For the purposes of having a response undermined - lets assume that this is based on an estimated MHR. Like Citius says, there will be estimation without a reliable MaxHR figure but seeing as the majority of people have access to that reliable figure, then pinches of salt a plenty (who's not heard of MHR not being initially estimated as 200-age? Not science, but good enough the for the majority of novice/amateur fitness enthusiasts).
Sitting on Z4 does seem pretty hardcore but its not implausible (although yes that Zone is based on generally accepted research as far as I know). I've spoken to a lady who I think is Cat2 and she says she can do that as one of her strengths.
Ride with a mate yesterday who is getting over a chest infection and his HR was higher than normal. I wouldn't go thinking that in itself it means something, but as has been said, taking it into account as to whether your perception of your effort was more, a headwind was prevalent or perhaps even crapper surfaces? Different tyres etc.
In the last couple of months I've put my MHR from 188 (average of all the calculations on brianmac.co.uk) to 196 as I saw that a couple of times while really gunning it. Now I find it hard to get over 190 - I'm not about to go changing it down all of sudden, I just have to try and work harder as I'm fairly sure I just haven't had the opportunity to get it up there. BUT I've also stopped using water tabs with caffeine - so many things to consider. Good suffer score though :-)
