Sprinting every effort
Above roughly one and a half times your FTP, the session changes character and mainly trains anaerobic capacity. You blow up mid-block, and VO₂max takes a back seat. Stay in the range.
CyclingVO2maxIntermediate 64 min
15/15 VO₂max intervals on the bike (3 × 10 min)
15/15 intervals for cycling: three 10-minute blocks alternating 15 seconds at 125–140% of FTP and 15 seconds easy, with 5 minutes' recovery between blocks. A 64-minute session. A way into VO₂max work for intermediate riders.
Session profile123 blocks
At a glance
- Duration
- 64 min
- Main target
- 125–140%FTP6 Z6 · Anaerobic
- Time at target
- 15 min
- Estimated load
- 68
- Structure
- 3 × (20 × 15″ at 125–140% FTP, 15″ recovery), 5′ between sets
- Level
- Intermediate
- Equipment
- Power meter · Smart trainer
The session in detail
FTP
Time in zone
- Z139 min
- Z210 min
- Z615 min
What the targets mean
| Target | Zone | Time | FTP 200 W | FTP 250 W | FTP 300 W |
|---|---|---|---|---|---|
| 125–140% | 6Anaerobic | 15 min | 250–280 W | 313–350 W | 375–420 W |
| 50–60% | 1Recovery | 14 min | 100–120 W | 125–150 W | 150–180 W |
| 45–55% | 1Recovery | 15 min | 90–110 W | 113–138 W | 135–165 W |
Put this session on your bike computer or watch
Wattson works out the targets from your FTP and sends the session to your Garmin, Wahoo, Coros or Suunto.
What this session is for
Fifteen seconds is too short for your legs to run out, even above your maximal aerobic power. But fifteen seconds of recovery is also too short for your heart rate and breathing to come down. Block after block, they stay high, and that's what trains your VO₂max: your heart and lungs run flat out, while your muscles only deal with brief efforts.
The format was first studied in running, by Véronique Billat and then by Jan Helgerud's group, who saw it raise VO₂max. On the bike, training platforms popularised it as 'microbursts', with no dedicated study found. The principle is sound; the cycling version rests on practice.
Who it's for
Intermediate riders with a recent FTP, especially those who struggle with long, hard efforts: 15 seconds always feels doable. It's a good first VO₂max session before 30/30s or Rønnestad's 30/15.
When to do it
One or two VO₂max sessions a week at most, with an easy day between hard sessions and none the day before a race. To progress, add a fourth block or extend the blocks to 12 minutes, then move to longer efforts such as the 30/15 or 40/20.
How to do it
Warm-up
15 min5′ at 45–55% FTP + 5′ at 56–65% FTP + 5′ at 66–75% FTP
15 minutes building from 45 to 75% of FTP, in 5-minute steps. Two or three short pick-ups at the end get your legs ready for the surges.
Main set
39 min3 × (20 × 15″ at 125–140% FTP, 15″ recovery), 5′ between sets
Three 10-minute blocks, with 5 easy minutes at 45–55% between them. Each block runs 20 times through 15 seconds at 125–140% of FTP, or 313 to 350 W for an FTP of 250 W, and 15 seconds at 50–60%. Hold an even power across the block: the twentieth rep should look like the first. Stay seated, spin a high cadence and pick up smoothly. On a turbo, ERG mode takes 2 to 4 seconds to reach the target, a big slice of the effort: resistance or slope mode is often a better fit.
Cool-down
10 min10′ at 40–55% FTP
10 very easy minutes at 40–55% of FTP. If your power dropped off noticeably at the end of the blocks, aim for the bottom of the range next time.
How it should feel
Each effort feels short, almost too easy at first. Then your breathing climbs, and around the fifteenth rep the 15 seconds of recovery are gone before you've caught your breath. The end of each block is hard, but never the feeling of dying on the spot. Guide: 8 out of 10.
Common mistakes
Letting ERG mode handle the surges
Over 15 seconds, the turbo's reaction time eats the effort: you hit the target just as it's time to ease off. Switch to resistance or slope mode and surge through your gears.
Freewheeling during recovery
The 15 seconds at 50–60% are pedalled. Stopping completely makes the next surge more brutal and lets your heart rate drop further than it should.
Treating 15/15s as sprints
Depending on the source, 'microbursts' means this format or any short on-off work. This one targets VO₂max, not sprint power: evenness first.
When to skip it
Before you start
- A recently measured FTP.
- A few weeks of regular endurance riding and one or two threshold sessions.
Skip it if
- You're tired, or a race or long ride is planned for the next day.
- Your road has junctions or traffic: 60 surges in 30 minutes need a clear route or a turbo.
- You have doubts about your health: get medical advice before near-maximal efforts.
Origin and evidence
Level of evidence
Moderate evidenceLevel 3 of 5
Helgerud et al. (Med Sci Sports Exerc 2007): in running, a 15/15 at 90–95% of maximum heart rate, with recovery at 70%, raised VO₂max by 5.5% in moderately trained men. Billat et al. (Int J Sports Med 2001) also studied the 15/15 in runners. On the bike, the format is well established on training platforms ('microbursts'), which use about 140% of FTP, with no dedicated study found. Above roughly 150% of FTP, the session mainly trains anaerobic capacity. The 125–140% range, the number of blocks, the 5-minute recoveries, the warm-up and the cool-down are Wattson choices. Bottom line: good evidence for the principle in running, moderate for the cycling version.
Sources
- Helgerud J. et al., Med Sci Sports Exerc 2007;39(4):665-671
- Billat V. L. et al., Int J Sports Med 2001;22(3):201-208
Frequently asked questions
What power should I ride 15/15 intervals at?
125–140% of FTP for the 15-second efforts, and 50–60% for the recovery. For an FTP of 200 W: 250 to 280 W. For 250 W: 313 to 350 W. For 300 W: 375 to 420 W. Start at the bottom of the range and go up if the ends of the blocks stay even.
15/15 or 30/30 on the bike: which should I choose?
The 15/15 is ridden a bit higher but with shorter efforts, which often makes it more bearable for riders who hate long efforts. The 30/30 asks you to hold on twice as long at a slightly lower intensity. Both lead nicely to Rønnestad's 30/15.
Are cycling 15/15s the same as running 15/15s?
The principle is the same: 15-second efforts and recoveries to keep your heart rate high. But the reference studies are in running, at vVO₂max pace and by heart rate. On the bike, the % of FTP intensity comes from training platforms' practice, not from a cycling study.
Which turbo trainer mode should I use for 15/15s?
Resistance or slope mode, surging through your gears. In ERG mode, resistance takes 2 to 4 seconds to reach the target, a big share of a 15-second effort. If you stay in ERG, expect a slightly lower average power than planned, and don't make up for it by forcing.
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