The data should earn a decision
Plenty of apps display last night’s sleep or a readiness score. Fewer let that number change the session you are about to do. Flux can use Apple Health sleep, heart-rate variability and resting heart rate — when you grant those reads — together with how hard recent training has been and how you say you feel. The output is a more conservative or more normal session, not a medical verdict.
| Signal | Useful as | Not useful as |
|---|---|---|
| Sleep last night | A reason to leave more reps in reserve or shorten the session | Proof you must skip training |
| HRV vs your own baseline | One witness among several | A red/green coach you obey blindly |
| Resting heart rate | Context when it is unusually high with poor sleep | A substitute for symptoms |
How to act on a bad night is in the poor-sleep workout guide. How to treat a low HRV reading is in the low-HRV guide. This page is only whether the app you pay for can use those inputs.
What Flux actually reads
Apple Health is off until you ask. In onboarding or Profile you choose what to share. With permission and available data, Flux can read sleep, HRV, resting heart rate and related fields, and can write the strength session back to Health. You can disable Health Sync in Profile; that does not change the system permissions in the Health app.
- Optional. The app trains without Health. You lose a refinement, not the planner.
- Not a prompt every morning. Ordinary refreshes do not beg for access again.
- Not sold. Health data is not a product Flux turns into ads.
A worked week with the Watch on
| Day | Health picture | Training response |
|---|---|---|
| Monday | Sleep and HRV look like your baseline | Run the planned session |
| Wednesday | Short sleep, HRV a bit low, warm-ups feel normal | Train, but keep the easy reps in reserve. Do not add a set |
| Friday | Second poor night, bar speed down, mood off | Minimum version of the session or rest. The ring did not decide; the whole picture did |
If several readiness signals stay poor across a block, that is a programme problem, not a Watch problem. Use a deload or a shorter week rather than collecting more scores.
When a Health-connected app is the wrong buy
If you want a medical HRV coach, Flux is the wrong product. If you want a social logger, Strong or Hevy may fit better. If you want an adaptive plan without a membership, that is the no-subscription offer.
Frequently asked questions
Does Flux require an Apple Watch?
No. Sleep and HRV from Health help when they exist. You can train from the recovery slider, recent work and how the warm-ups feel.
Will a low HRV cancel my workout?
No. The app never forces rest. A low reading is a prompt to check the rest of the picture. You can still generate and edit the session.
Is this medical advice?
No. Consumer sleep and HRV are training context. Fever, chest pain, faintness or other concerning symptoms are a person problem, not an app setting.
Does the data leave my iPhone?
HealthKit access is on-device unless you also use other apps or Apple’s own sharing. Flux’s personalisation models run on the iPhone or iPad. Cloud backup of Flux training data is a separate, optional signed-in feature.
How do I see it on a real session?
Download Flux Free. Leave Health off for a few sessions if you want a baseline, then switch on the reads you actually use. If the next workout does not change when sleep was clearly poor, the integration is not doing work for you.