WHOOP uses a logarithmic algorithm to calculate your strain score. For example, if your current strain score is 5.0 and then you go for a run that clocks in at 10.0, your total strain score is likely lower than the sum of both (15.0). WHOOP does that so it can top out at 21 — the maximum strain you can achieve. Consequently, increasing your strain from 10 to 11 is much easier than from 11 to 12.