In searching, it appears it was Joe Friel who originally had the recommendation of staying within -10 to -30 TSB zone for optimal training Part 3: Training Stress Balance—So What? - Joe Friel
In this blog post he says “If you spend much time in this -10 to +10 TSB range your training is stagnant. Not much is happening.” It’s interesting for me in light of the fact that from Feb 26th through mid-April I followed Sustained Power HV and barely dipped below -10. I don’t know what evidence Friel has that training is stagnant in that -10 to +10 range, but my curiosity is piqued on maybe trying to plan my training next year to stay more within the -10 to -30 TSB range for blocks. Even doing SSB2 HV the 2nd time around from late April-end of May I was staying in that grey zone, so would be curious if Friel would say if I wasn’t maximizing that training block by not staying in that alleged optimal range and if he would have suggested some more TSS to stay more consistently in that TSB range. I’m headed into CX season and won’t be super focused on any of this and just maintaining fitness while racing every weekend.