My guess: sounds like a case of insufficient warm-up.
Warming up slowly is valuable because it prevents fatigue accumulation in the muscles while the metabolic machinery in the muscle cells is still getting ramped up and put into action. If you exceed your aerobic metabolism ability initially (which is lower than your "z2" because that z2 value is a "warmed up aerobic ability", you'll accumulate fatigue quite quickly which will probably stay with you the rest of the effort. Or the fatigue will stay with you or until your cellular machinery is fully up and running, and able to sufficiently exceed the output currently required by whatever amount you previously exceeded it during your overly-short, or non-existent warm-up.
This is the same reason that pacing the first half mile or mile of a race is critically important for best possible race outcome. In racing, presumably, there will never be a time where your aerobic ability sufficiently exceeds your current output level to "pay pack" the debt.
Scenario with numbers:
z2 = 200-210W
Resting aerobic ability = 160W
20min warm-up needed for you personally to do 200W aerobically, without acid accumulation.
The first 20min of riding at 200W you're slowly accumulating fatigue while your cells use too much anaerobic metabolism to get the job done.
If you're riding at 200W for the duration of your ride, and you can produce 210W worth of energy aerobically, once fully warm, you may take as much as an hour to dissipate some of the fatigue at the onset of training.
That said, if you're pressed for time, and you don't mind a bit of discomfort in the onset of your rides, not much harm in skipping any warm-up and just getting to the more stimulative part of training.
Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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