This week a revision to the Crickles model for cardiac stress is being rolled out. This retains the same characteristics as the current model so that Cardiac Stress and related metrics such as Fitness and Fatigue remain consistently calculated – unlike with some other sites, you won’t find that training load and fitness measures vary dramatically according to whether you use a power meter, a heart rate monitor or both or neither.

The main driver for the revision is that we originally relied on information from papers and public sources pertaining to the shape of critical heart rate and power curves. In this revision those curves have been now determined from hard data, using orders of magnitude more observations than most, if not all, published sources.

The changes will filter in over the coming weeks. In general, more weight is now given to shorter activities as well as to activities that break prevailing intensity levels. You may notice that CSS scores tend to rise a little, especially if you do a lot of short, intense activities. Conversely, LTHR estimates may tend to reduce. These are average population effects and your particular data may be different; it is also likely that you won’t notice any change. FTP estimates are somewhat problematic due to the de facto quasi-standard of reporting FTP as 95% of 20 minute critical power (CP20) rather than proven 60 minute critical power (CP60). To side-step this, as the new calibration plays through we will replace FTP curves with CP20 curves.

Please do let me know if you see anything that looks wrong or that you would like to discuss regarding this update.

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