lactic acid isn’t actually a real thing. There’s lactate that is derived from anaerobic metabolism of energy, which can be used as fuel, and then the leftover lactate that isn’t used makes your blood acidic.

So when you work out, your muscle fibers tear, lactate is sent into the blood, you use what u can of it and then the leftover makes your blood acidic.

I’m kind of spit balling here from what I learned in EMT school, but when your blood is acidic (for any reason), I know your heart pumps faster and your respiratory rate increase slightly, when this happens your pushing oxygen from your bronchioles into your capillaries at a faster rate than usual which attaches onto the hemogoblin in your red blood cells and perfusion occurs and when this happens it allows the blood to be cleansed of the acidic effect the lactate has.

So yeah if you work out and get super sore, you will generally have a slightly elevated heart rate and respiratory rate.

If you have something like a whoop band, when you see it gives you a low recovery rate, that’s because it’s taking into account what your respiratory rate and heart rate are at.

It’s a good way to see if you need a rest day or not