WHOOP measures strain based on cardiovascular output. In other words, the more time you spend in high heart rate zones, the more strain you accumulate with WHOOP. On the flip side, if you do a workout consisting of single repetitions — for example, if you're trying to beat your back squat 1 rep max — your heart rate will not stay elevated long enough to cause any cardiovascular strain. WHOOP will be releasing an update in spring of 2023 to take muscle strain into account for its overall strain score. Stay tuned for more details on that feature. As a side note, if your weightlifting session causes muscle damage (which is normal) and inflammation, you'll see that reflected in a lower HRV, and thus, in your recovery score. Additionally, muscle fatigue might cause a higher strain the following day because your system is working hard to recover.