Strain and Training Load

Strain Score is a 0–21 daily rating of cardiovascular exertion. It compresses your cumulative heart-rate workload into a simple scale so you can compare hard days, easy days, and rest days at a glance.

The 0–21 scale

Higher numbers mean more cardiovascular strain:

• 0–6: light day, mostly recovery
• 7–12: moderate exertion
• 13–17: hard day
• 18–21: very high strain, usually reserved for races or peak sessions

The scale is intentionally logarithmic, so the gap between 15 and 18 represents a larger absolute workload than the gap between 5 and 8.

ACWR: Acute versus chronic load

Strain Score includes an Acute:Chronic Workload Ratio (ACWR) when enough history is available:

• Acute load: average over the last 7 days
• Chronic load: average over the last 28 days
• ACWR: acute load divided by chronic load

An ACWR near 1.0 means your recent workload is well balanced with your long-term baseline. A value above 1.3 suggests a rapid spike in load and a higher risk of fatigue or overreaching.

Heart-rate zone minutes

The app also records how many minutes you spend in each heart-rate zone during the day. These zone minutes feed the raw load calculation and explain why a short, intense session and a long, easy session can produce different strain shapes.

Readiness and strain together

Use Strain Score alongside Readiness Score and Body Battery:

• High readiness + low strain = room to push.
• Low readiness + high strain = prioritise recovery.
• A rising ACWR over several days is a signal to back off before fatigue accumulates.

How it is produced

Strain Score is computed by on-device models from heart-rate samples and activity data. It measures the cardiovascular cost of your movement. It is not a simple calorie or step count.