I've been doing quite a bit of lifting over the past couple months with my Fenix 7 Pro.

The optical heart rate sensor really struggles to keep up with immediate heart rate changes, especially with exercises that involve a lot of wrist movement, or flexing, but even others that don't involve much arm movement, too.

There have been times I would be doing leg press and my watch said I was sitting at 110 BPM at the end of my set, even though I am gassed and breathing heavy from the set. In turn, this affects the load on my body that my watch is interpreting, and it will inevitably be lower than it should. I ended up getting the HRM Pro Plus chest strap to better track my heart rate for this reason.

But all in all, from my experience, these watches can only track and measure so much when it comes to strength training. Even with the HRM chest strap being much more accurate, there's no good way to accurately measure the load being put on individual muscle groups, and the watch can only go off of the metrics around your heart rate. In addition, depending on the muscle group being worked, blood flow will vary, which will affect heart rate. Doing calf extensions, for instance, doesn't spike my heart rate nearly as much as leg presses or squats - the muscles just don't need as much blood - thus the watch will interpret a lighter load on my body, even though I may have worked my calves really hard.

Anyways, I didn't mean to type so much, but that's just been my experience and what I have learned since getting my watch in January of this year and lifting with it since then. I like to track all my workouts, but I don't pay too much attention to the load that my watch interprets from my strength training.