Ten weeks of 30/15 intervals raised trained cyclists’ VO2max (the most oxygen your body can use per minute) by 8.7%. Five-minute repeats raised it 2.6%. Before you rebuild your plan around that, check the sample: nine riders in one group, seven in the other.
That’s the catch with the most famous cycling interval study. The result is real. It’s also thin.
What the Ronnestad 2015 Study Found
Norwegian researchers split trained cyclists into two groups. Both did two hard sessions a week for 10 weeks. The work time was matched at about 20 minutes per session. One group rode 30 seconds hard, 15 seconds easy, in three sets of 13 reps. The other rode long intervals of about 5 minutes.
The short-interval group gained more VO2max. They also gained more mean power in all-out efforts of 30 seconds, 5 minutes and 40 minutes. Effect sizes ran from 0.86 to 1.54, which are large.
In short: 30/15 won, but sixteen riders can’t speak for the whole sport.
Where the Evidence Gets Weaker
The follow-up in elite cyclists (VO2max about 73 ml/kg/min) ran 3 weeks with 18 riders. Twenty-minute power rose 4.7% with short intervals and fell 1.4% with long ones. But VO2max itself didn’t differ between groups.
Translation: 30/15 may buy you watts. It hasn’t reliably bought you a bigger engine.
Then there’s the 4x4. Helgerud’s famous protocol (4 minutes at 90-95% of max heart rate, 3 minutes of easy movement between) raised VO2max 7.2% in 8 weeks. But those were 40 moderately trained men, and they ran. Nobody has tested 30/15 against 4x4 head to head in age-group cyclists.
So the honest summary is this. 30/15 has cycling data in small groups. 4x4 has strong data in runners. Neither format has been proven best for you.
Time Near Max: Why Pacing Changes VO2max Gains
VO2max training works best when you spend lots of time near it. Researchers count minutes above 90% of VO2max. In one 9-week study of 22 cyclists, each extra minute above that line was linked to a small power gain of 0.02 W/kg.
Think of it like a kettle. You only get steam once the water is boiling. Short rests let the water dip, then the next burst brings it straight back up. The question is which format keeps it boiling longer.
A 2025 study put 12 well-trained riders through both formats. When both were set at a fixed power, 30/15 won easily. When riders paced themselves all-out, the gap closed.
The fixed-power result flatters 30/15. At the same watts, 5-minute repeats fade before you finish. Self-paced, long intervals catch up.
For you, that means your execution matters as much as your format. If you ride the long efforts as hard as you can sustain, you lose little.
30/15 vs 4x4 vs 4x5: Side by Side
Your FTP (Functional Threshold Power) is the highest power you can hold for about an hour. All targets below are based on it.
| Protocol | Structure | Hard time | Power target | Evidence |
|---|---|---|---|---|
| 30/15 | 3 sets of 13 x (30 s on, 15 s off), 3 min between sets | ~19.5 min | ~110% FTP on, ~50% off | Cyclists, n=16 and n=18 |
| 4x5 min | 5 min on, 2.5 min off | 20 min | Hard but finishable (coaching rule of thumb) | Cyclists, n=16 |
| 4x4 min | 4 min on, 3 min easy | 16 min | Hard but finishable (coaching rule of thumb) | Runners, n=40 |
The 30/15 sets total about 35 minutes with breaks. A 4x4 takes about 25. With a 15-minute warm-up and 10 minutes of cooldown, both fit inside 60 to 75 minutes.
Heart rate can’t guide any of these well. In the 9-week study, time above 90% of max heart rate was a poor predictor of time near VO2max. Use watts.
How to Run 30/15 Intervals Well
Warm up for 15 minutes. Add two 1-minute efforts at 110-120% of FTP. Start with 2 or 3 sets of 9 reps, not the full 3x13.
Ride the 15-second breaks as true soft-pedaling. Don’t stop. Stopping resets the effort and ruins the rhythm.
Set your trainer to ERG mode if you have one. It locks the watts for you, so you can’t drift into feel-based riding. On the road, use a long climb or a flat road with no traffic.
Don’t skip the soft-pedaling. Easy breaks keep the next effort possible.
How to Run 4x4 or 4x5 Well
Start the first rep a bit under your ceiling. The first minute should feel controlled. By rep three, you should be near your limit.
The mistake is starting too hard. You blow up in minute three, and the rest of the set turns into survival. Steady beats fast-then-slow.
Take the full recovery. Pedal easy between reps. Those 3 minutes are part of the dose.
A Real-World Example
Take a cyclist I’ll call Dan, 41, training for a 100-mile Gran Fondo on 7 hours a week. He did 4x4 minutes every Thursday by feel. His last rep always faded, and his 20-minute power had been stuck at 262 watts for two months.
Dan has a smart trainer but never used ERG mode. He switched to 2 sets of 9 reps at 110% of his FTP in ERG mode, twice a week for 6 weeks, then built to 3 sets. His workouts were shorter, at 45 minutes total, but he held power the whole way. This is a hypothetical example, not a measured result. It shows how many watts can leak out of a poorly paced set.
The lesson is not that 30/15 is magic. Dan stopped losing watts to fade.
Who Should Pick Which Format
Pick 30/15 if you have a smart trainer or a reliable power meter. Pick it if you hate staring at a clock for 4 minutes. Pick it when time is tight and you want a hard session that doesn’t wreck the next day.
Pick 4x4 or 4x5 if you ride outdoors by feel. Pick it if your power meter data is noisy. Pick it if you want a simple session you can repeat on a hilly road.
Either is fine for most amateurs. The best format is the one you’ll do well twice a week.
Both formats belong inside a plan that protects your easy days. Zone 2 base work and polarized vs pyramidal training cover that side. If you want the running view of long versus short efforts, read why long VO2max intervals beat short ones for runners. And for the 4x4 basics, see the 4x4 rule.
Once you’ve built the threshold side, sweet spot vs threshold training shows where VO2max work fits next.
AthleteOS programs the VO2max interval format for your FTP, training history and session length. You can start a free 14-day trial and see which one it picks for you.