Short vs Long HIT Intervals – Here’s What Actually Matters

Roberto Vukovic standing thoughtful in the middle wearing a light orange adidas tshirt while he's surrounded by different types of short and long HIT interval formats

If you’re only half a serious cyclist, you might’ve certainly stumbled across the Rønnestad 30/15 interval studies in some form or another. 

It was all big headlines about what a magic workout these short intervals are compared to other interval styles. Everyone was just ripping 30/15s from there on. Other VO2max intervals were lame. 

But no matter if coaches, scientists, or cyclists themselves, everyone who jumped on the 30/15 bandwagon missed an important point: There is an inherent problem with the study design. And it’s not just that but there’s a way more important point in that whole short vs. long HIIT intervals discussion. A point way too many cyclists lack awareness of. 

Here’s what I mean.

Rønnestad Intervals Are Overrated

Rønnestad did two consecutive studies with the only difference being the participants. The first study was done with well-trained cyclists. The second was done with elite-cyclists. Where was the difference between well-trained and elite? Well, the well-trained cyclists had a mean VO2max of around 66 ml/min/kg. The elite cyclists had a mean VO2max of around 73 ml/min/kg. 

The protocol in both studies involved dividing participants into a long interval group (LI) completing 4×5 minute intervals and a short interval group (SI) completing 3x13x30/15 intervals. However, the well-trained cyclists study lasted for 10 weeks with 2 weekly HIT sessions while the elite-cyclists study lasted 4 weeks with 3 weekly HIT sessions. Just a reminder for you at this point: Please don’t commit to three weekly HIT sessions frequently. This was a study setting and if you do decide to do three, do it occasionally and otherwise stick to one or two weekly HIT sessions. HIT intervals are a double-edged sword. You improve performance rather fast, but on the other hand you can run into diminishing returns after as little as 6 total sessions.

Anyway, in the well-trained cyclists study the SI-group did 363 watts in the on-bout while the LI-group performed intervals at 324 watts average. In elite cyclists the gap was even bigger: 441 watts for short intervals and 368 watts for long intervals.

Essentially, they compared 19.5 minutes at 441 watts with 20 minutes at 368 watts. If you ask me, this is everything apart from surprising that the 30/15 intervals came out on top. 

But the only error isn’t in comparing higher with lower power. No. It’s the whole interval programming. Because from a metabolic and cardiovascular standpoint your body is still working hard during the 15 seconds off period. Therefore, rest periods are part of total work in intermittent intervals. 

So what they actually did was comparing 29.5 minutes with 20 minutes of interval work. 

I mean come on, even with a basic understanding of training I don’t need to tell you that 30 minutes of threshold intervals is a higher workload than 20 minutes of tempo. 

Making Sense of HIIT Sessions

I did some more research on that short vs. long HIT intervals topic and luckily came across an unpublished study that was a master thesis in the circles of Seiler’s group. In this study they took 30 cyclists (28 males, 2 females) with a mean VO2max of 64 ml/min/kg and split them into the three following groups:

  • 1. Long Intervals: 4×8 minutes HIT = 32 minutes total
  • 2. Short intervals 1: 4x12x40/20 HIT = 32 minutes total (40sec bouts)
  • 3. Short intervals 2: 4x8x40/20 HIT = 32 minutes total

In contrast to the Rønnestad studies these guys accounted for the importance of total work duration. 

Surprisingly, when total intensity was matched, there was no difference in VO2max or power output improvement among the three groups. The only difference researchers found in this study, though, was that power during the training sessions tended to increase more in the short interval groups. However, they don’t know if this was a learning effect or specific adaptation. 

The thing we have to keep in mind, however, is that the higher the exercise intensity, the higher the muscle fiber type recruitment. Basically, the short interval group activated more muscle fibers as a result of intensity. 

It’s All About Absolute Intensity Exposure

In a recent article by Dr. Stephen Seiler published in “Applied Physiology, Nutrition, and Metabolism” he once again outlined the importance of accumulated-work-duration or the total time spent at that particular intensity. 

If we go back in time, for example, French researcher Véronique Billat discovered that what drives most of the adaptations around VO2max is driven by spending more than 10 minutes above 90% of your VO2max. But you need time to ramp up your body’s oxygen consumption. In general, 12-24 minutes of total time are a good measure to get to this point depending on your fitness level. I use 2-4 minute intervals as I found athletes are able to pace a higher power at this interval length compared to longer 5-6 minute intervals – and research is backing this up. 

Now to illustrate what total time at intensity can look like here’s an example of different intervals that all equal 24 minutes of total work:

  • 3×8 minute HIT intervals = 24 minutes
  • 3x8x40/20 intermittent intervals = 24 minutes
  • 8×3 minute HIT intervals = 24 minutes

Find What You Love and Let it Kill You

What I want you to take home from this article is that the 3x13x30/15 workout is not a magic bullet that will make you as powerful as owning the ring to rule them all. Because the truth is there are no magic workouts. Instead, it’s about finding interval designs that you enjoy, find easy to do and that have a useful structure. After making a choice you just repeat them for long enough and then let the rest take care of itself. Even intervals are worth nothing without consistency.

And honestly, who can keep the overview with 30/15s anyway. 

I usually go with standard 2-4 minute HIT intervals and 30/30s as my go to intermittent intervals. Remember to think about total time at intensity and make sure to choose at least 8 minute sets for the 30/30s to allow your body to drive up the system toward VO2max. 

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Studies Used in This Article

  1. Aerobic short or long high intensity interval training does it matter?
  2. It’s about the long game, not epic workouts: unpacking HIIT for endurance athletes
  3. Superior performance improvements in elite cyclists following short-interval vs effort-matched long-interval training
  4. Short intervals induce superior training adaptations compared with long intervals in cyclists – an effort-matched approach
  5. Effect of work duration on physiological and rating scale of perceived exertion responses during self-paced interval training
  6. Very short (15s-15s) interval-training around the critical velocity allows middle-aged runners to maintain VO2 max for 14 minutes
  7. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration
  8. 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
  9. Interval training at VO2max: effects on aerobic performance and overtraining markers
  10. Defining the number of bouts and oxygen uptake during the “Tabata protocol” performed at different intensities
  11. Skeletal muscle buffering capacity and endurance performance after high-intensity interval training by well-trained cyclists