iOS app that reads heart rate from AirPods with heart rate monitoring via HealthKit and announces it aloud — no watch, no chest strap, eyes and hands free.
Built by Halfmarble LLC as a proof of concept for ambient on-device biometric feedback, and as a prior art vehicle for the consumer-earbud-as-closed-loop-biometric-sensor pipeline. It was also a testbed for what it actually takes to keep user data protected and never released — to prove we can deliver on the privacy we promise.
Prior art published: 2026-06-04 — two public-domain defensive publications, in the Technical Disclosure Commons (Defensive Publications Series): PRIOR_ART_EARBUD_CLOSED_LOOP.md (dpubs_series/10440) and PRIOR_ART_REST_GATING.md (dpubs_series/10441).
- Voice announcements — current BPM announced at a configurable interval (continuous / 2s / 5s / 15s / 30s / 1 min) and immediately when HR changes by a configurable threshold (±3 / ±5 / ±8 / ±10 BPM). Both triggers use independent baselines so interval announcements never suppress drift detection.
- Live chart — full-session HR history, chart fills the screen, auto-scales to the session's HR range. Chart time axis is at least 15 minutes and grows as the session continues.
- 5-zone color gradient — the HR line is colored green (Zone 1 recovery) → chartreuse (Zone 2 fat burn) → yellow (Zone 3 aerobic) → deep orange (Zone 4 anaerobic) → red (Zone 5 maximum effort), with quadratic easing in the warning zone. Zone boundaries are computed automatically from your date of birth via the Tanaka formula (maxHR = 208 − 0.7 × age) read from Apple Health.
- Zone coaching — optionally name your training zone in each announcement ("142, zone 4"), and with a target zone set, append a steering nudge — "push" when you're below it, "ease off" when above. Composed on-device in the native layer so it keeps coaching while backgrounded with the screen off. Requires heart-rate zones (age or date of birth configured); falls back to the bare number otherwise.
- Pre-workout readiness snapshot — shows resting HR, HRV (SDNN), and VO₂ max from Apple Watch (when available), color-coded against age-banded reference norms (see References), with data freshness timestamps.
- Live metrics overlay — current BPM centered on the chart with calories (yellow, right) and breathing rate (blue, left) as subscripts. In portrait, the BPM number drifts 1 px per sensor sample away from the heart rate line. In landscape, it's always dead center.
- Workout type — select Boxing 🥊, Cycling 🚴, Running 🏃, or Other before starting. Sets the correct
HKWorkoutActivityTypefor Apple Fitness rings. Saving the finished workout to Apple Health is on by default and can be turned off in Preferences (Apple Health → Save workouts to Apple Health) — with it off, the workout is discarded and never leaves the device. - Boxing round timer — for boxing workouts, an optional round timer with amateur (3 × 2:00) and pro (12 × 3:00) presets, plus configurable round length, rest length, and round count. Calls rounds and rest aloud through the same voice channel, with an optional "ten seconds" warning before each round ends.
- Post-workout summary — max HR, average HR (time-weighted), duration, calories, effort score (avg/maxHR × 100), zone time distribution, and a 1-BPM resolution HR histogram colored by zone.
- Session history — every completed session is saved as JSON to the app's Documents directory. Tap the clock icon in the app bar to browse past sessions with mini histograms.
- Screen wakelock — screen stays on during monitoring, returns to normal auto-lock when stopped.
 Pre-workout readiness — resting HRV, resting HR, VO₂ max — then pick a workout type |
 Set a target zone and hear "push" / "ease off" coaching as you train |
 Boxing round timer — amateur/pro presets, configurable rounds, rest, and "ten seconds" warning |
The full-session HR chart fills the screen, colored by training zone (green recovery → red maximum effort) — turn the phone landscape. Below, the same hike recorded on two people:
A higher-intensity profile — sustained Zone 4 with Zone 5 peaks (max 150, avg 131)
A long aerobic effort bookended by two climbs (max 128, avg 96, over two hours)
Requires AirPods with heart rate monitoring (AirPods Pro 3 or later) and iOS 26 or later. Heart rate monitoring requires HKWorkoutSession on iPhone, which is only available on iOS 26+.
For the clearest spoken readout, install Apple's Ava (Premium) voice before your first workout: Settings → Accessibility → Spoken Content → Voices → English → Ava, then choose the Premium quality (one-time download). SteadyHeartBeat automatically prefers the highest-quality installed voice (premium → enhanced → default), and you can also pick it explicitly in Preferences → Voice & announcements → Voice. Without a premium voice the app falls back to the standard system voice, which sounds noticeably more robotic.
AirPods Pro 3 optical sensor
│
▼
HKWorkoutSession / HKLiveWorkoutBuilder
(iOS HealthKit — on-device only)
│
┌────┴────┐
│ │
Heart Respiratory Active
rate rate calories
│ │ │
└────┬────┴───────────────┘
│
WorkoutProvider (Dart)
│
┌────┴──────────────────────┐
│ │
Voice TTS Live chart
(AVSpeechSynthesizer) + overlay
│ │
└─────────AirPods───────────┘
(same device)
All computation is on-device. No data leaves the iPhone — halfmarble never receives it. Session files are stored in the app's private Documents directory and are explicitly excluded from iCloud/iTunes backup, so they stay on the device.
These are deliberately independent decisions. A change to one does not affect the others:
- Prior art (public domain). Selected methods and techniques are disclosed as dated, public-domain defensive publications — PRIOR_ART_EARBUD_CLOSED_LOOP.md and PRIOR_ART_REST_GATING.md (effective 2026-06-04; also published in the Technical Disclosure Commons at dpubs_series/10440 and dpubs_series/10441 for examiner-database findability). Publishing them as prior art keeps them freely practicable by anyone and prevents third parties from patenting them. That is their only purpose — they do not give away the source code or the application.
- Code license. The source code is governed separately by LICENSE, and may be relicensed in future without affecting the prior-art dedication above.
- The application. SteadyHeartBeat the product — its distribution, pricing, and availability — is a halfmarble product decision, independent of the two layers above.
So: the ideas are public-domain prior art, the code has its own license, and the app is a product. Publishing the prior art secures the defensive goal (unpatentability) without requiring the app to be free or the code to be open.
Requires a physical iPhone (HealthKit and HKWorkoutSession are unavailable in Simulator). Minimum deployment target: iOS 26.0.
flutter pub get
flutter run --device-id <your-device-id> --releaseAfter changing ios/Runner/Runner.entitlements, confirm the HealthKit capability is enabled in Xcode under Signing & Capabilities for both Debug and Release.
All physiological thresholds are deterministic and traceable to published science (no opaque models). The metric grading lives in lib/health_norms.dart.
- Max heart rate / zones — Tanaka formula,
maxHR = 208 − 0.7 × age. Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. J Am Coll Cardiol. 2001;37(1):153–156. doi:10.1016/S0735-1097(00)01054-8 - VO₂ max grading — age- and sex-banded norms from the American College of Sports Medicine / Cooper Institute Aerobics Center Longitudinal Study (decade percentiles by sex; "excellent" ≈ 80th, "good" ≈ 60th, "fair" ≈ 40th). Biological sex is read from HealthKit (with a manual override in Preferences); it defaults to men's norms when unknown. ACSM. ACSM's Guidelines for Exercise Testing and Prescription, 11th ed. Wolters Kluwer; 2021.
- Resting HRV (SDNN) grading — age-banded short-term reference ranges (age-only; SDNN sex differences are small and inconsistent). Sammito S, Böckelmann I. Reference values for time- and frequency-domain heart rate variability measures. Heart Rhythm. 2016; and short-term (5-min) HRV reference ranges from the Multi-Ethnic Study of Atherosclerosis (MESA). PMC5010946
- Measurement-validity caveat: consumer-wearable HRV is supported only at the device-class level. It is markedly less reliable than heart rate; the reference ranges above were not derived from Apple Watch; validated wearables typically report rMSSD, not the SDNN Apple exposes; and no study has validated Apple Watch HRV specifically, nor in a Parkinson's/RBD cohort. Treat the grade as a coarse within-person trend, not a validated readout.
All processing is on-device — halfmarble never receives your data, and nothing is sent to any server. Your session files and personal health data (age, sex, self-reported conditions, and biometric values) stay in the app's private storage and are excluded from iCloud/iTunes backup. App settings (voice, intervals, units) are the only thing kept in standard preferences.
The one exception is Apple Health: by default each finished workout is saved there for your Activity rings, after which it follows your own iCloud Health sync settings. You can turn that off in Preferences (Apple Health → Save workouts to Apple Health), and the workout will stay on this device only.
Your data is yours — locked in is not the goal. Protecting data this aggressively has a failure mode: it can lock out the owner too. So Preferences → Your Data gives you two unconditional actions: Export My Data — a complete, readable copy of everything the app stores (sessions and health profile), handed over through the system share sheet to a destination you choose — and Delete All Data, which erases it from the device. The export sheet also offers Anonymize for Research…, which produces a de-identified copy (random IDs, identity and absolute timestamps stripped) if you choose to donate it. Nothing uploads anywhere; both paths end at the share sheet. The full reasoning — including why a readable export is more respectful of ownership than an encrypted one — is in docs/DATA_PORTABILITY.md.
For a complete breakdown of every way data could leave the device and how each is handled — verifiable against the code — see DATA_PRIVACY.md. The full privacy policy is at halfmarble.com/steadyheartbeat/privacy.html.
Halfmarble LLC — halfmarble.com — privacy@halfmarble.com