The Magic Interval? What the Research Says About 30/15s

GCN recently called one interval session "magic": a set of 30 seconds hard and 15 seconds easy, repeated 13 times, done three times. The presenter found it both more effective and easier to get through than classic 5-minute intervals, and interviewed the Norwegian scientist who designed it, Professor Bent Rønnestad^[1]. It's a good video and worth watching.

"Magic" is a big claim, so we went back to the papers behind it. The headline numbers mostly check out. But some are weaker than they sound, a newer study from the same lab adds an important catch, and Rønnestad himself says plainly that it isn't magic.

Summary

The session: 3 sets of 13 × 30 seconds hard / 15 seconds easy, with 3 minutes between sets: about 20 minutes of interval work.
The idea behind it: fitness gains track how long you spend near your maximum oxygen uptake (VO₂max). The 15-second recoveries are too short for breathing and heart rate to settle, so you stay near the top for longer at a power you can tolerate.
The training trials favour it. Over 10 weeks in trained cyclists, 30/15s raised VO₂max by 8.7% against 2.6% for effort-matched 4 × 5-minute intervals. A 3-week study in elite riders also favoured them.
But not every number is as strong as quoted. The often-repeated 12% gain in 40-minute power was only a statistical trend against the long-interval group, and both trials are small and from one lab.
The catch: when riders go all-out in both formats, the advantage in time near VO₂max disappears.
Our verdict: an excellent, well-supported VO₂max session that many riders find easier to get through. Not magic. Use it as one of your two hard sessions a week, alongside longer intervals.

1What a 30/15 actually is

The full session from the research is three sets of 13 repetitions of 30 seconds hard and 15 seconds easy, with 3 minutes of easy riding between sets^[2][3]. Each set lasts 9½ minutes, so the whole block is about 20 minutes of interval work plus recoveries. The odd 13 has no deeper meaning: Rønnestad explains in the GCN interview that it was simply the number that got the session closest to 20 minutes, to match the long-interval sessions he wanted to compare it with^[1].

The recoveries are ridden at about half the work power^[2], so this isn't sprinting. In the studies the riders went as hard as they could sustain for the whole session. For everyone else, Rønnestad's rule of thumb is to ride the 30-second efforts at roughly the power you could hold for 5 minutes, and the 15-second recoveries at about half that^[1].

The idea isn't new. Rønnestad says he took the 2:1 work-to-rest ratio from a classic 1960 study of intermittent running by Christensen, Hedman and Saltin^[1][10], and wanted something fairer to compare with 5-minute intervals than the all-out 30-second sprints with long recoveries that dominated research at the time.

2Why it might work: time near VO₂max

The theory is that one of the strongest stimuli for aerobic fitness is time spent close to your maximum oxygen uptake, usually measured as time above 90% of VO₂max. The best direct evidence came in 2024: Odden and colleagues measured oxygen uptake in every interval session of a 9-week programme in 22 well-trained cyclists. The riders who worked at a higher fraction of their VO₂max in those sessions improved the most, in VO₂max, peak power and a combined performance index^[5]. Notably, those sessions were 5 × 8-minute intervals, not 30/15s, so this supports the principle rather than the specific session.

Short intervals can raise that time because oxygen uptake doesn't fall much in 15 seconds. When cyclists rode to exhaustion at the same power in different formats, the 30-second version let them keep going longer and spend more time above 90% of VO₂max than longer work periods^[4]. And at the same average power, 30/15s gave more time above 90% of VO₂max than steady riding (about 19 minutes against 15 across six 8-minute blocks), in women as well as men^[6].

3What the training trials found

Trained cyclists, 10 weeks

In the first trial, 16 trained male cyclists added two interval sessions a week to their normal easy training for 10 weeks. One group did 30/15s, the other 4 × 5-minute intervals with 2½-minute recoveries. Both were told to ride as hard as they could sustain, and both rated their sessions the same, around 17–18 on the 6–20 effort scale^[2].

Figure 1. Effort-matched short versus long intervals, twice a week for 10 weeks in trained cyclists. *Difference between groups statistically significant; the others were trends or moderate effect sizes only. Data from Rønnestad et al., 2015^[2].

The 30/15 group improved more on everything measured. But it matters which differences were statistically solid. VO₂max (+8.7% against +2.6%), peak aerobic power (+8.5% against +1.6%) and the average across all the performance tests (+10.0% against +3.1%) were clearly in favour of the 30/15s^[2]. The headline that gets quoted most, a 12% rise in 40-minute power against 4%, was a strong trend but fell just short of significance between the groups (p = 0.056). The 5-minute result, +8% against +3%, wasn't statistically different either^[2]. With only nine and seven riders, that's not surprising, and the direction is consistent. It just isn't as conclusive as it sounds.

Elite cyclists, 3 weeks

The follow-up tested the same session in 18 elite cyclists with a VO₂max around 73 mL/kg/min: nine sessions in 3 weeks, against effort-matched 4 × 5 minutes. The 30/15 group improved peak aerobic power (+3.7% against −0.3%), power at a blood lactate of 4 mmol/L (+2.0% against −2.8%) and 20-minute performance (+4.7% against −1.4%). VO₂max itself didn't differ between the groups^[3]. At elite level, a few percent in three weeks is a large effect. The GCN video's "3% at threshold" refers to the share of their VO₂max riders could use at that lactate level, which rose by 3 percentage points^[3].

4The catch

In both trials, everyone was told to ride as hard as they could. That raises a fair question: are 30/15s better, or were the long-interval groups just ridden too conservatively? Evenly paced 5-minute efforts are hard to judge, and it's easy to start cautiously.

A 2025 study, with Rønnestad as a co-author, tested exactly that. Twelve well-trained cyclists and triathletes did 3 × 13 × 30/15 and 6 × 5 minutes twice each: once at fixed powers, once all-out and self-paced^[7].

Figure 2. Twelve well-trained cyclists and triathletes rode both sessions twice. At fixed powers, 30/15s gave more than twice the time near VO₂max. All-out, both sessions gave a lot, and the difference was no longer significant. Data from Appelhans et al., 2025^[7].

At fixed powers, 30/15s won clearly. Ridden all-out, both sessions produced far more time near VO₂max, and the difference between them disappeared^[7]. In trained runners, a set of 30-second efforts even gave less time above 90% of VO₂max than 3-minute intervals^[8]. And long intervals can be improved too: starting each one harder and easing back gave cyclists more time above 90% of VO₂max than evenly paced efforts at the same average power, without feeling any harder^[9].

So the honest reading is that 30/15s make it easier to spend a lot of time near VO₂max, because you never have to hold a horrible effort for long. That's a real advantage, and probably why riders find them easier to get through. But a determined rider can get a similar stimulus from well-paced long intervals.

Rønnestad himself says as much in the GCN interview: 30/15s are one good tool among several, longer intervals still belong in a plan (especially for time trials), and the best interval session is the one you'll actually do^[1].

5How to ride them

Power: about your best 5-minute power for the 30-second efforts, and roughly half that for the 15-second recoveries^[1]. The first few reps should feel too easy. By the end of a set they won't.
Volume: the research session is 3 sets of 13 with 3 minutes between sets. If that's new to you, start with two sets and build up.
Frequency: count it as one of your two hard sessions a week, at least 48 hours from the other one, with genuinely easy riding in between.
Mix it up: alternate with longer intervals (4–8 minutes), especially if your goal is a long, steady effort such as a time trial.
On a smart trainer: ERG mode takes a second or two to change resistance, so the first moments of each 15-second recovery can feel heavy. Keep your cadence up through the switch and the trainer catches up.

Build it in JustZone2. The Interval Builder on the web has a 30/15 preset that loads the full research session: 3 sets of 13 × 30 s / 15 s, with 3 minutes between sets. Sets are coming to the app in its next update, under Ride in sets in the session editor. Until then, build one set of 13 in the app and repeat it with an easy spin in between.

6Limitations

Both training trials are small (16 and 18 cyclists), short (10 and 3 weeks), mostly male, and come from the research group that designed the session. In both, the long-interval comparison was evenly paced, and later work suggests better-paced long intervals close much of the gap in time near VO₂max. Time above 90% of VO₂max is a strong candidate for what drives the gains, but it has only been linked to improvements in one training study, which used 8-minute intervals. We found no long-term trial of 30/15s in recreational or masters riders. The practical power guidance comes from Rønnestad's interview rather than a published protocol.

Related reading. How many hard sessions a week, and why recovery matters: How Many Hard Sessions a Week? Why the easy days count too: The Rise and Reassessment of Zone 2.

References

GCN (Global Cycling Network). This training session is so good it blew my mind [video, including an interview with Professor Bent Rønnestad]. YouTube, 2026. youtube.com/watch?v=NSzAYj43wK8
Rønnestad BR, Hansen J, Vegge G, Tønnessen E, Slettaløkken G. Short intervals induce superior training adaptations compared with long intervals in cyclists – an effort-matched approach. Scand J Med Sci Sports. 2015;25:143–151.
Rønnestad BR, Hansen J, Nygaard H, Lundby C. Superior performance improvements in elite cyclists following short-interval vs effort-matched long-interval training. Scand J Med Sci Sports. 2020;30:849–857.
Rønnestad BR, Hansen J. Optimizing interval training at power output associated with peak oxygen uptake in well-trained cyclists. J Strength Cond Res. 2016;30:999–1006.
Odden I, Nymoen L, Urianstad T, et al. The higher the fraction of maximal oxygen uptake is during interval training, the greater is the cycling performance gain. Eur J Sport Sci. 2024;24:1583–1596.
Urianstad T, Hamarsland H, Odden I, et al. The higher oxygen consumption during multiple short intervals is sex-independent and not influenced by skeletal muscle characteristics in well-trained cyclists. Eur J Sport Sci. 2024;24:1614–1626.
Appelhans D, Rønnestad B, Skovereng K. Multiple short intervals induce longer time above 90% of maximal oxygen consumption than long intervals when matched by similar fixed intensity, but not during self-paced cycling ergometry. Int J Sports Physiol Perform. 2025;20:848–855.
Fleckenstein D, Braunstein H, Walter N. Faster intervals, faster recoveries – intensified short VO₂max running intervals are inferior to traditional long intervals in terms of time spent above 90% VO₂max. Front Sports Act Living. 2024;6:1507957.
Bossi AH, Mesquida C, Passfield L, Rønnestad BR, Hopker JG. Optimizing interval training through power-output variation within the work intervals. Int J Sports Physiol Perform. 2020;15:982–989.
Christensen EH, Hedman R, Saltin B. Intermittent and continuous running. Acta Physiol Scand. 1960;50:269–286.