I didn't say I could sustain Z5 at 90 cadence for a long time, just that I could do it at all. I've usually been able to sustain Z5 for 5-10 minutes but at a much lower cadence. Actually I've been putting in 159 as my MHR even though I haven't seen over 156 for a long time. I've been using 143 as my LTHR, but that's probably technically too high: I don't think I could sustain that for an hour, maybe only 20 minutes. I do Z5 intervals at ~146 which I can hold for ~8 minutes in a set, longer if I only do one.