Last updated: 30 July 2026
Runnerd computes with models that have names and sources — Banister, Karvonen, Daniels. This page orders them not by technical term but by the question each one answers. Four questions every runner reaches eventually, and the number in charge of each.
That is what the readiness score is for: a value between 0 and 100 built from heart rate variability, resting heart rate, sleep and your current form. Each factor is compared against your own baseline over the past 60 days, not against a standard table.
Below 40 means a rest day. From 60, normal training holds; from 80, hard sessions too. In between: keep the intensity down.
The readiness score: the 4 factors and the maths
That is what CTL, ATL and TSB are for. CTL is the 42-day average of your training load and stands for fitness. ATL is the 7-day average and stands for fatigue. The difference between them is your form.
The gap between 42 and 7 days explains a feeling everyone knows: in a hard week, fatigue grows faster than fitness. You are fitter and slower at the same time.
CTL, ATL, TSB: fitness, fatigue and form
That is what your heart rate zones are for. Runnerd computes them from your pulse, not from your age. Everything below 70 % counts as easy — and that boundary decides whether your ratio of easy to hard runs holds up.
Set the boundary too low and you run the easy days too fast. That is the most common mistake in endurance training.
Calculate heart rate zones: calculator and formula
That is what VDOT from Jack Daniels’ Running Formula is for. From one real race time it derives predictions for other distances, plus the paces for easy, tempo and interval work.
A 10 km time therefore tells you what is possible over 5 km and over the half marathon.
Three constraints apply to every one of these numbers. Knowing them beats believing them.
They estimate, they do not measure. Three of the four compute from duration and heart rate, VDOT from a race time. Muscle damage, heat, short sleep and the stress outside training appear in none of them.
A wrong maximum heart rate spoils three models at once. It sits inside the zones, inside training load, and through form inside the readiness score — a value from “220 minus your age” plants the same error into all three. Only VDOT is immune: it needs no heart rate, just a real race time.
Only runs count. Cycling, swimming and strength work do not feed in. Do a lot of them and your real load is higher than the curves show.
What your Apple Health data holds
From your own runs. Runnerd reads them out of Apple Health, computes on your Mac and your iPhone, and syncs through your own private iCloud. No Runnerd server, no account.
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 · The models with their sources · Frequently asked questions
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.