Watts @ VO2 max is a shifting target due to rate of energy demand – and rpm matters here, as well.
What does that mean? Ok, I haven’t done a hard, hard 4min effort for over two months. I did a six weeks of just going out and riding at 60% for a couple of hours, and then for the last three weeks, I’ve done a long 75-80 mile ride each week, and a sweet spot session of an hour at 90% FTP.
Now, I haven’t tested FTP – I hate tests. But, after 20 years of using powermeters, I know that an hour at a HR of 140 is 90% of my current FTP. Those sweet spot chunks were at 315w, so ok, 350-ish, down a little from the end of last season, which it should be.
But here’s the rub – I’m a slow twitch grinder, and my FTP tends to be a high % of my VO2, especially in Base. I was getting slow component VO2 at the end of those long rides, but reaching slow component VO2 at 85rpm and 220w is a whole lot different than trying to hit 370+ at 110rpm on my fixed gear trainer setup. So, last night’s 2min on, :30 off set was done at a damp-squibby 360w, and I was cracking at the end of each interval.
Simply, my old legs were not used to doing supra-threshold power, and not used to trying to do it at high rpms, so the watts were lower. But, as Andy Coggan says, sometimes “alls you can do is alls you can do.”
There’s something to be said, I think, by just doing the intervals at the highest steady output that your legs can do that day – and to vary rpm for those zone 5 intervals, rather than just doing them on the same hill every session, or in the same gear on the trainer. Just because you have the VO2 capacity doesn’t mean you can channel it to the pedals in all situations, at all force and rpm loads. Vary the stimulus.