Last updated: 30 July 2026
Every run from the past few years sits in Apple Health. Distance, duration, every heartbeat. The data is complete — and it sits there quietly. Health archives it but never does the maths. This page shows what your runs hold, which 4 calculations turn them into training analysis, and where those models reach their limits.
When your Apple Watch ends a run, a surprisingly complete record lands in Health: distance, duration, average and maximum heart rate, the full heart rate curve, per-kilometre splits, elevation, cadence.
Alongside it sit the numbers that have nothing to do with that single run and still say everything about it: your overnight heart rate variability, your resting heart rate, your sleep.
As an archive, this is excellent. Nothing is lost, everything stays on your device. Run for 4 years and you quickly have 300 or 400 runs in the archive. Health has all of them.
Open Health and look at last Tuesday’s run. You see distance, pace, heart rate. You do not see whether that run made you fitter or merely tired. Whether it was hard by your standards or easy. How it relates to the 11 runs before it. Or whether tomorrow has any speed in it.
One example makes it obvious: 10 km easy and 6 × 1000 m at race pace both take about an hour. In Health they are two similar rows. For your body they are two completely different days.
Health answers questions about a run. Runners have questions about their training. 4 calculations sit between the two.
TRIMP (Training Impulse) is a load measure for a single run. It multiplies duration by your heart rate reserve — the share of your heart rate range you were working in — and weights intensity exponentially.
Concretely: 60 minutes at half your heart rate reserve gives 50 points. 60 minutes at 85 % gives 230. A tempo run therefore contributes around 2.5 times more than an easy run of the same length — much closer to physiological reality than any sum that only counts minutes.
That is what finally makes the easy 10 km and the 6 × 1000 m comparable.
Three curves grow out of your daily TRIMP. CTL (Chronic Training Load) is the 42-day average and stands for your long-term fitness. ATL (Acute Training Load) is the 7-day average and stands for your current fatigue. The difference between them is TSB (Training Stress Balance) — your form today.
This explains a feeling every runner knows: 3 hard weeks make you fitter and slower at the same time. CTL rises, ATL rises faster, TSB slips into the negative. Run 5 easy days and you watch form come back without losing fitness.
The Karvonen formula builds your zones from your resting heart rate and your maximum heart rate, not from your year of birth. Both are available in Apple Health or in your runs.
The difference is not a detail. “220 minus your age” is 10 to 20 beats off for many runners. Derive your zones from it and you run the easy days too fast and the hard days too slow — then wonder why nothing improves.
VDOT comes from Jack Daniels’ Running Formula. From a single race time it derives 2 things: realistic times over other distances, and the paces you should train at — easy, tempo, interval.
A 10 km time tells you what is possible over 5 km and over the half marathon. And it tells you how fast your easy run may be for it to stay easy.
All 4 models, with their sources, are listed in the methods overview.
Garmin has Body Battery, Whoop has its Recovery Score. Apple Health has no answer to the question every morning starts with: how fit am I today?
Runnerd computes a value between 0 and 100 from 4 factors: heart rate variability at 30 %, resting heart rate at 20 %, sleep at 25 %, and TSB from your form curve at 25 %. Each factor is compared against your own baseline over the past 60 days, not against a standard table. A resting heart rate of 52 is good or bad for you — depending on what is normal for you.
The score does not need all 4 factors. If the watch spends the night on its charger, Runnerd redistributes the sleep weight across the others. Without an Apple Watch, TSB from your runs remains — the score is then purely training-based, but it is there. The more data flows in, the more precise it gets.
What the number means:
TRIMP is a model, not a measurement. It estimates load from duration and heart rate. It knows nothing about muscle damage, neuromuscular fatigue, heat, or how the rest of your day went.
The constants are population averages. The model is calibrated for good relative values: your tempo run in relation to your easy run is right. The absolute value stays an estimate. This is why Runnerd computes with sex-specific constants — one average across everyone would cost exactly the advantage the model is there for.
A wrong maximum heart rate makes every TRIMP wrong. Runnerd estimates it from your runs on first import. If you want it exact, override it in settings.
CTL needs 42 days to warm up. It starts at 0 and grows slowly into your actual fitness. That is not a bug — it is how every tool doing this maths behaves.
Runnerd only counts runs. If you cycle, swim or lift a lot, your real load is higher than what you see here.
This section exists because it belongs here. A number whose limits you know is worth more than one you are asked to believe.
Apple Health lives on your device. Runnerd reads from there and computes on your Mac and your iPhone. There is no Runnerd server your runs land on — syncing between your devices runs through your own private iCloud.
No account, no email, no password. Apple’s privacy label for Runnerd reads “Data Not Collected”. The details are in the Privacy Policy.
Download Runnerd and let it read your Apple Health data once. That takes 10 to 30 seconds. The maths then runs backwards across your whole history — not from today, but across everything already there.
From then on: your readiness score each morning, your form curve across the season, every run classified. Runnerd is free. Pro unlocks the full analytical depth, a one-time 8.99 € for Mac and iPhone together. No subscription.
Get Runnerd on the App Store — or read the frequently asked questions first.
Runnerd is not a medical product and does not replace medical advice. Apple, Apple Health, Apple Watch, HealthKit and App Store are registered trademarks of Apple Inc.