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High-intensity intervals: is the bubble about to burst?
High-intensity interval training using short-duration work periods is now the interval session of choice for many athletes. However, new research suggests that neglecting more traditional interval sessions may not be wise when seeking maximum endurance performance
As most of our readers will already know very well, interval training sessions consist of a number of high-intensity efforts of a predetermined duration interspersed with short periods of rest, and are now considered an essential tool for building and maximizing fitness. This is not surprising because a large body of excellent empirical evidence has accumulated demonstrating that a wide variety of interval training protocols (ie varying interval durations and intensities) are very effective for enhancing the efficiency of both the aerobic and anaerobic energy producing systems in the muscles(1).
How are interval durations determined?
Until the late 1990s, most endurance athletes performing intervals relied on interval durations that reflected the demands of their event. Since endurance efforts lasting more than around two minutes – ie most endurance events - rely very heavily on the aerobic energy producing system(2), and efforts of up to 10 minutes require over 90% of energy demands to be met from the aerobic energy system(3), this meant that training sessions were typically modelled around intervals lasting from 2-10 minutes, with intervals of 3-5 minutes becoming a popular and well-documented method for athletes and fitness enthusiasts seeking to improve aerobic fitness and endurance performance(4).
The rationale for using 3-5 minute intervals is that there is a lag between starting a high-intensity effort and the cardiovascular system and muscles ‘catching up’ - a phenomenon that arises from the way the body responds to increase oxygen demand (more technically known as ‘oxygen kinetics’)(5). This determines the lower limit for interval duration of around three minutes. By the same token, the accumulated fatigue resulting from repeated intervals means that these intervals cannot be too much longer in duration than three minutes (because it becomes impossible to perform a decent set of repeats)(6), which explains why the upper limit typically chosen was around 5 minutes.
The short interval revolution
Around the beginning of the millennium, sports science researchers became interested in playing around with interval lengths and were surprised to discover that athletes can also benefit from interval sessions that don’t appear to fit the principles described above. In particular, evidence emerged that endurance athletes whose events rely primarily on aerobic energy production can actually benefit greatly from short, intense interval repeats of under a minute, sometimes referred to as ‘sprint intervals’ or HIIT. The enthusiasm for shorter, high-intensity intervals stemmed from research by a Japanese professor called Izumi Tabata of the National Institute of Fitness and Sports in Kanoya, Japan(7).
In his landmark study published in 1997, Tabata compared athletes performing one hour of steady-state, moderate-intensity (70% of maximum oxygen uptake - VO2 max) endurance training on a stationary bicycle for five days each week with another group who also performed stationary cycling but using a 4-minute period of eight very hard intervals (170% of VO2 max) with a 2:1 ratio between work and rest (ie 8 sets of 20 seconds of work followed by 10 seconds of rest). Amazingly, the sprint-interval trained athletes made substantial gains in maximum oxygen uptake (VO2 max), indicating that the very short, high-intensity protocol had actually been able to boost aerobic endurance (as well as anaerobic performance), equalling the aerobic gains achieved by the steady state group!
Subsequent research has since confirmed that high-intensity sprint-intervals can be an effective tool for developing aerobic fitness(8). But if their duration is so short, how is this aerobic performance-boosting effect possible? The reasoning is that while the aerobic intensity and oxygen demand of each (short/sprint) interval is not particularly high (due to the slow oxygen uptake kinetics), by keeping the recovery intervals in between each interval effort short, the overall aerobic intensity through the session can be raised because there’s insufficient time in each recovery period to repay the oxygen debt. This in turn means the body has to respond by keeping the oxygen supply system ramped up, and getting better at delivering and using oxygen (exactly what happens during steady state training).
Shortcomings of short intervals
As mentioned above, compared to a larger body of steady-state training, sessions of short-duration, high-intensity intervals can deliver the same aerobic benefits for a reduced training volume – and also seriously enhance anaerobic performance(9,10). Given the performance benefits and time savings (which make it easier to slot in a quick interval session), it’s perhaps not surprising that many athletes now choose to perform interval sessions using short-duration, high intensity intervals rather than the traditional sessions using longer-duration intervals.
Although this approach seems logical, some researchers have questioned whether the use of short-duration intervals (under one minute) is optimal when it comes to generating the best endurance adaptations. This is because some more recent studies have concluded that the best way to improve oxygen uptake and endurance performance is to train with a view to maximizing the time in the training session spent near maximal oxygen uptake (ie over 90%–95% of VO2max)(11-13).
The problem with using short intervals is that several studies have concluded that the use of short interval durations (under 60 seconds) in interval sessions tends to be less successful in achieving high levels of maximal oxygen uptake and maintaining these levels above 90% or 95% of VO2max compared to longer intervals(14,15). For example, one study compared the physiological strain of long intervals (4 × 3 minutes) vs. short intervals (21 × 30 seconds) performed at the same intensity, the same work-rest ratio (1:1) and the same total duration (21 minutes) in highly-trained endurance athletes. Averaged over the entire training session, there was the same overall elevation in heart rate across both sessions, but a higher oxygen uptake and higher subjective exertion in the longer intervals(14).
Some researchers have suggested that this shortcoming of short intervals (not enough time spent above 90-95% VO2max) can be overcome by increasing further the intensity of each short interval(16). However, the research on this approach is mixed to say the least. In a study on endurance-trained athletes, upping the intensity of 30-second intervals from 100% of maximal aerobic velocity to 110% of maximal aerobic velocity did not lead to a significant increase in time spent above 90% VO2max(17). In fact, because of the more anaerobic nature of the intervals, the athletes couldn’t complete as many reps, which meant that the total time accumulated over 90% of VO2max actually fell by over a third!
New research on short vs. long intervals
Another theoretical way of manipulating a session of shorter, high-intensity intervals in order to produce more accumulated time at 90% or more of VO2max in the session could be to up both the interval intensity and change the nature of the recovery period in between intervals from passive to active (see this article). This should slow the drop in heart rate and oxygen recovery during the rest period, placing more oxygen demand in the following interval, thus increasing overall oxygen uptake. However, no study has ever been carried comparing this approach with traditional longer interval durations – until now that is, thanks to brand new research by a team of German scientists(18).
Published in the journal ‘Frontiers in Sport and Active Living’, this study investigated whether short running intervals using a fairly intense speed of 100% vVO2max (the speed when running at maximum oxygen uptake) combine with active recoveries in between was sufficient to boost the time spent above 90% VO2max, and how it compared to long intervals where the total work duration and intensity was the same.
What they did
Twelve highly-trained middle-distance runners (7 males: and 5 females) took part in the study. All the runners were part of national training groups and had competed in 400m and 800m at national and in some cases international championships. Over a period of five days, the subjects completed an initial assessment then following this, two VO2max interval sessions. These were as follows:
- A VO2max incremental ramp test on the treadmill to determine each runner’s maximal oxygen uptake and the running speed at which their maximal oxygen uptake was reached.
- A traditional long-duration interval session consisting of 4 repeats of 3-minute intervals performed at 95% vVO2max, each interval separated by an active recovery of 3 minutes jogging at 50% vVO2max.
- A high-intensity session of short interval durations, consisting of 24 repeats of 30-second intervals at 100% vVO2max, each interval separated by an active recovery of 30 seconds jogging at 50% vVO2max.
Note that each interval session contained exactly the same total amount of work and rest periods, but the short intervals used a slightly higher work intensity (100%vVO2max) compared to the long intervals (95% vVO2max). Each runner performed both interval sessions but the order of the two sessions was chosen at random to avoid any bias. A time interval of 48 hours was maintained between all tests, and on the days between the tests, the athletes were asked to complete only easy training (up to 30 minutes) or to take a day off. During the two interval sessions, the athletes’ respiratory gases were measured breath-by-breath to determine oxygen consumption, and therefore what proportion of VO2max they were maintaining through the sessions. In addition, blood lactate and heart rates were constantly monitored.
What they found
When the data was analyzed, what the researchers discovered surprised them. The main findings were as follows:
- The time accumulated above 90% VO2max was significantly lower in the 30-second intervals compared to the 3-minute intervals (see figure 1 - 201.3 seconds in the short intervals vs. 327.9 seconds in the long) despite the intensity of the shorter intervals being higher than that of the longer intervals. In other words, despite their lower intensity and same total interval work time, the long intervals were much more effective at generating oxygen uptakes above 90% VO2max.
- When it came to heart rates, the situation was reversed because the short intervals generated more accumulated time with heart rates above 90% maximum (see figure 2).
- Despite the lower intensity, the longer intervals generated higher levels of blood lactate, indicating a higher degree of physiological loading of the cardiovascular system.
- In terms of perceived exertion, there was no difference between the short and long intervals.
When concluding their study, the authors commented: “Increasing the intensity of short VO2max intervals may not always lead to the desired effect, particularly if achieving a high time spent above 90% VO2max is the primary training goal.”
Figure 1: Time spent above 90% VO2max threshold
Green line = 4 × 3-minute protocol (long intervals); blue line = 24 × 30-second protocol (short intervals). The red area indicates the range between 90 and 100% of VO2max. Note how the long interval protocol results in a considerable amount of time above 90% of VO2max whereas the short protocol results in far less.
Practical implications for athletes
This study shows that even when short intervals are made harder and have an active recovery period in between, they are less effective at generating accumulated time above the all-important 90% VO2max threshold – a pivotal marker of how effective a training session is at inducing endurance performance gains. Therefore, if all your intervals sessions consist of short intervals (under a minute), that training may not lead to the performance gains you desire. In this respect, traditional long intervals of 2-4 minutes’ duration are definitely superior in terms of time spent above 90% VO2max.
An important point to make is that athletes who use heart rates to reflect session intensity may erroneously believe that their short-duration interval sessions are generating the all-important 90%+ of VO2max response. But as we can see from the above study, this is not the case at all because a 90%+ heart rate does not correspond to 90%+ VO2max. In fact, it is likely that you will be missing out on a 90%+ VO2max response because your oxygen delivery system is just not taxed long enough in each interval.
In practice therefore, when you are using heart rate as a training monitoring marker, these findings suggest that you should aim for heart rates that exceed 93%–95% of your max heart rate – ie to ensure that you are generating time 90% VO2max. Importantly, you should try to ensure that you establish your maximum heart rate accurately via a maximal test rather than by using an approximate formula or performing a submaximal test and extrapolating upwards. That said, there is the potential for risk associated with a flat-out maximal test to exhaustion; those who are unaccustomed to training at near maximal effort, especially older athletes, are recommended to seek medical clearance first to be on the safe side (see this article for a more in-depth discussion of introducing intense training into a program)(19).
Finally, it’s important to emphasize that you don’t need to make an either-or choice with regard to interval lengths. Mixing up your sessions between long and short-duration intervals is a great strategy. Not only does it add variety, the incorporation of short intervals will be effective for helping to develop anaerobic power. These short-duration interval sessions are also easy to slot in when time is tight. However, making sure you perform long-duration interval sessions as well will help fine tune your aerobic engine for ultimate performance!
References
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2. Journal of Sports Medicine and Physical Fitness. May 2015;55(5):377-82
3. J Sports Sci. Oct 2005;23(10):993-1002
4. Med Sci Sports Exerc. Apr 2007;39(4):665-71
5. J Appl Physiol (1985). Jul 2002;93(1):167-74
6. Eur J Appl Physiol. Jun 2000;82(3):178-87
7. Med Sci Sports Exerc 1997; Volume 29(3), pp 390-395
8. Scand J Med Sci Sports. Oct 2009;19(5):687-94
9. Front. Physiol. 2018;9:1012
10. Limits Hum. Endur. 2013;76:51–60
11. Sports Med. (2013) 43(5):313–38
12. Eur J Appl Physiol. Jul 2019;119(7):1513-1523
13. Physiol Behav. May 15 2018;189:10-15
14. J Sports Sci Med. (2017) 16(2):219–29
15. Eur J Appl Physiol. (2007) 102(1):19–26
16. Eur J Appl Physiol. (2003) 90(1–2):50–7
17. Eur J Appl Physiol. (2007) 102(1):19–26
18. Front Sports Act Living. 2025 Jan 6:6:1507957
19. Medicine & Science in Sports & Exercise November 2015. 47(11):p 2473-2479
Andrew Hamilton
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