Micro-burst intervals — such as 40/20s, tabatas, on/offs — have gained traction among the endurance community as one of the most effective training sessions for improving top-end fitness. Why are these types of intervals so effective?
Micro-Burst Interval Training Helps Increase VO2Max
Research has shown that micro-burst intervals (aka, burst training) can allow you to maintain an effort close to VO2max for longer than traditional intervals (e.g., 4×5 minutes w/ 4 minutes of rest). A hallmark study showed that runners who performed a 30/30 protocol to exhaustion, (i.e., 30 seconds at 100% of VO2max velocity alternating with 30 seconds at 50% of VO2max velocity) allowed the test subjects to hold maximal oxygen uptake for more than twice as long than those who ran continuously at 100% VO2max velocity until exhaustion.
When performing several intervals during a workout, the amount of time that one can accrue at VO2max would be substantially higher with micro-burst intervals than with traditional VO2max sets. Cumulative time close to VO2max has been shown to be one of the most important factors of effective HIIT workouts.
Micro-Burst Training Helps Increase Power and Velocity
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 VO2max elicits the same amount of gene transcription as a session at 100% of VO2max 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% VO2max session had a higher overall muscle activation and thus a higher absolute gene transcription relative to the 73% session, meaning that higher power output will cause aerobic improvement in a greater number of muscle fibers.
When performing micro-burst intervals, one can accumulate more time at a much higher power or velocity than with traditional intervals. For example, an athlete might have a 6-minute best power at 400w. If they perform 3×10-minute 40/20 intervals targeting 400w during the “on” portion, they can accumulate a total of 20 minutes at 400w during the session and have a higher total muscle activation.
If the same individual does a 6×5-minute “VO2max session,” they would likely only be able to hit around 90% of their 6-minute best power across all intervals (approximately 360w). It would be very difficult to accumulate 20 minutes at 400w with any other type of VO2max interval session than the 40/20 workout.
Micro-Burst Training Increases Muscle Activation
A higher muscle activation means that your muscles will be prepared to hold higher powers/velocities. HIIT workouts are very effective because you can improve both your aerobic capacity and musculature at the same time.
Shorter intervals (i.e., those less than three minutes) are great for training high power outputs but not as effective at training your aerobic capacity, as they are too short to allow you to reach VO2max. Compared to longer VO2max style intervals (e.g., ones that are 5-6 minutes long), micro-burst intervals will allow you to stay close to your VO2max for a longer period of time and spend more time at a higher power output. The result is that both your aerobic system and your musculature will be able to handle more.
The nature of HIIT training allows for a wide range of customization. You will be able to 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. Like any good hot sauce, interval training will give you a good kick — but more isn’t always better!
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/
