Here's something that should bother you more than it does. Two groups of trained cyclists can do the same amount of hard interval work, and only one of them improves its VO2 max.
I open this week's episode with exactly that finding, because it changes the question. We spend a lot of time arguing about which interval session is best: 4x4, 4x8, 30/15, the Norwegian this, the Rønnestad that. The more useful question is what you actually did inside the session you chose.
What the format research says
Let's start with what's on the table, because it's more mixed than the internet lets on.
In a seven-week study of trained recreational cyclists, groups did 4x4, 4x8 or 4x16 minutes twice a week alongside low-intensity training. All three improved. The 4x8 group had the largest average gain across the reported measures. Small groups, one study, seven weeks.
In a 2015 trial, 16 cyclists did short or long intervals twice a week for ten weeks. The short-interval group, on 30 seconds on and 15 off, improved VO2 max and several performance measures more than the long-interval group. Nine riders against seven.
Then the 2025 crossover with 12 well-trained cyclists and triathletes. When the intensity was fixed, 30/15 produced more time above 90% of VO2 max than 6x5 minutes. When both were ridden as hard as the riders could manage, the difference went away.
And a 2026 review of 239 HIIT protocols found that longer work intervals generally produced more time near VO2 max, while the authors were careful to say the link between that time and long-term adaptation is still unsettled.
So which is best? The answer is that it depends on how the intensity is controlled. That's not a cop-out. It's the finding.
Why pacing is the thing
Think about what happens in a typical 4x8 on a Tuesday evening. You're keen. The first repetition goes out at a power you'd struggle to hold for five minutes, never mind eight. By minute five your legs have gone. The second repetition is a survival exercise. The third is half-hearted, and the fourth you finish because you've said you would.
On paper you did 4x8. In practice you spent one repetition in the zone that moves the needle and three below it. The data says "VO2 max session." Your body says otherwise.
An evenly paced session looks boring by comparison. Four repetitions at close to the same power, each one a bit uncomfortable and none of them a disaster. That's the one that delivers what the plan intended.
A simple pacing check
I'm not going to hand you a formula, because the right number depends on your testing and your history. Use this instead.
Ride the first repetition at a power you believe you could repeat for every repetition in the set. Look at the last one afterwards. If it's close to the first, you paced it well. If it's far below, the first was too hard, or the recoveries were too short, or you started tired.
Do that for a few weeks and your numbers will tell you what your target actually is. It's a better method than a percentage from a spreadsheet, as I said in the VO2max intervals guide: the percentage is not the athlete.
The four-week block
There's an unusual VO2 max block in the episode that reported a 4.6% improvement in four weeks. I'm sceptical in the good sense: interested, and unwilling to treat one result as a promise. If you want to try something like it, do it as a planned block with the rest of your week easy, not on top of your usual training. And ask what you'll give up to recover from it. The block that works is the one you can absorb.
How often
One session a week is a sensible place to start. Plenty of trials use two, and some riders handle that well. But a VO2 session is expensive to recover from, so the question isn't "how many can I fit in?" It's "what does the week around it look like?" If you do a hard session on Tuesday and a group ride on Thursday that turns into a race, you've got two VO2 sessions and no plan.
The best and worst cycling sessions walks through how to score a session against the cost of recovering from it, which is the same logic.
Fatigue changes the answer
Everything above assumes you start the session fresh. You don't get the same stimulus on tired legs. If you're carrying a hard weekend, a poor night or a heavy week at work, the first repetition will tell you. If the power you'd normally pick for the first one feels like a maximal effort, stop the session, or turn it into something easier. The best VO2 session you can do that day might be a good endurance ride.
For riders over 40, that matters even more, and VO2 max workouts for cyclists over 40 covers how to adjust the dose.
What to take from this
Choose one format. Pace it evenly. Do it where you're freshest, and repeat it for four to six weeks before you judge it. If the numbers don't move after a fair trial, change one thing: the format, the frequency or the recovery around it.
And keep VO2 max in perspective. It's one input. Your sustainable power, your fuelling and your durability decide a lot more of what happens on the day.
Want a second pair of eyes on your sessions? Come and share one in the free Roadman community on Skool: https://www.skool.com/roadmancycling
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