I think adding an option to recalculate the relative effort of the past activities according to current heart rate zones would be very useful.
MOTIVATION: After changing the HR zone setting from default to custom, the Relative Effort chart on Fitness & Freshness page gets messed up because the past activities are not recalculated according to the new setting. Consequently, the overall chart is not useful in comparing the current fitness level with the fitness in the past. Similarly, Relative Efforts before and after the HR zones modification are not comparable.
EXAMPLE: In the past few weeks, I trained more intensely than before, but my Relative Effort chart is constantly in a downhill trend since updating my HR zones a month ago. See the picture below.
IMPLEMENTATION: The recalculation of past activities should be made optional and not happen automatically when HR zones are changed. After all, HR zones need to be adjusted regularly because they naturally change over time. In this case, when HR zones are regularly adjusted to reflect the actual physical change in HR zones, the global recalculation of past activities would not be desirable and the HR zones adjustment should impact only the newly added activities. On the other hand, when HR zone setting is changed initially to match the real HR zones, the recalculation is necessary. Ideally, to make it maximally flexible, the option to recalculate past activities should not be global but rather it would come with another option to specify the timeframe of the activities which should be recalculated on the basis of the current HR zone setting. This would be especially useful for anyone who got new HR zones some time ago, but for any reason did not adjust them in Strava settings immediately after he got them.