I’ll throw pathways back on the pile.
(Chad wannabe deep dive)
The “long, low, and slow” higher volume (say 12 hrs/week+) approach accumulates a lot of time in which the muscles release calcium, so the CA2+ enzyme (calcium-calmodulin protein kinase) can break that ionic bond and convert to ATP in the mitochondria. That pathway is the best indicator of aerobic performance – how fast are you at 2mmol/AeT? The problem is that it takes a lot of time – more than many career-and-family-committed adults have – for training.
If you do have the time, though, do the volume – the main effects of low intensity are 1) increasing muscle calcium levels and maximizing the efficiency of that pathway and 2) cycling through the small motor units as they fatigue, so by the end of a long single session – like 4-5 hours+ – the large motor units are having to work aerobically, and at this point, you’ve hit the slow component of VO2 max without your HR cracking 70% or so.
Now, the first objective is reached just through volume. 14 hours of low intensity in one week is 14 hours, no matter how you split it up. The training effect on the CA2+ pathway is the same. The second objective can only be reached through the long ride if you are staying below AeT and not kicking in the AMPK pathway very much.
That second aerobic pathway – AMPK – converts glycogen to ATP through the other aerobic enzyme, adenosine monophosphate kinase. You need to work somewhere between AeT and MLSS (maximal lactate steady state, or – if you buy into the FTP concept, FTP), and the lower in the range, the less glycogen demand, and the higher in that range, the higher the demand. The big motor units have to get involved to help meet the power output, so if you do those 2 x 20s or Hours of Power, you’ll get some larger motor unit recruitment and some slow component VO2 Max (like the last 15 min of a 40k TT – ouch). Sweet Spot emphasizes this pathway, and if you don’t have 14 hours a week to train, then emphasizing this pathway is the best compromise between long-term aerobic development (which is a direct consequence of how many hours you plug away below AeT) and realistic time availability. You’re hitting AMPK, but you’re not getting the “ideal” hit to CA2+ because you’re not getting as much low intensity volume to really work that system most effectively. It’s a compromise, not a substitute, but it’s a hell of an effective compromise, until you hit the point where you’re just not going to be able to add more time in SST zone and you have to add more low intensity stimulus to force adaptations.
Personally, I would not do more than a month during the training year without poking above AeT. My October was “just ride and let the CTL drop.” I still did 60 hours, but it was all puttering around just hard enough to be power zone 2 – so CTL dropped 20 points.
Even doing 14-15 hours a week, I think it would be best for me (N=1 warning) to do two days a week of accumulating 40-60min at 85-89% of FTP for the next four weeks. So a traditional base with some sweet spot seasoning. In that respect, there is probably also some AMPK “compromise” to be done if you can’t do 20 hour+ weeks on the bike during the Foundation/Base phases.
Those big motor unit bears need poked, aerobically. Find the best way to poke them that fits in your available training time. But poke 'em.