“Will I improve both my endurance performance and my V̇O2max more by including high intensity interval training (HIIT) above maximum metabolic steady state (MMSS) threshold in my training plan?”
HIIT refers to interval training with continuous work bouts longer than 2 minutes (or intermittent work bouts adding up to longer than 2 min) performed in the severe intensity domain above MMSS, e.g. above “threshold”; critical power/speed (CP/CS), functional threshold power (FTP), maximal lactate steady state (MLSS), or any number of other ways of operationalising MMSS.
We thought the answer would be obvious. Do you?
This is a quick re-write of a twitter thread I posted in September, 2023. Since X née Twitter is more difficult to read these days without an account, and even less worth it than ever to create an account.. it might be worth transferring over some of my threads back here to the old blog. We’ll see!
Surprisingly this question hadn’t been meta-analysed specifically in endurance-trained athletes, meaning athletes who might have very different fitness levels (different V̇O2max), but who are experienced and adapted to their sport.
My colleague Dr. Michael Rosenblat is an expert in performing meta-analysis on training data, and we were lucky enough to have Dr. Stephen Seiler as our senior author for this project. So we decided to perform that meta-analysis and establish what the current evidence suggest!
We sought out studies investigating the effects of training interventions in groups of trained runners, cyclists, or rowers performing either:
1) Training exclusively below MMSS in either moderate intensity domain below the first lactate/ventilatory breakpoint, or heavy domain between the first and second breakpoints. This might have looked like moderate intensity continuous training (MICT), ‘zone 2’, fat-max training, tempo, sweet spot, or even sub-threshold training. We’ll see later how this variety may have affected (or not) the resulting outcomes.
2) Training which included some volume above MMSS in severe (or extreme) intensity domain. This was mostly some kind of HIIT training. Some groups performed sprint interval training (SIT) in the extreme domain.
By comparing studies which included these two groups we could examine the additional effects of including high-intensity training on our primary outcome measures, which where:
A) The peak or maximal rate of systemic oxygen uptake in ml·kg-1·min-1 at the end of a ramp test (V̇O2peak; note, there were no differences between V̇O2peak or V̇O2max, so I will call it peak from here as we do in the manuscript)
B) Time-trial (TT) performance in seconds in the same modality in which V̇O2peak was tested (running, rowing, or cycling). We also looked at peak workload at the end of a ramp test in either cycling power output 
or running or rowing (km·hr-1). But I will focus on TT as our performance outcome, since it is more relevant to real-world performance.
14 studies with 299 total participants (51 female athletes) were included in the review. This includes well known studies such as Stepto et al, 1999:
And Dr. Seiler’s own 4×4, 8, and 16 min interval study:
What does the research show?
The Additional Benefit of HIIT to V̇O2peak
Including training above MMSS led to significantly and meaningfully improved V̇O2peak at the meta-group level, compared to training exclusively below MMSS.
Important to note, both groups might have improved with training, but the HIIT groups improved more.
The standardised mean difference between groups was equal to 0.44 standard deviations.
Think of it like this: if you were to measure V̇O2peak values in a group of athletes before and after a training intervention, you can calculate the mean and SD of the group. Let’s say a group of well-trained cyclists might have a mean V̇O2peak of 60 ml·kg-1·min-1 and the SD of the group might be 5 ml·kg-1·min-1 (range ≈ 50-70).
If that group performs high intensity training above threshold, we would expect them to increase by an additional ~2 ml·kg-1·min-1 (0.44 * 5), compared to if they only performed low/intermediate intensity training below threshold.
That would be equivalent to the group mean increasing say from 60 to 62 ml·kg-1·min-1 with lower intensity training, and 60 to 64 ml·kg-1·min-1 by including some high intensity in their training!
When we are talking about trained athletes, a few points of V̇O2peak represents a meaningful change… or so we would think?
No Additional Benefit of HIIT to Endurance Performance?
If V̇O2peak was increased more, we might expect to also see a greater improvement in TT performance in the high intensity training groups?
Instead, there were no detectable differences between the groups for TT performance (or Wpeak)!
Groups may have improved performance by training either exclusively below MMSS or including above MMSS, however there were no differences between groups.
This might be unexpected to you? It was to us. We speculated a few possible explanations
Speculatively Explaining the Dissociated V̇O2peak and TT Outcomes
The sample size of athletes studied for TT was less than that for V̇O2peak, meaning all else equal, the statistical power to detect a significant effect was lower in TT performance than V̇O2peak.
According to the Joyner and Coyle model of endurance performance, TT performance is proportional to the interaction between V̇O2peak, fractional utilisation of V̇O2max at threshold, gross mechanical efficiency (exercise economy), and as we understand more recently; fatigue resistance or durability.
Therefore, it may be that while V̇O2peak improved significantly more in the high-intensity training groups, less change in the other factors may have washed out the net improvement in TT performance, leading to a non-significant difference between groups.
Another idea is that generally there are trade-offs between improvements in energetic capacity, i.e. an increased maximal rate of energy provision, and efficiency of that energy provision. It could be that the increased V̇O2peak (capacity) came at the trade-off of (transiently?) increased oxygen cost of work or reduced efficiency elsewhere along the bioenergetic pathways.
There is good research on the time course of different physiological adaptations which contribute to endurance performance. Early improvements could drive ⬆️V̇O2peak, which could itself allow for slower processes that drive ⬆️TT performance.
This possible capacity-efficiency trade-off could be temporary. We know that utilising a taper period of reduced training volume after an intensive training block will improve performance in subsequent competition. It could be that if the studies had re-tested athletes after a taper period, their newly improved V̇O2peak might have more effectively translated to improved TT performance. This is of course speculative.
Another possible explanation is that the timeframe of the training studies was not sufficient to ‘convert’ the improved V̇O2peak to TT performance. The studies ranged from 2 to 12 weeks, and the time-trial tests were between ~5 and ~60 minutes for either rowing, running, or cycling.
There were other differences between studies that could possibly have affected results, such as the participants’ baseline fitness level, sporting modality, or programming variables of the training sessions (frequency, duration, intensity, etc.). These potential sources of variability will appear in the measure of statistical heterogeneity (I2) of the meta-analysis, as a percent value.
I2 for TT performance was 0%, meaning that in fact none of these, or any other potential sources for variability between studies appeared to matter to the results. Basically, after the primary analysis, there was no remaining unexplained variance to account for.
Implications for Training Prescription
Meta-analysis is a very good method to find robust group-level effects. If we see that group-level differences are persistent across a number of studies which demonstrate those findings in different ways, we can be quite confident that for most individuals within the target population (i.e., endurance trained athletes) we would expect to see a real effect.
However, group-level research is not designed to, and is therefore not particularly good at predicting individual-level outcomes. This is always a challenge with interpreting group-level research for individual-level application, something I’ve written about before.
To improve individual-level prediction, we would need to perform something called an individual patient data (IPD) meta-analysis. This would require collecting original participant data from every athlete in all the studies we are interested in. Consider this a teaser, because this is exactly what Michael and I are doing for our next project on training programming.
For now, what can we take away from this research?
High intensity interval training (HIIT) above the maximal metabolic steady state seems to be important to help drive improvements in V̇O2peak within 2-12 week timeframes. Thus, athletes should be including some high intensity sessions within their total training volume.
But certainly not every session, nor every week needs to have high intensity. Periodised training blocks can include periods of exclusively lower intensity below MMSS without necessarily sacrificing any performance capabilities. These could look like ‘zone 2’, tempo, sweet-spot, sub-threshold, or however we want to program our long duration low-intensity, and intermediate duration x intensity sessions.
Given this flexibility over any few periodised mesocycle timeframes, we can interpret this to give us permission to do the training we enjoy, the training we know our athlete responds well to, and the training that will more specifically prepare us for the demands of our target event.
This resembles how I used to think about ‘physiological’ and ‘performance training’. The former being the general, non-sport-specific phase of training consisting of high-intensity interval training through the winter. Leading into a more specific phase of extensive (sub-)threshold intervals as we would approach road racing season.
TLDR
- 2-12 weeks of training in endurance trained athletes leads to improvements in both V̇O2peak and time-trial performance
- Groups that performed some of their training at high intensity above the maximal metabolic steady state (MMSS) improved their V̇O2peak more than groups who only trained below MMSS
- However, there were no detectable group-level differences in TT performance outcome or Wpeak.
- This finding could represent methodological limitation, or it could be explained by a time-dependent interaction between capacity and efficiency to express the improvements in V̇O2peak as improved endurance performance.
“The Additional Effect of Training Above the Maximal Metabolic Steady State on VO2peak, Wpeak and Time‐Trial Performance in Endurance‐Trained Athletes: A Systematic Review, Meta‐analysis, and Reality Check“ (Available free to download)
Spare Cycles 
Interesting article. Thank you!
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