Please check your max heart rate and heart rate zones in the settings! There is an ongoing bug that Strava changes the max heart rate and zones without reason or notifying the user, see 

For the the activities in question the only known way to correct the relative effort after setting the heart rate zones right is to temporarily change the sport type, that leads to a recalculation after some wait time.

Besides of what ​@Jan_Mantau said, I’ve also noticed that after reaching certain level in the “Fitness” chart, the increase slows down and pretty much stops - it only keeps growing when I’m pushing myself harder than usually. When only maintaining my fitness, the graph would stagnate or go down - so e.g. last summer, when I’d run a race pretty much every week, the graph would look like a plateau, slightly oscillating but not growing (despite of new PRs). But then I had all kinds of health issues and got back into training only after ~6 months: and since this April re-start, my Fitness graph has reached about 20-30% higher than last summer's “plateau”, and keeps growing. BUT, I’m nowhere near my last summer's abilities yet, and my 5k/10k pace is almost a minute/km worse. 

Bottom line - the Relative Effort used to be called “suffer score” if I remember correctly, and tbh that's a better description of what it actually means. Not an actual fitness level, rather a quantification of “how much I’ve suffered” doing those activities. And while my actual fitness grows thanks to the training, the “suffering” decreases… and from this point of view, what ​@Kiwidane described kinda makes sense...

Thanks ​@Jan_Mantau and ​@Jana_S. I’ve checked the max HR and HR zones and they are correct. 

Today I did a hard interval session, for which I was awarded an RE of 55 and an increase of 1 in Fitness 🙄Maybe my data is starting to find a new balance now, as my Fitness score is steady over the last week and the RE scores from session to session seem to be relatively consistent now. 

Good to know that you weren’t affected by the bug. Then it seems you just got fitter in the last months  (lower heartrate for the same performance) and therefore your fitness has decreased. If you find a contradiction in the last sentence that is Strava for you, they misnamed the cumulative training load (or as ​@Jana_S wrote the “suffering”) as fitness. Someone who gets sick and trains as before is rewarded with a higher fitness in Strava because their heart rate is higher than usual 🙃

Thanks ​@Jan_Mantau Yes, I think lower HR for similar distances and times is probably the reason. It’s just a bit demoralising to see the chart decline steadily! I’m sure Strava didn’t intend for that. 

They indeed intend for that and it even makes sense. Now that you are better than before you have to train harder accordingly to get the same relative effort.

There is logic to that, but intuitively, I don’t think it makes sense to punish actual progress with a chart that says your fitness is decreasing. But at least I understand it now!

This is bizarre… I came to discuss the exact opposite. I have been training with same heart rate monitor for last three years and have had expected/consistent results until July ‘25 when my fitness score and RE have exploded to nearly double that of last three years doing similar activities. 

I suspect that it's related to the recent automatic change of HR max, some people report that the value set by Strava is incorrect / too low - one of the consequences could be that Strava considers the same effort to be much harder (let's say that your maximal HR is 190, but Strava sets it to 175, and your activity’s avg HR is 172… so while you perceive it as a relatively hard but manageable effort, Strava will think it was extremely hard and maximum performance). So you might want to check it in your profile and correct it, but hard to tell how long the correction will last - mine has changed at least twice lately, it might be getting updated automatically every month or so (though no change for me in August just yet).