Heart rate variability
Last night
61 ms
7-day trend
Deterministic · not AI-driven
An honest read
on your heart.
CardialOS turns the biometrics your Apple Watch already records — HRV, resting heart rate, VO₂max, sleep and workouts — into a clear picture of your recovery, training load and fitness.
No hype, no vanity scores. Deterministic formulas, computed on your phone, explained on the Methodology screen.
Sign in with Apple or Google to set up your profile. Your Apple Watch data is read from Health and calculated on your device — see the privacy policy.
- Recovery, training load and cardio fitness — computed on your phone
- Every number has a formula, readable on the Methodology screen
- Reads the data your Apple Watch already records
- Not a medical device
01 · Deterministic, not AI
How CardialOS reads your heart
Every figure CardialOS shows is a formula over the biometrics your Apple Watch already records — never a model guessing at your recovery. The same inputs always produce the same output.
The Methodology screen in the app names the formula behind every number: TRIMP-based training load, CTL/ATL/TSB, acute:chronic ratio, VO₂max trends and the rest — nothing hidden behind "AI insights."
Recovery
Optimal- HRV vs. baseline +8%
- Resting HR vs. baseline −3 bpm
- Sleep 7h 42m
Result 78 / 100
Formula · HRV · resting HR · sleep
- 01
Recovery, daily
A daily read built from HRV, resting heart rate and sleep — never a vanity score dressed up as insight.
- 02
Training load
Acute vs. chronic load and your acute:chronic ratio, so you build fitness without digging a hole.
- 03
Cardio fitness
VO₂max trends, a heart-rate-to-VO₂ view, and a race-time predictor.
- 04
Zones & vitals
Heart-rate zones and training distribution, plus HRV, resting heart rate, blood oxygen and sleep, with trends.
02 · How it works
From your Watch to an honest read, in three steps.
- 01
Connect Apple Watch
- 02
CardialOS reads your data
- 03
Get an honest read
03 · No hype, no vanity scores
Cardiovascular fitness you can actually trust.
Most fitness apps turn a number into a mood — a streak to keep, a score to chase. CardialOS is built the other way round: a plain read on what your body is telling you, so you can decide what to do with it.
Every figure traces back to a formula on the Methodology screen — never a model, never a guess.
- 01
Deterministic, not hype
Every number is a formula you can read on the Methodology screen — never a vanity score dressed up as insight, and never AI-driven.
- 02
Honest, supportive voice
On a hard day CardialOS tells you to hold steady and protect your sleep — not that you are crushing it. A read you can trust over months, not a dopamine hit.
- 03
Built for training with intent
Acute vs. chronic load and your ratio, heart-rate zones and training distribution — so fitness gets built without digging a hole.
- 04
Your Apple Watch data stays on your device
HRV, heart rate, VO₂max, sleep and workouts are read from Apple Health and calculated on your phone, not on a server.
- 05
Works with the watch you already wear
The more your Apple Watch records — heart rate, HRV, sleep, VO₂max, workouts — the richer the picture CardialOS builds.
04 · Questions
Frequently asked questions.
What does CardialOS measure?
The biometrics your Apple Watch already writes to Apple Health: heart-rate variability, resting heart rate, VO₂max, respiratory rate, blood oxygen, wrist temperature, sleep, and your workouts. CardialOS turns those into a daily recovery read, training load, cardio fitness trends, heart-rate zones and a cardiovascular-age estimate.
Does CardialOS use AI?
No. The metric pipeline is deterministic — the same inputs always produce the same output, through formulas you can read on the Methodology screen. Nothing here is a model guessing at your recovery.
How is Recovery calculated?
From heart-rate variability, resting heart rate and sleep, each compared against your own baseline — never against someone else's. It is a daily read, not a cumulative score, and it is built to tell you the truth on a bad night rather than soften it.
What is training load, and why does the ratio matter?
CardialOS tracks acute load (roughly your last week) against chronic load (roughly your last month) and reports the ratio between them. Climbing too fast relative to your base is exactly how people dig themselves into overtraining; the ratio is what makes that visible before it becomes an injury.
Do I need an Apple Watch?
CardialOS works best with one. It reads the heart-rate, HRV, sleep, VO₂max and workout data your Apple Watch writes to Apple Health — the more your Watch records, the richer the picture. Without a Watch, most of the metrics have nothing to compute from.
Does my health data leave my phone?
The HealthKit readings and every metric computed from them stay on your device. Signing in with Apple or Google sets up your shared profile (the details you enter during setup — date of birth, sex, weight) with the family of apps, which does go to the family API. CardialOS also uses PostHog for product analytics and Firebase Crashlytics for crash diagnostics, but your health values are never included in what either one receives.
Do I need to sign in?
Yes. CardialOS requires sign-in with Apple or Google before any other screen. It sets up the account-linked profile that the rest of the HealthOS family can read back, so you do not re-enter your basics in each app.
Is CardialOS a medical device?
No. It is a personal fitness and wellness tool. It does not diagnose, treat, cure or prevent any condition, and it is not a substitute for professional medical advice — always consult a qualified professional about your health.
Your next workout is all the data it needs.
Connect your Apple Watch. Read your recovery. Train with intent.