High intensity interval training and time at VO2max

Toolbox: Interval training is popular in cycling and many other sports. High-intensity interval training (HIIT) involves short periods of hard efforts separated by low-intensity recovery runs. Let’s dig deeper into how these workouts can help your physique.


High-intensity intervals are great for improving your body’s maximum ability to take in and process oxygen, which is often abbreviated as VO2max. When combined with adequate recovery, high-intensity workouts are a great way to boost your fitness. We reviewed an article a few months ago that Compare different high-density protocols.

It is especially important to make high-intensity training effective, since most athletes can only handle 2 or 3 high-intensity sessions per week. As we saw in my previous Toolbox article, sports scientists continue to investigate the relative importance of interval intensity, duration, and recovery to making HIIT training as effective as possible. In this month’s Toolbox article, I wanted to review some of the classic high-intensity workouts and how they paved the way for a new and effective high-intensity protocol.

Interval intensity versus duration

In 2013, Dr. Stephen Seiler and colleagues published one of the groundbreaking papers to advance our understanding of the role that intensity and duration play in improving performance (Seiler, et al., 2013). Their study compared high-intensity intervals of different lengths (4 x 4 minutes, 4 x 8 minutes, 4 x 16 minutes) to determine the most effective intervals for improving physical fitness. Their findings concluded that more time spent above 90% HRmax (4 x 8) was more effective than less time at a higher intensity (4 x 4 minutes), as well as a lower session rate of observed exertion (RPE). For this reason, the authors also stated that the 4×8 courses provided the “greatest pain gain.”

For those using Xert, we can model and compare the effectiveness of these three exercises using the Workout Designer. Based on the analysis on each exercise, the 4×8 workout yields more xert degree of stress (XSS), compared to the 4×4min and 4×16min protocols, so we can reasonably expect that—all else being equal—the 4×8 workout will lead to greater improvements in fitness over time.

Long periods versus short periods

Seiler’s results have been well received in the cycling community. In fact, you’ll still find some variations of the 4×8 minute workouts on most training platforms, including Xert! Another new paper that has advanced our understanding of the role of intensity versus duration was Dr. Ronstadt’s 2016 paper, which compared the effectiveness of long periods to short periods interspersed with short recall periods. For extended periods, they followed Dr. Zeller’s lead and used the 4 x 8 minute protocol. For the short interval protocol, they suggested 3 sets of 13 repetitions of a 30-second hard effort followed by a 15-second recovery.

The short-interval group experienced significant improvements in performance compared to the long-interval group, although there was no significant difference in total training volume. The authors suggested that short runs were a greater motivator for VO2max, as more time was spent at or above 90% of VO2max than long runs. This finding confirms findings by Seiler, who suggested that time accumulated above 90% of HRmax (VO2max) is a key motivator for training.

You can find a similar exercise to the Ronnestad protocol in the Xert exercise library: SMART – pain I’m used to. This XSS workout offers more than Seiler’s 4×8 workout, but it also performs at a higher difficulty level. Just like in the original study, these hard efforts should be done for 30 seconds with 15-second recoveries by feel. If you try this exercise on your own, use your gears and cadence to push yourself fully for 30 seconds before shifting into an easier gear and pedaling easy for 15 seconds to recover before the next effort.

Another case for short periods

Working with runners, Veronique Pellatt also recognized the importance of maximizing the training time spent near your VO2max. Similar to the Ronnestad group, Billat made use of short (30-second) intervals with evenly short recoveries to keep VO2 levels near maximum for relatively long periods. By doing this, runners were able to train above 90% VO2max for 2-3 times longer than a steady pace/effort (Billat, et al., 2000). Short recovery periods allow enough time for the muscles to recover from the all-out effort, but not long enough for your heart rate or VO2 levels to drop before the next repetition begins.

SMART – breakthrough (to the other side) Does Xert take 30-30 Pilates intervals. As with many of Xert’s all-encompassing SMART exercises, this mixed-position exercise has to be done by feel. For 30-30 efforts, you’ll need to use your gears and cadence to push yourself hard for 30 seconds before shifting into an easier gear to recover for 30 seconds before your next hard effort.

Put it all together

Based on the research we’ve reviewed so far, it appears that the effectiveness of high-intensity training is often determined by the amount of time an athlete spends near VO2max. If time at 90% VO2max is critical to improving our physical fitness, then we must improve our high-intensity training protocol to do two things:

  1. Accelerate the way we can reach 90% VO2max
  2. Continue to maintain 90% VO2max for as long as possible

There are several ways to address the first point. The first option, called “Fast Start” intervals, starts with about 1.5 minutes at a higher intensity to speed up your VO2 response. Ronnestad’s lab showed that periods of fast starts led to a higher peak and average VO2 response in skiers (Ronnestad, et al., 2019).

Above you’ll see the VO2 response to high-intensity efforts in the skaters. Note how the black dots (fast start interval) reach high VO2max relatively faster than the white squares (traditional steady power effort). Figure from Ronnestad, et al., 2019.

Xert has had fast start periods for several years now! You can see them in action in the SMART – Closer Exercise Series (example shown above), as well as the SMART – Gasoline Exercises.

Alternatively, a longer interval can be used to bring heart rate and VO2 to ~90%. In practice, longer high-intensity intervals (3+ minutes) are not used as frequently for several reasons. Remember, Seiler found that the 4 x 4 min protocol was less effective than the 4 x 8 min protocol. Furthermore, prolonged efforts at high intensities can only be sustained for a few repetitions before exhaustion is reached, making it difficult for athletes to accumulate time >90% VO2max.

Once ~90% VO2max (or ~90% HRmax) is reached, the goal for a high-intensity session should be preservationthat intensity for as long as possible. This brings us back to findings by Billat, who showed short cap, 30-second efforts with equally short recoveries can be used to keep VO2 near maximal levels for extended periods of time.

Putting these two concepts together leads us to the findings of Filippo Vacari and colleagues, who proposed a high-intensity interval training (HIDIT) protocol (Vacari et al., 2020). HIDIT combines two longer intervals to speed up the body’s HR and VO2 response before switching to shorter intervals to maintain near-maximal effort for as long as possible.

As expected, the HIDIT protocol resulted in significantly longer time spent >90% VO2max compared to the short interval protocol (30 seconds work with 20 seconds recovery) and the long interval protocol (3 minutes work with 2 minutes recovery).

Here you can see the HIDIT protocol proposed by Vacari and colleagues. 3:2 ratio of work: recovery is used throughout, starting with 3 minutes work and 2 minutes recovery. The duration of each repetition is gradually decreased until you are stable at 30 seconds, 20 seconds recovery. Power is shown as solid black and VO2 is shown as black dots – 90% of VO2max is shown as a dashed horizontal line. Figure from Vaccari, et al., 2020.

We modeled HIDIT using Xert in SMART – Bow Down work out. After an extended warm-up with 4 short, challenging intervals, this workout uses 2 relatively longer intervals to reduce your maximum available energy (MPA) and speed up your VO2 & HR response. Once the MPA is reduced, the workout shifts to 30-30 minute intervals to keep heart rate and VO2 near their limits. The goal is to stay in these minute intervals for as long as possible.

Final thoughts

Many of the high-intensity protocols we reviewed today are designed to be (near) maximum efforts. These workouts provide a powerful stimulus to your cardiovascular and respiratory systems because they push you to your limits, which also means that they will tax your available maximal energy and result in a relatively high difficulty score. Most of the exercises shared above are close to the Xert’s 5-Diamond difficulty rating. This means that these exercises are best performed when your training status is fresh and you are highly motivated to complete a high-difficulty exercise. Doing this once a week can reap some amazing benefits for your fitness! I hope this article was interesting! Stay safe, ride fast, and I’ll see you next month!



Pellatt V, Slawinski J, Bouquet F, et al. Intermittent running at the maximum oxygen uptake rate enables subjects to remain at maximum oxygen uptake for longer than intense sessions but without maxing out. Eur J Appl Physiol 81, 188–196 (2000). https://doi.org/10.1007/s004210050029.

Rønnestad BR, Hansen J, Vegge G, Tønnessen E, Slettaløkken G. Short intervals lead to superior training adaptations compared to long intervals in cyclists – an effort proportional approach. Scand G Med Sports Sciences. 2015 Apr; 25 (2): 143-51. doi: 10.1111/sms.12165. Epub 2014 Jan 1. PMID: 24382021.

Rønnestad BR, Rømer T, Hansen J. Increased oxygen uptake in well-trained cross-country skiers during periods of work with a fast start. Int J Sports Physiol Perform. 2019 Oct 15:1-7. doi: 10.1123/ijspp.2018-0360. Electronic publishing before printing. PMID: 31621643.

Seiler S, Jøranson K, Olesen BV, Hetlelid KJ. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scand G Med Sports Sciences. 2013 Feb; 23 (1): 74-83. doi: 10.1111/j.1600-0838.2011.01351.x. Epub 2011 Aug 3 PMID: 21812820.

Vaccari F, Giovanelli N, Lazzer S. High-intensity interval training (HIDIT) increases time over 90% [Formula: see text]O2peak. Eur J Apple Physiol. 2020 November; 120 (11): 2397-2405. doi: 10.1007/s00421-020-04463-w. Epub 2020 Aug 11, PMID: 32780251; PMCID: PMC7560936.

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