Slowing the recovery
The mistake that empties the session of its point. Jog at 50% instead of running at 70% and your breathing settles between efforts. Rozenek and colleagues measured a clearly lower share of VO₂max in a 15/15 with slow recoveries.
RunningVO2maxIntermediate 47 min
15/15 running intervals (3 × 12)
15/15 running intervals: three sets of 12 × 15 seconds at 100% of your vVO₂max, with 15 seconds of steady running at 70% between reps. 9 minutes at that speed in a 47-minute session. The efforts are short; the hard part is never easing off on the recoveries.
Session profile74 blocks
At a glance
- Duration
- 47 min
- Main target
- 100%vVO₂max5 Z5 · VO₂max
- Time at target
- 9 min
- Estimated load
- 48
- Structure
- 3 × (12 × 15″ at 100% vVO₂max, 15″ recovery), 2′30 between sets
- Level
- Intermediate
- Equipment
- GPS watch · Track (optional)
The session in detail
vVO₂max
Time in zone
- Z125 min
- Z213 min
- Z59 min
What the targets mean
| Target | Zone | Time | vVO₂max 14 km/h | vVO₂max 16 km/h | vVO₂max 18 km/h |
|---|---|---|---|---|---|
| 100% | 5VO₂max | 9 min | 4:17/km 58 m per rep | 3:45/km 67 m per rep | 3:20/km 75 m per rep |
| 70% | 2Base endurance | 8 min | 6:07/km 41 m per rep | 5:21/km 47 m per rep | 4:46/km 53 m per rep |
| 25–35% | 1Recovery | 5 min | 17:09/km–12:15/km 146–204 m per rep | 15:00/km–10:43/km 167–233 m per rep | 13:20/km–9:31/km 188–263 m per rep |
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What this session is for
The 15/15 trains your vVO₂max, your aerobic power, with efforts so short they cost your legs very little. The trick isn't in the 15 fast seconds but in the 15 that follow: at 70% of vVO₂max you're still running, and your breathing doesn't have time to settle. From one rep to the next, your oxygen uptake stays high.
That's what Billat's 2001 study showed: at the same average intensity, a 15/15 at 100% with recoveries at 70% gave roughly twice as much time at VO₂max as one run faster on the effort and slower on the recovery. Running faster on the effort and then recovering more slowly throws away most of what makes the format work.
Who it's for
Runners who've already done a few short-interval sessions, a 30/30 for instance, and who prefer short reps to long efforts. It also works when you're coming back to training, with fewer sets, and it's widely used in team sports. You need a recently measured vVO₂max: over 15 seconds, being out by a km/h or two changes the whole session.
When to do it
One vVO₂max session a week is usually enough: the 15/15 can alternate with a 30/30 or a 1-minute session from week to week. Put it after an easy day or a rest day, never the day before a long run, another hard session or a race. To progress, add a fourth set while keeping the 100%/70% pairing, then move on to longer efforts.
How to do it
Warm-up
15 min10′ at 60–65% vVO₂max + 5′ at 66–72% vVO₂max
10 minutes of easy running at 60–65% of vVO₂max, then 5 minutes at 66–72%. Meanwhile, set out your cones: at 16 km/h, one at 67 metres for the effort, then another 47 metres further on for the recovery.
Main set
22 min3 × (12 × 15″ at 100% vVO₂max, 15″ recovery), 2′30 between sets
Three sets of 12 × 15 seconds at 100% of your vVO₂max, each followed by 15 seconds at 70%, with 2 min 30 of walking between sets. At 16 km/h that's 67 metres in 15 seconds at 3:45/km, then 47 metres of recovery at 5:21/km. The recovery isn't a gentle jog: it's steady running, and it's what makes the session. Don't go faster than your vVO₂max on the efforts, even if it feels slow at first.
Cool-down
10 min10′ at 55–65% vVO₂max
10 minutes of very easy running at 55–65% of vVO₂max. If you couldn't hold the 70% recovery to the end of the last set, make a note: next time, drop a set rather than slowing the recovery.
How it should feel
The 15-second efforts feel almost easy. The recoveries, though, never let you catch your breath, and by the second set the fatigue settles into your legs. Thirty-six reps: counting them becomes an event in itself. Guide: 7 out of 10.
Common mistakes
Running faster than your vVO₂max
Over 15 seconds it's easy to end up sprinting. In Billat's study, running faster than vVO₂max on the efforts halved the time spent at VO₂max. The right speed is your vVO₂max, no more, from the first rep to the last.
Running without markers
Over such a short time, your watch can't give a reliable pace. Put out cones at your distances, or use the markings on a track: it's the only way to hold the right speed.
Mixing it up with the supramaximal 15/15
Another 15/15 is run at 115–120% of vVO₂max with walked recoveries: that's a speed session, not a vVO₂max one. Same notation, different effect.
When to skip it
Before you start
- A recently measured vVO₂max: every distance depends on it.
- A few short-interval sessions already done, such as a 30/30.
Skip it if
- You have pain or a recent injury, especially in your calves or hamstrings: the repeated accelerations load them.
- A long run, another hard session or a race is planned for the next day.
Origin and evidence
Level of evidence
Moderate evidenceLevel 3 of 5
Billat and colleagues (Int J Sports Med 2001) compared three 15/15s with the same average intensity, about 85% of vVO₂max: 90/80, 100/70 and 110/60%, in 7 runners aged around 52 (mean vVO₂max 15.9 km/h). The 90/80 and 100/70 formats gave about 14 minutes at VO₂max, against 7 for the 110/60; the authors conclude you shouldn't go above 100% of vVO₂max. Helgerud and colleagues (Med Sci Sports Exerc 2007), in 40 moderately trained men over 8 weeks at 3 sessions a week with equal work: VO₂max +5.5% with a 15/15 (47 reps, heart-rate targets) against +7.2% with 4 × 4 minutes. Conversely, Rozenek and colleagues (J Strength Cond Res 2007) measured a low fraction of VO₂max (71.6%) in a 15/15 at 100% with recovery at 50%. The 100%/70% pairing comes from Billat; the 3 × 12 split, the walking breaks, the warm-up and the cool-down are Wattson choices. Bottom line: good evidence on the immediate response, moderate on training gains.
Sources
- Billat V. L. et al., Int J Sports Med 2001;22(3):201-208
- Helgerud J. et al., Med Sci Sports Exerc 2007;39(4):665-671
- Rozenek R. et al., J Strength Cond Res 2007;21(1):188-192
Frequently asked questions
How fast should you run 15/15 intervals?
At 100% of your vVO₂max on the effort and 70% on the recovery. At 14 km/h that's 58 metres in 15 seconds, then 41 metres of recovery; at 16 km/h, 67 then 47 metres; at 18 km/h, 75 then 53 metres. Put cones at those distances: over 15 seconds, they're more reliable than your watch.
Active or passive recovery in a 15/15?
Active, and brisk: 70% of vVO₂max, proper running, not a shuffle or a walk. That recovery keeps your breathing high and is what makes a 15/15 worthwhile. Only the 2 min 30 breaks between sets are walked.
15/15 or 30/30: which should I choose?
The 30/30 is simpler to start with: 30 seconds gives you time to feel the pace, and the recovery is a jog. The 15/15 is choppier and needs a faster recovery, at 70%. Both target vVO₂max; you can alternate them from one week to the next.
Is a 15/15 as effective as 4 × 4 minutes?
In Helgerud's 2007 study, over 8 weeks and with equal work, a 15/15 raised VO₂max a little less than 4 × 4 minutes. Both worked. The 15/15 remains a good option if long efforts don't suit you, or simply for variety.
How many sets of 15/15 should a beginner do?
Two sets of 12 if the format is new to you, then three once the 70% recovery holds to the end. Always keep the 100%/70% pairing: better to do fewer sets than to slow the recovery.
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