Running index dropping

Illness that you might've not noticed, getting hotter outside, overtraining/lack of easy runs (at 40 RI it's kinda annoying to get easy runs in, been there after COVID, I know how it feels going at 6:30-7 mins/km, sluggish and boring), some chronic condition like rheumatism/gastritis etc., life stress/lack of sleep/bad diet/gaining weight.

It's not that uncommon to get these periods, which can be awful, the difference between 40 and 48-ish which were your best runs in the pic is very large (considering accurate tracking of HR and distance).

I'd say take a chill week with easy runs, sneak in some longer ones. Try to get some 9-10hrs sleep nights, monitor the HRV with your H10 to see what's going on, and eat well, but not too much. Monitor weight too. If you gained weight (which is unlikely since you seemed to run often enough), that will impact you quite a bit. Just 2-3 kgs can make a huge difference, and sometimes you can have 1.5kgs of water retention or full intestines, and that's gonna impact your runs.

Curious if your higher HR "bad" runs are usually after day(s) of rest? If yes, try some long walks on your rest day, and also like a half hour of strength training and leg stretching to keep your muscles active. Some biking or spinning in the first 2 HR zones should do fine too. Just get the blood flowing.

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