Maximum heart rate now uncapped
UI follow-up to a change last year
Last year we wrote about unfiltered heart rate data, and changed the way Crickles obtains detailed heart rate data from intervals.icu. The reason, in short: intervals.icu treats any reading above the maximum heart rate held for you as a spike and removes it. That is a sensible default for their purposes, but it discards readings that are sometimes useful to Crickles. We therefore began taking the unfiltered version instead.
Alongside the detailed second-by-second data, intervals.icu supplies a summary for each activity, and the maximum heart rate in that summary is a filtered figure. That summary value is what Crickles has been showing in the Dashboard and on the Zones page. It was therefore possible for Max HR on the Dashboard page and Maximum Heart Rate shown in Zones to be lower than some high heart rate readings that Crickles has observed and used in the raw data feed.
That is now fixed. The maximum heart rate Crickles reports is read from the same unfiltered trace as everything else, on the Zones page, on the Dashboard and in the spreadsheet you can obtain from the Download CSV button.
This does not affect the analysis
It is worth being clear that this affects what is reported, not what is calculated.
Crickles does not use maximum heart rate for anything. Your zones are built from your sustainable heart rate, not your maximum - the top of Zone 4 is your sustainable level, and the other boundaries sit at fixed proportions of it. The same is true of every other measure on the site. Nothing about your zones, your sustainable levels, your heart rate intensity or your cardiac stress score changes because the maximum heart rate now displays differently. This is deliberate and long-standing: maximum heart rate is observed rarely, is easily corrupted by a single bad reading, and barely moves as you get fitter, whereas sustainable heart rate is measured constantly and tracks your actual condition.
Maximum heart rate is shown only for interest.
What you may notice
The first change that you may notice is that your maximum heart rate shown in the Navigator may now go up. If the maximum heart rate held for you at intervals.icu is set conservatively, every reading above it has been trimmed off before Crickles and the maximum HR value passed to Crickles will have reflected this. Uncapping it bases maximum heart rate on what your monitor actually recorded, which may be several beats higher.
The second is the flip side. We no longer inherit the heart rate cap that you set (or take as a default) in intervals.icu, so where a chest strap or optical sensor has genuinely misfired, you will now see the misfire rather than a capped number that concealed it. For Crickles, this is helpful. As we proposed in the arrhythmia paper, oddly high readings are not just noise: they may carry signal, and sporadic mis-reporting by sports devices is common enough that hiding it may lose useful information.
A recording anomaly severe enough to distort a maximum will usually disturb the trace in ways that Crickles picks up, and the activity will carry a Regularity flag other than Regular. That flag sits beside the maximum in the Dashboard’s activity table (see screenshot), and it is the thing to look at if a figure surprises you. For that reason the reference maximum on the Zones page counts only activities flagged Regular, while an individual activity shows what was recorded for it. The two can therefore differ, and that is intended rather than an inconsistency.
Your earlier activities
New activities pick this up automatically. Activities already analysed keep the figure they were given until they are looked at again, so if you would like your history brought up to date, refresh your data as described here and hit Confirm Refresh.
Where Crickles is working from detailed data it stored before the June 2025 change, that stored copy was filtered, and refreshing will re-read the same filtered values. Most activities are unaffected, and anything recorded since is fine.
As ever, do get in touch if anything doesn’t look right.