Continuing from my discussion of Exercise Efficiency, I want to dip back into the research and look at the metabolic adaptations to high-intensity interval training (HIIT).
Metabolic adaptations to short-term high-intensity interval training: a little pain for a lot of gain?
High-intensity interval training (HIT) is a potent time-efficient strategy to induce numerous metabolic adaptations usually associated with traditional endurance training. As little as six sessions of HIT over 2 wk or a total of only approximately 15 min of very intense exercise (approximately 600 kJ), can increase skeletal muscle oxidative capacity and endurance performance and alter metabolic control during aerobic-based exercise.
It’s long been felt or argued that the only way to reach the pinnacle of endurance performance is through years of grinding effort, usually involving absolute piles of low-intensity training. To a great degree, outside of the occasional period when programs based around intensification have become popular, this has been the basic approach to endurance training.
‘Miles build champions’ is a commonly held adage and after my comments about Exercise Efficiency and how takes YEARS of training to have any real impact on it, there is probably some merit to this idea.
However, efficiency is only one of several components of performance, the other two major ones being VO2 max and lactate threshold. To a great degree, similar ideas have been held regarding the time course of changes in those components with years being expected to maximize them. But is this true?
Research going back to the 1970’s (by Hickson) had questioned that idea, at least as it pertained to things such as VO2 max. In that study, subjects performed high intensity intervals (6X5 minutes at VO2 max with a 2′ break) three days per week on a bike alternated with 40 minutes running as fast as possible on the other three. In that study, VO2 max increased linearly week to week with one subject approaching an elite (for then) VO2 max score. So clearly there is some indication that at least some aspects of endurance training can improve quite quickly.
High-Intensity Interval Training for Performance
In fact, a number of recent studies, typically using cyclists have found that substituting about 15% of the total weekly volume with high intensity intervals can improve performance significantly, by up to 5%, in a matter of weeks (interestingly the maximal effect seems to occur at the three week mark with no further benefits occurring if intervals are continued beyond that point). Clearly this points to something very beneficial going on with interval training and certain aspects of performance. It was this topic that today’s research review examined in some detail.
Their initial comments are essentially identical to mine above pointing out that, while it’s usually been thought that only long-duration endurance training could improve aerobic energy metabolism, more recent work has shown similar adaptations can be induced with a much lower volume of interval training. Interestingly, the adaptations may actually happen faster with the intervals.
The researchers rely primarily on work from their own lab where they utilize a standard interval protocol consisting of a 30 second Wingate test (30 seconds all out against a braked cycle ergometer) repeated 4-6 times with a 4 minute rest period. This workout is performed three times per week for anywhere from 2-6 weeks (the earlier work I mentioned used intervals of varying lengths twice per week for three weeks, or six total sessions). I’d note that most of the work on this lab was performed on essentially untrained college aged men and women NOT highly trained endurance athletes.
I’d note that this type of training protocol looks exactly nothing like the types of interval training being currently propagated for fat loss. Rather, the fat loss type programs are usually based on longer intervals (60-90 seconds) with relatively short rest periods (60-90 seconds); this makes it difficult to apply the research results.
The results from their work are somewhat interesting, in one, a mere two weeks of the above protocol improved exercise tolerance (the ability to sustain a given workload) by nearly double. However, there was no increase in VO2 max suggesting that the adaptations were peripheral (e.g. occurring primarily in the muscle) and not central (e.g. occurring in the body’s ability to transport oxygen via the bloodstream). The researchers note that other studies using HIIT have found an increase in VO2 max but invariably used a larger volume of training.
What Causes the Metabolic Adaptations?
Looking at the cause of the adaptations, the researchers found increases in aerobically producing energy enzymes that were significant with only the short interval group. In a related study, they compared the above interval protocol to a group that performed standard steady state cardio for 90-120 minutes for the same 6 sessions. Identical results were seen in both groups but the interval group exercised only 2.5 total hours compared to 10.5 hours in the steady state group.
Additional adaptations in the interval group also include changes in carbohydrate metabolism including increased glycogen storage, reduced glycogen utilization and lactate production during exercise along with increased glucose uptake into skeletal muscle. Markers of fat oxidation were unchanged, at least over the 2 week studies (in this vein I’d refer readers back to my research review on Endurance Training and Obesity: Effect on Substrate Metabolism and Insulin Sensitivity which found that only steady state cardio increased fat oxidation rates and interval training did not).
In contrast, the review mentions another interval training paper (which used a much higher volume of training) that did show an increase in fat oxidation over only 2 weeks. The subjects in that study performed a massive 10 sets of 4 minute intervals at 90% of VO2 max, significantly more volume than the small amount used in the studies described above (and far far more than any of the popular interval programs for fat loss).
Although the paper went into some detail on the signaling mechanisms by which interval training may work, I don’t think a detailed discussion will be that valuable so I’m going to skip over it.
Jumping to the practical implications of their work, the researchers comment that the main complaint about exercise programs is a lack of time and, as such, exercise innovations that allow for similar benefits in less time are clearly of interest from a health point of view. Interval training may play a role here (I mention time efficiency as one of the ‘pros’ of interval training in the article Steady State and Interval Training: Part 1) and at least one study suggested greater long-term adherence to exercise programs requiring less time investment.
At the same time, the researchers mention explicitly that it’s unclear how longer term interval programs will work compared to more traditional training programs and there may be different times courses of adaptations for each kind of training (e.g. perhaps interval training causes more rapid initial gains which then plateau sooner).
As well, the type of interval program they use in their lab requires not only specialized equipment but, to quote them “…an extremely high level of subject motivation.” As others have pointed out, the intensities used in many interval training studies are essentially unachievable by beginners which raises many more questions such as:
Can a beginner even achieve the intensities and durations used in these studies to get the benefits
Would a modified interval training program, using less intense or lower duration intervals achieve the same benefits in the first place?
You get the idea.
There’s no doubt (and I haven’t intended to suggest otherwise) that high intensity interval training can have benefits. It’s time effective and may induce similar performance adaptations to longer duration traditional cardio. With regards endurance athletes, it’s clear that even short periods of low volume interval training can have rather large benefits for performance.
But with most of the benefits seeming to occur with only a handful of sessions per week (2-3 is the norm) and with benefits appearing to end fairly quickly (3-6 weeks), we might ask what a trainee should do when either
- They need to train more frequently than that
- They are looking at their training over a period longer than a few weeks. That is, if interval training stops providing benefits after 3-6 weeks, what should a trainee do for the remaining 46-49 weeks out of the year?
There is also the issue of how intervals integrate with training when OTHER TYPES OF TRAINING (e.g. weight training) are being done. That is, what happens if someone is training their legs heavily in the weight room twice/week. How realistic is it to then add high intensity interval training to that workload?
As well, what happens when someone (e.g. an athlete or obsessive exerciser) is trying to train daily? What happens then in terms of how they structure their week? If you take much of the current guru-speak (e.g. intervals are the only beneficial way to train) at face value, you end up developing a training week that no human being can survive.
These are critically important topics that I’ll discuss in Summing Up Steady State vs. HIIT
- A Look at The Tabata Protcol
- Endurance Training and Obesity: Effect on Substrate Metabolism and Insulin Sensitivity.
- Does Cumulating Endurance Training at the Weekends Impair Training Effectiveness?
- Stead State vs. Intervals: A Conclusion
- Steady State vs. Intervals: Explaining the Disconnect Part 2