I am currently in the final week of sweetspot base low volume two. I did these two rides outdoors approximately two weeks apart. The area where I live has a rolling terrain so it is not possible to maintain a steady pace sometimes. The average heart rate for these two rides varies by almost 10bpm which is pretty big. I use a vector 3s powermeter and calibrate them before every ride. I wouldn’t completely agree that this is due to increased fitness as 2 weeks is a small-time to make huge gains in my opinion. The average power and NP for both the rides are similar.
Temperature: 3ºC on average.
Temperature 6ºC on average
I felt really comfortable in ride#2