From today, references in Crickles to Lactate Threshold Heart Rate (LTHR) and Functional Threshold Power (FTP) are replaced by the terms Sustainable Heart Rate (Sustainable HR) and Sustainable Power respectively. The LTHR-FTP page is now renamed SUSTAINABLE LEVELS. The calculations themselves have not changed but the new terms more accurately reflect their meaning and derivation.
In the case of the Sustainable Power metric, the general usage of the term FTP is variable – see here – and does not always reflect what Crickles measures. In Crickles, Sustainable Power is a measure of the level of power that the athlete can be expected to sustain for an hour on a recent evidential basis. This means that if the athlete does not engage in any effort sessions for a while then the recent evidential basis for the estimate of sustained power will decay and the estimate will start to fall until a new, high power effort is recorded. It is not necessary to do a full hour’s effort to get an updated estimate of Sustained Power: Crickles uses empirical information to determine the relationship between the power that can be maintained over different time windows.
Sustainable Power on Crickles will typically be lower than other estimates of FTP, which typically either define FTP differently or rely on relationships between different power windows that we do not see supported by data.
The term LTHR is less contentious as fewer people hold a view about their current LTHR level. However, it enshrines a theory regarding blood lactate that, while it may well be true, cannot be empirically established by Crickles as we never draw blood! Instead, the term Sustainable HR references facts that can be determined on a recent evidential basis and this is what we do. Again, this is done by examining the heart rate levels sustained over different time windows and using empirically-derived data from a relevant population to express the totality of recent activity in an hour-equivalent number.
As before, when we expressed this as LTHR, Sustained HR is preferred to maximum heart rate as the basis for calculating cardiac stress and defining heart rate training zones because it is more stable and more reliably measured.