>>/19055/
Yeah precisely, or rather the upper end of your Z2. This January my FTP was around 360-370 and my LT1 was around 240, or just 65% of ftp. Now it's closer to 75%. I think it can go as high as 80-85% for really trained athletes, triathletes etc.
LT1 is tough to raise because you really just need a lot of volume under LT1 over the years to raise it, doing vo2max work etc. isn't going to help there. I think that's why so many of those old geezers and ex pros are so good in long gravel stuff, their LT1 is absolutely elite and thus they can just keep on grinding away at high power for long stretches even if their top end power isn't as high.