SteadyHeartBeat is built on a simple principle: your health data belongs to you. It is processed entirely on your device, and halfmarble never receives it.
Last updated: June 11, 2026
SteadyHeartBeat reads health data through Apple's HealthKit framework. All processing happens on your iPhone. No data is transmitted to halfmarble or any third party.
| Data type | Source | Leaves device? | Purpose |
|---|---|---|---|
| Heart rate (live) | AirPods with heart rate monitoring | NEVER | Real-time voice readout and chart display during workout |
| Respiratory rate (live) | AirPods with heart rate monitoring | NEVER | Displayed during workout as breaths/min |
| Active calories burned | HealthKit (calculated from HR) | NEVER | Displayed during workout; saved to your workout session |
| Date of birth | Your Health profile | NEVER | Calculates your personal heart rate zones using the Tanaka formula (maxHR = 208 − 0.7 × age). Never stored on our servers. |
| Resting heart rate | Apple Watch (background) | NEVER | Shown on the pre-workout screen as a cardiovascular baseline indicator |
| Heart rate variability (HRV) | Apple Watch (background) | NEVER | Shown on the pre-workout screen as an autonomic health indicator |
| Cardio fitness (VO₂ max) | Apple Watch (background) | NEVER | Shown on the pre-workout screen as a fitness context indicator |
| Biological sex | Your Health profile | NEVER | Selects age- and sex-based reference ranges for cardio fitness (VO₂ max) |
| Body mass (weight) | Your Health profile | NEVER | Estimates the energy cost of elevation gain during a workout |
| Step count | HealthKit (during workout) | NEVER | Shown during and after the workout |
| Walking + running distance | HealthKit (during workout) | NEVER | Shown during and after the workout |
| Flights climbed | HealthKit (during workout) | NEVER | Shown during and after the workout |
When you complete a monitoring session, SteadyHeartBeat saves a workout record to Apple Health by default — including the session duration, activity type (boxing, cycling, running, walking, hiking, or other), heart rate, and active calories burned — so it counts toward your Apple Fitness rings. This data belongs to you, is governed by Apple's Health privacy settings, and then follows your own iCloud Health sync settings. You can turn this off in the app under Preferences → Apple Health → Save workouts to Apple Health; with it off, the workout is discarded and never written to Apple Health.
Completed session summaries — HR timeline, zone distribution, and histogram — and your on-device profile (age, biological sex, any self-reported conditions, and manually entered metrics) are saved to the app's private storage on your iPhone for the Sessions history and pre-workout screens. This storage is explicitly excluded from iCloud and iTunes backups, so it stays on your device.
Keeping your data on-device must not lock you out of it. Preferences → Your Data gives you two unconditional actions — no account, no request form, no waiting:
Export My Data assembles everything the app stores — your workout sessions and on-device profile — into a complete, readable copy and hands it to the iOS share sheet. Where it goes is your choice; the app does not upload it anywhere. The export sheet also offers Anonymize for Research…, which produces a de-identified copy (random identifiers; identity and absolute timestamps removed) for you to donate to a research effort of your own choosing. halfmarble receives nothing in either case. (Where halfmarble itself runs opt-in research, it never takes individual records — only differential-privacy-protected aggregates; see our data policy.)
Delete All Data immediately erases every session and the on-device profile. Workout records you already saved to Apple Health remain in Health, under your control there.
This website (halfmarble.com) uses Plausible Analytics, a privacy-first analytics tool that collects no personal data, sets no cookies, and is fully GDPR-compliant. Aggregate page view counts are the only data collected. The SteadyHeartBeat iOS app does not include any analytics SDK.
SteadyHeartBeat is not directed at children under 13 years of age and we do not knowingly collect personal information from children under 13.
If we materially change how we handle your data, we will update this page and change the "Last updated" date above. Continued use of the app after a material change constitutes acceptance of the updated policy.
Halfmarble LLC
privacy@halfmarble.com
For data deletion: use Delete All Data in the app (Preferences → Your Data), or simply delete the app from your iPhone. For Apple Health data, use the Health app's data management features. For any other privacy concern, email the address above.
SteadyHeartBeat is built under halfmarble's Glass Box principle: all algorithmic logic is deterministic, traceable to peer-reviewed science, and runs locally on your device. Read the full data philosophy at halfmarble.com/glass-box/data.html.
