We all know that HIIT workouts are highly effective, but what types are best? For endurance athletes looking to improve top-end fitness, micro-burst intervals are worth the effort.
Micro-burst intervals, such as 40/20s, Tabatas and other “on/off” workouts, have gained traction in the endurance community as one of the most effective training sessions for improving VO2 max, power and velocity. But why are these short, spicy intervals so effective?
The answer comes down to how much time you can spend close to VO2 max, how much power or speed you can produce during the workout and how much muscle activation you can accumulate along the way.
What is micro-burst training?
Micro-burst training is a type of high-intensity interval training that alternates short, hard efforts with short recoveries. Common examples include 30 seconds hard/30 seconds easy, 40 seconds hard/20 seconds easy and Tabata-style intervals.
The goal is not to sprint every repetition until you fall apart. The goal is to spend more total time at a very high intensity than you could during one long, continuous effort.
In other words, micro-bursts let you take a VO2 max workout, chop it into bite-sized pieces and keep the heat turned up for longer.
Micro-burst interval training helps increase VO2 max
Research has shown that micro-burst intervals, also known as burst training or short-short intervals, can allow you to maintain an effort close to VO2 max for longer than traditional intervals, such as 4×5 minutes with 4 minutes of rest.
A hallmark study showed that runners who performed a 30/30 protocol to exhaustion (30 seconds at 100% of VO2 max velocity alternating with 30 seconds at 50% of VO2 max velocity) were able to hold maximal oxygen uptake for more than twice as long as those who ran continuously at 100% VO2 max velocity until exhaustion.
When performing several intervals during a workout, the amount of time you can accrue at VO2 max can be substantially higher with micro-burst intervals than with traditional VO2 max sets. That matters because cumulative time close to VO2 max has been shown to be one of the most important factors of effective HIIT workouts. And the more quality time you can spend near VO2 max, the more potent the workout becomes.
Micro-burst training helps increase power and velocity
Micro-bursts are not only effective because they keep your aerobic system working hard. They also allow you to accumulate more time at a much higher power or velocity than you typically could with traditional interval formats.
Research has shown that the amount of gene transcription for the creation of new mitochondria is proportional to the amount of muscle activation during a training bout. One study concluded that an interval session at 73% of VO2 max can elicit the same amount of gene transcription as a session at 100% of VO2 max when expressed relative to total muscle activation.
Put more simply, the amount of gene transcription for each muscle fiber activated was the same for both workouts. However, the 100% VO2 max session had a higher overall muscle activation and therefore a higher absolute gene transcription relative to the 73% session. That means a higher power output can cause aerobic improvement in a greater number of muscle fibers.
This is where micro-bursts get interesting.
For example, an athlete might have a 6-minute best power of 400 watts. If they perform 3×10-minute 40/20 intervals targeting 400 watts during the “on” portion, they can accumulate a total of 20 minutes at 400 watts during the session and have a higher total muscle activation.
If the same athlete does a 6×5-minute VO2 max session, they would likely only be able to hit around 90% of their 6-minute best power across all intervals, or approximately 360 watts. It would be very difficult to accumulate 20 minutes at 400 watts with any other type of VO2 max interval session than the 40/20 workout.
That is the beauty of micro-bursts: they let you touch very high outputs again and again without asking you to hold the whole thing in one uninterrupted block.
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Micro-burst training increases muscle activation
Higher muscle activation means your muscles are being prepared to hold higher powers or velocities. This is one reason HIIT workouts are so effective: you can improve both your aerobic capacity and your musculature at the same time.
Shorter intervals, especially those less than three minutes, are great for training high power outputs. However, they are not always as effective at training aerobic capacity because they can be too short to allow you to reach VO2 max.
Compared to longer VO2 max-style intervals, such as 5- or 6-minute repeats, micro-burst intervals allow you to stay close to VO2 max for a longer period of time while also spending more time at a higher power output. The result is that both your aerobic system and your musculature learn to handle more.
Think of it as getting the aerobic burn of a VO2 max workout with the muscular snap of high-power intervals. Spicy, but effective.
Example micro-burst workouts
Micro-burst training can be customized based on your sport, fitness level and target event. Here are a few common examples:
30/30s:
30 seconds hard, 30 seconds easy. Repeat 8–12 times per set for 2–3 sets.
40/20s:
40 seconds hard, 20 seconds easy. Repeat for 8–10 minutes per set for 2–3 sets.
Tabata-style intervals:
20 seconds very hard, 10 seconds easy. Repeat 6–8 times per set.
These workouts can be prescribed by power, pace, heart rate or RPE, depending on the athlete and the sport. The key is to make the “on” portions hard enough to create the desired stimulus, but not so hard that the workout turns into survival mode after the first few reps.
Training Tip: Build and analyze micro-burst workouts in TrainingPeaks
Micro-burst intervals sound simple on paper, but once the workout starts, they can be hard to execute well. The transitions come quickly, the recoveries are short and it is easy to overshoot the early efforts before the workout has really begun to bite.
This is where TrainingPeaks Workout Builder can help. You or your coach can build structured 30/30, 40/20 or Tabata-style workouts ahead of time using targets for power, pace, heart rate or RPE. Then, instead of staring at your watch and trying to remember whether you are on rep 7 or rep 17, you can follow the workout directly on your device.
Afterward, use the completed workout file in TrainingPeaks to see how well you handled the session. Did your power or pace stay consistent during the “on” portions? Did your recoveries stay controlled? Did your heart rate continue to rise across the set, or did the wheels come off halfway through?
Analyze 360 gives you a clearer view of your workout, allowing you see heart rate, pace, and power, or other metrics all in one view. It also has automatic interval detection when using a power meter on your bike or your running shoes, making it easier to see when you start and stopped.
When to use micro-burst training
The nature of HIIT training allows for a wide range of customization. You can set target powers, velocities and interval durations that closely mimic your target event.
While micro-burst intervals are very effective, they are also very taxing and should only be done at specific times of the year. They work best when you are trying to improve VO2 max, top-end power, race-specific speed or your ability to repeat hard efforts.
For most endurance athletes, one micro-burst workout per week during a focused high-intensity block is plenty. These sessions should be balanced with aerobic endurance work, recovery and lower-intensity training.
Like any good hot sauce, interval training can give you a good kick, but more isn’t always better. Use the right amount at the right time, and micro-bursts can add exactly the spice your fitness needs.
References
Almquist, N.W. et al. (2020, April 30). Systemic and muscular responses to effort‐matched short intervals and long intervals in elite cyclists. Retrieved from https://pubmed.ncbi.nlm.nih.gov/32267032/
Billat, V.L. et al. (2000, January). Intermittent runs at the velocity associated with maximal oxygen uptake enables subjects to remain at maximal oxygen uptake for a longer time than intense but submaximal runs. Retrieved from https://link.springer.com/article/10.1007%2Fs004210050029
Edgett, B.A. et al. (2013, August 12). Dissociation of Increases in PGC-1α and Its Regulators from Exercise Intensity and Muscle Activation Following Acute Exercise. Retrieved from https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0071623
Faria, E.W. et al. (2012, September 23). The Science of Cycling. Retrieved from https://link.springer.com/article/10.2165%2F00007256-200535040-00002
Laursen, P.B. & Jenkins, D.G. (2002). The scientific basis for high-intensity interval training: optimising training programmes and maximising performance in highly trained endurance athletes. Retrieved from https://pubmed.ncbi.nlm.nih.gov/11772161/
Rønnestad, B.R. et al. (2014, January 1). Short intervals induce superior training adaptations compared with long intervals in cyclists – An effort-matched approach. Retrieved from https://onlinelibrary.wiley.com/doi/10.1111/sms.12165
Rønnestad, B.R. et al. (2020, February 5). Superior performance improvements in elite cyclists following short‐interval vs effort‐matched long‐interval training. Retrieved from https://pubmed.ncbi.nlm.nih.gov/31977120/
