Strain Score

The science of cardiovascular strain

Not all exercise creates equal physiological stress. A 30-minute easy run and a 30-minute threshold interval session have very different impacts on the body, even if their durations match. Heart rate — specifically how it relates to your personal maximum and resting heart rate — is the most practical real-time indicator of cardiovascular effort.

Flux uses the concept of Heart Rate Reserve (HRR), which normalises your current heart rate between your individual resting and maximum heart rate. This makes the measure personal and comparable across different athletes regardless of their absolute heart rate values.

Importantly, strain accumulates non-linearly — time spent at high intensity contributes disproportionately more strain than time at easy effort. This reflects the real physiological cost of intense exercise, which creates far greater metabolic, muscular, and autonomic stress than low-intensity movement.

How Flux calculates Strain

Flux reads minute-by-minute heart rate data from Apple Health for the entire day — not just during workouts. Minutes below a minimal effort threshold are excluded. For every active minute, the relative cardiovascular effort is weighted according to intensity, with higher efforts contributing more to the final score. The total is then normalised to a 0–100 scale.

This means a day with several low-intensity activities can accumulate moderate strain, while a single high-intensity workout can push strain well above 70–80 even without other activity.

Training Zones

Flux categorises every minute of activity into one of five heart rate zones (plus a rest zone), based on the percentage of your maximum heart rate. Zone 0 does not count towards your strain score. The Strain detail view shows the time distribution across zones for the day.

Target Strain

The Strain detail view shows a Target Strain range— a recommended window of cardiovascular load for the day. This is calculated based on your morning Recovery Score, recent HRV trend, and recent training load to provide personalized guidance on how much strain you should aim for today.

• High recovery → higher target strain (your body is primed to absorb hard training).
• Moderate recovery → moderate target strain (productive training without excessive load).
• Low recovery → low target strain (light movement or rest is advisable).

This feedback loop between Recovery and Strain is the core of Flux's training guidance — hard days when your body is ready, easy days when it is not.

Interpreting your Strain Score

The Strain Score follows a logarithmic progression, meaning each additional point becomes progressively more difficult to achieve. This reflects the real physiological cost of high-intensity training: as your strain increases, each additional increment requires disproportionately greater effort.