al., 2001) |
They all try to answer the same question: how much did today cost?
Why the curves bend
Most scores are non-linear: getting from 0 to 5 is easy, and from 8 to 10 takes a huge day. That mirrors physiology. The first hour of easy movement matters, but the cost of very hard efforts climbs steeply.
Load needs recovery to mean anything
Sports scientists have modelled performance as fitness minus fatigue since the 1970s (Banister et al.). In practice:
- High recovery + high load: productive training.
- Low recovery + high load: where overreaching starts.
- Several weeks of high load with falling HRV: time for a lighter week. See signs of overtraining.
How Pulse Coach does it
Pulse Coach uses your Apple Watch heart-rate zones (or estimates them from max heart rate, Tanaka: 211 − 0.64 × age), puts every heart-rate sample into one of five zones, weights each zone about twice the one below, and compresses the total onto 0–10. Each morning it sets a target range from your recovery: 5–8 when high, 3–6 when moderate, 1–3 when low. Read How it works.
Questions people also ask
Is a higher strain or exertion score better?
Not by itself. A high score on a well-recovered day builds fitness; the same score on a poorly recovered day digs a hole. Match load to recovery.
Can I convert WHOOP strain to Apple Watch exertion?
Not exactly. The scales and formulas differ. Compare each app with itself over time.
Sources
Not medical advice. This article is general information about wellness metrics. It is not a diagnosis or a substitute for professional care. If a reading worries you, or you feel unwell, talk to a doctor.