Anecdotally it seems that fatigue in one sport (swimming) doesn't necessarily impact on, say, running in the same way that it would impact, say, bike. how to measure fatigue for swimming then? more specifically how measure the fatigue IMPACT on another sport.

thank you for that. I had a medical potential issue where I was told to avoid high intensity training. I was pretty fit at the time. So I just did the base-type training Z1/Z2/Z3. I was surprised, but it just about maintained my shorter distance performance for about 6 months. maybe i'd neglected the longer stuff previously. after that it went a bit pear shaped. On the all-clear signal it took a LONG time to claw back time towards the glory days :-) - not sure if this observation helps the discussion!!

agreed 100%. If a day is down as 'rest' or 'recovery' then often athletes who follow a plan feel able to juggle days around in the plan. Using 'Adaptation day' emphasises the importance of the day being where it is for a specific and important reason. IMHO.