The funny thing about sweet spot base (I’m taking mid vol here) II is that there isn’t a lot of sweet spot in it! If we consider the TR sweet spot definition:
“The Sweet Spot power zone, which we define as 88% to 94% of FTP”
mid vol SSBII is characterized by Tues VO2max, Wed endurance, Threshold Thurs, Sat over/under, Sun SweetSpot. So really there is just one sweet spot workout in the plan & it comes at the end of a challenging week of training. Everybody loves to follow up Mary Austin with Tallac +3! Yeeha!
So maybe it’s not surprising that it starts to bite a little. If you do any sweet spot intensity interval long enough it’s gonna turn into a VO2max effort.
One other thing to consider, though, is that your minimum power to elicit VO2max & Threshold might not have improved in lockstep. Consider what would happen if your threshold has been improving at a slightly faster rate than VO2max over the past couple seasons. So that by now, threshold sits very close to the minimum power needed to elicit VO2max. Now, with a little bit of fatigue over the course of a few sweetspot intervals or maybe one long sweetspot interval you can go from sweetspot to VO2max.
If that’s the case, maybe it’s time to do a ‘VO2max intervention’ for a few weeks and give yourself some more room to grow that threshold/sweetspot intensity.