You can do exactly the same VO2 max session as the rider next to you and get a very different result. That's the finding I open this episode with, and it's the reason I think most of us are approaching interval training slightly wrong.
The study is simple to describe. Two groups of trained cyclists did the same amount of hard interval work. Only one group improved its VO2 max. Same sessions on paper, different outcome in the lab.
What the research says about formats
The episode goes through the main research on which interval sessions produce the biggest gains. The short version of the formats most riders argue about:
In a seven-week study of trained cyclists, groups doing 4x4, 4x8 or 4x16 minutes twice a week all improved, and the 4x8 group showed the largest average gain. In a 2015 trial of 16 cyclists, short 30/15 intervals beat longer efforts over ten weeks. Then a 2025 crossover found the 30/15 advantage showed up when intensity was fixed but not when both formats were ridden as hard as possible.
If that sounds like the research disagrees with itself, it does, and it's because the details of how the intensity is set change the answer. That's why I spend so much of the episode on pacing rather than on picking a winner.
Why pacing is the bit that matters
If you go out too hard on the first repetition and fade, the later repetitions land below the intensity that actually stimulates your VO2 max. You've done the session. You've logged the kilojoules. You've spent less time near your ceiling than you planned.
The fix is dull and it works: even pacing, with the last repetition close to the first. I cover how to set that up in the episode and in the companion article on pacing VO2 max intervals.
The four-week block
Towards the end I look at an unusual VO2 max block that reported a 4.6% improvement in four weeks. It's an attention-grabbing number and I treat it that way: worth knowing about, worth testing carefully if you're an experienced rider with a clear plan, and not worth rebuilding your winter around on the strength of a single result. Whether it suits you depends on how much recovery you can afford and what the rest of your training looks like.
What to do with it
Pick one format. Pace it evenly. Do it once a week where you're freshest, and repeat it for four to six weeks before you judge it. If you want the broader context, VO2max intervals for cycling lays out the sessions and the evidence, and the best and worst cycling sessions shows where VO2 work fits against the rest of your week.
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