Introduction to Flux
Flux is a science-based health and training app designed for endurance athletes. It translates complex physiological data into clear, actionable scores — helping you train smarter, recover better, and stay healthy.
What Flux measures
Flux integrates data from Apple Health, your Apple Watch, wearable devices that sync with Apple Health, a Bluetooth chest strap, and your iPhone camera to build a comprehensive picture of your daily physiology. Every morning, you receive three key scores and a set of monitoring metrics:
Recovery Score
How ready your body is to take on training today, based on HRV, resting heart rate, sleep, and breathing rate.
Sleep Score
A quality assessment of your last night's sleep, analysing sleep stages, total duration, awakenings, regularity, and efficiency.
Strain Score
The cardiovascular load accumulated during the day, reflecting how hard your heart worked across all activities.
Stress Monitor
Real-time tracking of cardiovascular stress throughout the day, updated continuously from heart rate data.
Health Monitor
An illness-risk assessment that combines biological signals with subjective symptom reports.
Training Load
Long-term tracking of acute and chronic training load to help you avoid overtraining and optimise fitness gains.
Data sources
Flux pulls data from multiple sources to build the most accurate picture possible:
- Apple Health: Heart rate, HRV, sleep stages, respiratory rate, SpO₂, and workout data.
- Apple Watch and wearables: Morning HRV readings and data from any wearable device that syncs with Apple Health (Garmin, Oura, WHOOP, Fitbit, and more).
- Bluetooth chest strap: Raw R-R intervals for high-accuracy HRV measurement (e.g. Polar H10).
- iPhone camera (PPG): Photoplethysmography-based HRV measurement using your phone's camera and flash.
- Manual input: Daily symptom questionnaire for health monitoring.
How to use this documentation
Use the sidebar to navigate between topics. Each section explains the science behind a specific feature, what data it uses, and how to interpret the results. If you are new to Flux, we recommend starting with the HRV Reading page to understand the core metric that powers most of Flux's calculations.