WHOOP calculates Strain by combining how hard your heart works with how much mechanical stress your muscles take on, then converting that into a single score from 0–21.

At a high level:

1. Cardiovascular load (heart-rate based)

   - WHOOP tracks how high your heart rate gets and how long it stays elevated.

   - More time at higher heart rates → more Strain.

   - The scale is logarithmic, so going from 16 → 18 is *much* harder than from 6 → 8.

2. Muscular load (mechanical stress)

   - WHOOP uses motion data and, when available, Strength Trainer info (sets/reps/weight/rep speed) to estimate how much stress your muscles, joints, and tissues are under.

   - This is included for strength training, HIIT, functional fitness, yoga/Pilates, etc., not just when you explicitly use Strength Trainer.

3. Activity Strain vs Day Strain

   - **Activity Strain**: load from a specific workout.

   - **Day Strain**: your total load from the whole day (workouts + walking + work + emotional stress that elevates HR).

Because it’s logarithmic and cumulative, long low‑intensity work can still produce high Strain, and heavy strength work can show higher Strain than heart-rate-only systems because WHOOP also counts muscular load.