@Stoke80 I’ve not analysed in detail but my gut feeling is ZS needs to ‘see’ a clear change in the RR data so blasting through into VO2max for a couple of minutes and hitting max hr would likely work but so would multiple ‘dips’ over into VO2Max and back into threshold. I mostly get LT2 (Zone 4 -> Zone 5) recommendations from ZS when I run threshold runs where my HR is dancing either side of LT2 (as I go up and down hills) and I sometimes get a recommendation when I over-cook the last step in a progression run. In the same way, I mostly get LT1 (zone2 -> zone3) recommendations when I do threshold intervals of a few mins with recovery of about 2 mins as my HR has time to drop back down to Zone 2 or Zone 1. Basically, the more your HR crosses the LT1 and LT2 thresholds within a workout (with enough time between that ZS can track it - so HIIT doesn’t work) the better it seems able to recommend zone settings.