Just having an ‘off forum’ conversation about this topic and some muscle oxygenation data I took a while back. We notice that if you just hope on the trainer and start pedalling at some low wattage (let’s say 75W) SMo2 gradually goes up until it reaches a plateau somewhere in the mid 80’s after about 35 minutes.
I suspect that more than anything this is what is happening in my data. After a half hour or so the blood’s ability to carry oxygen to the working muscle is just reaching it’s peak.