Sweetspot heartrate lower in the last block


When my wife is doing Sweetspot we see every time that the heartrate in the last block is lower than in the first blocks…how is that possible and and how should we act upon it? Increase power or not?

For example. Yesterday see did 40+37min SST with 3min rest in between.

First block:
Avg power: 166 (92% FTP)
Avg hf: 167

second block:
Avg power: 167 (92% FTP)
Avg hf: 160

Probably due to lack of a sufficient warm-up.

Thnx. Now only 10min from Z1 ramp to Z2 as WU. What is a good warming up? 15min with 2x 1min around FTP or something?