Podcast, Science

What types of intervals are most effective? A scientific analysis with Michael Rosenblat | EP#243

 July 20, 2020

By  Mikael Eriksson


In this Episode you'll learn about:

  • Long, medium, and short High-Intensity Interval Training (HIIT) compared with Sprint Interval Training (SIT)
  • The scientific evidence available for trying to find the "optimal" type of interval training
  • Why Long HIIT may have certain advantages over other types of HIIT as well as SIT
  • Should athletes try to optimise (find the one best workout) or mix things up to get the best of all worlds? 

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High intensity interval training (HIIT) v.s. sprint interval training (SIT)

05:05 - 

  • Let’s start by defining these two types of intervals.

    HIIT is taking place in the very intense training domain, which most people only can sustain for at max 10-20mins, even though the most common types of HIIT intervals would be around 4 mins long.

    SIT is even more intense and are typically performed as 30s all out efforts.
  • I wished to compare these two types of interval training and hence, I conducted a meta analysis of the research done within the area.

    This is also extra interesting because most meta analysis on the subject have not differentiated between HIIT and SIT.

    My main outcome measurement was time trial performance but as secondary outcome measurements I also looked at VO2max, mitochondrial density, respiratory capacity etc.

    Beforehand a college of mine had conducted a study in which he found that SIT improved VO2max, mitochondrial function, mitochondrial density as well as respiratory capacity more than HIIT, however, only HIIT lead to improved time trial performance.

    So this study raised many questions and therefore I wanted to study this more diligently.
  • In the meta analysis, 6 studies were included, which all had a time trial as a primary outcome (all secondary outcomes of the studies were, however, also included and analyzed as a bonus).

    In total 100-110 athletes were included in the pooled analysis and the subjects’ VO2max ranged from 46-64 ml/kg/min.
  • In time trial performance, there wasn’t any significant difference between the groups, and the same applied to VO2max (even though both types of intervals were effective ways to increase VO2max).

    However, looking at maximum aerobic power (MAP), we did see an almost 2.5 % increase in the HIIT group compared to the SIT group.
  • As we were taking a closer look at the HIIT that the subjects had performed in the included studies, we noticed that the duration of the intervals differed quite substantially between the studies, from only 1min long up to 5mins.

    This difference in interval length could have a significant impact on the results, so therefore we decided to divide the HIIT group into three sub groups: short HIIT, medium HIIT and long HIIT, for additional analyses.

    Now we could see a significant improvement in time trial performance between long HIIT and SIT.

    Another interesting finding was that the improvements in MAP was even greater in the long HIIT group compared to the medium and short HIIT groups.

Potential limitations

30:00 -

  • The studies included raised a few question marks, one being how workload should be determined.

    The total time in the high intensity zone differed in the HIIT group compared to the SIT group, with the SIT group naturally had less time in the high intensity zone.

    However, this may not mean that the workload was less for the SIT group since they performed the intervals at an even higher intensity compared to the HIIT.

    Therefore, it is hard to know, which of the groups that were subject for the highest total workload.

The training week of the study subjects

35:00 -

  • Some of the studies included in the meta analysis included subjects who were also doing some other easier recreational training besides the SIT or HIIT training.

    This may have the potential to influence the results but since the subjects were randomized I would say that this is unlikely.

Short summary of the findings

36:05 -

  • There were no significant difference between the HIIT and SIT group when looking at time trial performance.

    However, if one would only look at those in the HIIT group that performed longer intervals (> 4min), then a significant increase in time trial performance of about 2 % could be seen compared to the SIT group.

    This increase corresponded very well with a concurrent increase in MAP (maximum aerobic power).

Practical application of the results

40:30 -

  • The results suggest that in order to improve time trial performance, then longer intervals (> 4 mins) would be the way to go.
  • MAP could be a great indicator of performance, which is fairly easy to measure.

Future research

43:35 -

  • In the future I hope to do a larger study all on my own comparing SIT and HIIT.
  • I am also currently working on a meta regression analysis, with the aim to characterize baseline characteristics of the subjects of many different kinds of interval studies.

Rapid fire questions

46:45 -

  • What is your favorite book, blog or resource related to running, endurance sports or anything that we have basically discussed here? Pubmed is where I live these days and my favorite resource at the moment.
  • What is your favorite of gear or equipment? My Cervelo S2, it’s like my dream bike!
  • What is a personal habit that has helped you achieve success? I read a lot, reading all the latest science is like playing video games for me!


Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

Please contact me if you have feedback on the podcast or want to make suggestions for improvement or send in a question for a Q&A episode.

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