using the Humon oxygenation sensor has shown me that all of my old 15-20 min warm ups (or sometimes, I’d just jump right in to sweet spot, feel rough for the first five minutes, then get going) were inadequate.
Your oxygenation levels drop during the first 10-15 minutes of exercise, then recover to start level around 20 minutes, but then don’t elevate above starting level (meaning your capillaries are dilated and everything is ready to go) until about 25-30 minutes.
My warm up for an interval session (say, 2 sets of 5 x 2:00 @ 380w, :30 soft pedal recoveries) would be something like this, now (FTP is 350):
Starting SMO2 might be around 70%
5 minutes at 240w: SMO2 drops to 65%
5 Minutes at 260w: SMO2 drops to 60%
5 Minutes at 280w: SMO2 steady at 60%
5 Minutes at 300w: SMO2 rises to 65%
5 x :30 on, :30 off, ascending from 400-450 w: SMO2 drops to 60%
5 minutes at 200w: SMO2 rises quickly to 75%
A step warmup gradually raises oxygen demand and the system gradually opens up to compensate. A set of short hard efforts gets the capillaries dilated and fully primes the muscles for hard work.