What Are Micro-Bursts in Turbo Training? (Coach Guide)

Micro-bursts are short, repeated efforts, usually 10 to 30 seconds of hard riding, each followed by an equally short easy spin, performed in blocks of several minutes. The best known version is 15 seconds on, 15 seconds off. Because the recoveries are so brief, your aerobic system never fully unloads, so you accumulate far more time near your VO2 max than you could in one long interval.

Micro-bursts are one of the few sessions I rate more highly indoors than out, because the on and off pattern only stays clean when nothing interrupts it. Below is exactly what they are, why they work, and a session you can do tonight.

What is a micro-burst, exactly?

A micro-burst is a very short, very hard effort, typically lasting 10 to 30 seconds, paired with a recovery of similar length. You do not do one in isolation: you string them together into a block, for example 10 minutes of continuous 15s on, 15s off. The defining feature is the short, incomplete recovery. Unlike traditional intervals where you rest until you feel fresh, here the rest is deliberately too short to fully recover, which is what makes the session work.

People sometimes call these “30/15s”, “15/15s”, or “over-unders done fast”. The numbers describe the work and rest in seconds. A 40/20 (40 seconds hard, 20 easy) is a longer, grindier cousin; a true micro-burst sits at the shorter, punchier end.

The science: why short rests beat long ones

When you start a hard effort, your muscles draw on oxygen carried in the blood and stored in the muscle itself before your aerobic system fully ramps up. In a long single interval, you blow through that oxygen store early, then spend the rest of the effort in heavy anaerobic territory, which forces you to stop sooner.

With micro-bursts, the short hard effort dips into that oxygen store, and the short easy spin lets it partially refill before the next rep. The result is that your blood oxygen saturation stays high and relatively stable across the whole block. Your aerobic engine stays loaded near its ceiling for far longer than a single continuous effort would allow, while the brief surges still recruit fast-twitch fibres and develop your anaerobic capacity. In plain terms: you trick your body into spending more total minutes near VO2 max without the early blow-up.

This is why the format is so efficient. Ten minutes of 15s on/15s off might give you five full minutes of high-power work, where a single continuous effort at that power might fail after 90 seconds.

How hard should each burst be?

The on portion should sit well above your FTP, roughly 120 to 150 percent. It needs to feel hard from the first rep but be repeatable: if you cannot complete the whole block, you started too high. The off portion should be genuinely easy, around 40 to 50 percent of FTP, just enough to keep the chain moving. Resist the urge to keep pushing through the recovery; the easy spin is doing real work by refilling that oxygen store.

If you train without a power meter, use feel: the bursts should be at a “hard, controlled, slightly uncomfortable” effort you could not hold for more than a minute, and the recoveries should feel almost lazy. Heart rate is a poor guide here because the efforts are too short for it to respond meaningfully, so it tends to drift up steadily across the block rather than spiking with each rep. That steady climb is normal and expected.

A worked micro-burst session you can do tonight

Here is the session I prescribe most often for club riders building VO2 max in the off-season. It takes about 45 minutes including warm-up and cool-down.

1. Warm-up, 12 minutes. Start easy at around 50 percent of FTP and build gradually. In the last 4 minutes, add three 10-second openers at burst intensity with 50 seconds easy between them, to wake up the legs.
2. Block 1, 10 minutes. 15 seconds at 120-140 percent FTP, then 15 seconds easy at 40-50 percent. That is 20 reps. Hold a steady, repeatable power, do not sprint the first one.
3. Recovery, 5 minutes. Easy spinning at 50 percent FTP. Drink, reset, breathe.
4. Block 2, 10 minutes. Repeat block 1. Aim to match, not beat, your block 1 power.
5. Cool-down, 8 minutes. Easy, dropping to a gentle spin.

If you are new to this, do block 1 only the first two or three times, then add block 2. Strong riders can build to three blocks, but most people get the bulk of the benefit from two well-executed blocks rather than three ragged ones.

Why a turbo trainer is the ideal tool

These efforts live or die on rhythm, and that is exactly where a turbo beats the road. You can hit the burst and back off without freewheeling, without a junction interrupting rep 14, and without traffic forcing an early stop. A smart trainer in ERG mode will hold the target power for you, although for such short efforts I actually prefer resistance or level mode so my own surge defines the burst rather than the trainer chasing a number with a lag.

The catch is responsiveness. On a budget magnetic or fluid wheel-on trainer, the resistance and the read-out can lag the pedal stroke just enough to blur a 15-second effort. It still works, you just trust your legs more than the screen. If you are doing a lot of short, sharp work, this is one of the better arguments for a direct-drive smart trainer with fast power reporting. My power numbers here are cross-checked against Favero Assioma pedals, and on direct-drive units the trainer tracked the pedals closely at 100, 200 and 300 watts in steady state, but on wheel-on units the very short surges of a micro-burst showed more lag and smoothing, which is worth knowing before you obsess over the on-screen wattage.

If you want to understand why your numbers look the way they do during these sessions, my guide to understanding power readings on your turbo trainer explains accuracy and smoothing in detail, and understanding power smoothing in training apps covers why short bursts can read lower than they feel.

How micro-bursts fit your wider training

Micro-bursts are a VO2 max and repeatability tool, so they belong in a build phase, not every week of the year. They pair well with longer sweet-spot work on other days. If you are mapping out your week, see creating a weekly turbo training plan and read why you should do short intervals on a turbo trainer for the bigger case for this style of work. Newer riders should also read how long beginner turbo trainer sessions should be before adding a hard block like this.

To place these efforts correctly, it helps to know your zones: my guide to understanding training zones on the turbo shows where bursts sit relative to sweet spot and threshold.

The honest verdict

Micro-bursts are one of the highest-value sessions you can do indoors, and the 15s on/15s off format is the most beginner-friendly way into VO2 max work because no single effort lasts long enough to feel crushing. The trade-off is mental: keeping a clean rhythm for 20 reps takes focus, and on a laggy trainer the screen will not always reward you fairly. Do them on a responsive trainer if you can, judge the session by your last block rather than your first, and treat them as the hard day they are. Done twice a week in a build phase, they will lift your top end faster than almost anything else in the indoor toolbox.