Are 40-20’s (40s ON 20s OFF) the best work to rest ratios in HIIT intervals? Or should you consider 30-30 Billat intervals? Both are common in endurance sports like cycling and running, and recent scientific literature shows why. Continue reading to learn more about work-to-rest ratios in HIIT training.

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40-20 intervals are named after their work to rest ratio: 40 seconds ON, 20 seconds OFF. There are many similar HIIT intervals that work with slightly different work-to-rest ratios like:

  • 30 seconds on 30 seconds off, also known as Billat intervals, named after Veronique Billat, a French exercise physiologist.
  • 20-40s, which are the opposite from 40-20s and therefore have a work-to-rest ratio of 1:2 instead of 2:1.

Example 40-20 interval workout

Contrary to a HIIT workout like Tabatas, there’s not one specific 40 20 interval workout. As a result you need to decide on:

  • The intensity of your 40 second interval
  • The intensity of your 20 second recovery
  • The number of intervals per set and the number of sets in total
  • The duration and intensity of your recovery in between sets

Before we dive a little deeper into these parameters, let’s start with a typical 40 20 workout example:

  1. Warm-up
  2. Intervals: 10 intervals of 40 seconds, at an intensity of 120% AT (anaerobic threshold)
  3. Recovery: 20 seconds rest between each interval, at an intensity of 50% AT
  4. Sets: 3 sets with 5 minutes in between, at an intensity of 60% AT
  5. Cool-down

This workout would take roughly 40 minutes (ex warm-up and cool down).

Example 40-20s interval workout
Example 40-20s interval workout: sets of 10x40s at 120% anaerobic threshold, 20s recovery in between intervals.

The best work to rest ratio for interval training depends on your goal. There are 4 factors that you should take into account when thinking about interval work-to-rest ratios:

  1. Interval intensity
  2. Interval duration
  3. Recovery intensity
  4. Recovery duration

As mentioned, there are many similar HIIT intervals with slightly different work-to-rest ratios. You might wonder which work to rest ratio works best and which intensity you should pick for the intervals.

The truth is that there’s probably not a huge difference between 40 20s, 30 30s (Billat intervals), 20 40s etc. as long as the ON phase intensity is above threshold and the OFF phase is too short to fully recover. Here’s why.

The first interval after the warmup is heavily relying on anaerobic energy supply (glycolysis). That is because the intensity is high and the aerobic energy system is not up to speed. However, soon lactate accumulates, glycolysis gets hampered and aerobic energy supply becomes dominant. This will be the case in all interval sets similar to 40 20s, regardless of whether the work to rest ratio is 2:1, like in the 40-20s, 1:1 like in the 30-30s Billat intervals, 1:2 etc.

Again, this is true for all HIIT workouts with an ON phase above threshold and an OFF phase that is too short to significantly recover.

One thing you should take into account is the duration of the recovery between sets. If you want a higher anaerobic energy component in your workout, it’s important to use a long recovery between sets. If you prefer a higher aerobic energy component in your workout, it’s best to use a short recovery between sets.

Download our infographic on how to create highly effective interval training.

The benefits of 40-20s or Billat intervals (30-30s) are very similar to Tabata intervals, due to the above mentioned similarities in energy kinetics. You can expect these HIIT intervals to boost both your aerobic power (VO2max) and anaerobic power (VLamax).

A recent study looked at the performance improvements of elite cyclists who performed HIIT intervals within the same domain as 40-20s. More precisely, these elite cyclists did 13 intervals of 30 seconds ON 15 seconds OFF (2:1 work to rest ratio). They repeated this set 3 times with 3 minutes recovery between sets.

After 9 of these HIIT sessions in only 3 weeks, they significantly increased their:

  • VO2max from 73.3 ml/min/kg to 75.5 ml/min/kg
  • Mean power output during the last minute of the incremental VO2max test from 460 to 476 watts
  • 20-minute power (FTP) from 343 to 358 watts

Whether you’re an elite cyclist or not, training adaptations can vary. If you’re curious about the impact of short interval sessions like 40 20s or 30 30s on your performance, consider trying a similar 3-week training program.

But how can you measure these improvements? How can you truly understand the impact of these short interval sessions on athletic performance?

The answer lies in metabolic testing. Many coaches, labs and athletes use the INSCYD power performance decoder test for these kinds of progress tests, since it gives you all the information you need (from VO2max to Anaerobic power and beyond), but only requires a power meter in cycling or GPS watch in running.

What sets the PPD test apart is its ability to obtain crucial physiological data where it truly matters – on the road, on the bike, or on a track. This powerful tool not only saves you time, requiring just five minutes to deliver a comprehensive performance report, but also ensures lab test accuracy. It’s like having a personal performance lab at your fingertips, offering lab-level accuracy without the need for a physical visit. 

With INSCYD, you can also leverage lactate testing to gain a complete understanding of your athlete’s physiology. Whether you’re testing in the lab or in the field, INSCYD’s lactate testing can provide a holistic performance profile of any athlete. You can even add VO2 measurements if you want to INSCYD take data from any sport. This way, you’re not just guessing your progress – you’re measuring it with precision.

Don’t just guess your progress – measure it. Coaches and Labs book a free consultation with INSCYD today and discover how INSCYD can help you understand and enhance athletic performance. 

Whether short HIIT intervals are beneficial for you depends on your current fitness and your goal. As mentioned, 40 20s are likely to increase both your VO2max and VLamax. The latter might be beneficial for some, while it can be detrimental for others.

That’s why it’s important to first understand your so-called metabolic profile via a metabolic performance test. This test measures your current fitness in detail.

Once you have your metabolic profile metrics (like: VO2max, VLamax, anaerobic power, fat- and carbohydrate combustion etc.), you can compare your profile to the physiological demands of your goal (e.g. cycling race, running marathon etc.).

INSCYD metabolic testing results
INSCYD’s Metabolic Profile

This comparison will tell you exactly what to work on, and whether 40 20s are the way to reach your goal.

INSCYD helps you understand and enhance athletic performance. From VO2max and VLamax metrics to carbohydrate and fat combustion rates, and even lactate recovery and accumulation, INSCYD provides the insights you need to create highly individualized and effective training programs.

Get a full metabolic performance profile of an athlete right where it matters – on the road, on the bike, or on a track. No more guesswork, only lab-level accuracy without the need for a physical visit.

Moreover, INSCYD allows you to project future performances, aiding you in making informed decisions about your training and racing strategies. It even offers performance tracking, comparison, and automated reporting.

But don’t just take our word for it. Experience the power of INSCYD for yourself. We invite you to schedule a free consultation. Discover how INSCYD can transform your approach to training and help your athletes reach their full potential.

Short intervals vs Long intervals in elite cyclists

Rønnestad, B. R., Hansen, J., Nygaard, H., & Lundby, C. (2020). Superior performance improvements in elite cyclists following short-interval vs effort-matched long-interval training. Scandinavian journal of medicine & science in sports, 30(5), 849–857.


Human Movement Scientist | Content Marketing and Education

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