HIIT or MIIT for maximum endurance?

As all our regular subscribers will already know, interval training is an excellent method of improving fitness. Research shows that regular sessions of interval training can produce significant fitness gains in less time and with less effort than a higher volume of steady-state endurance training^(1-4). Even better, the benefits of interval training can also be realised by amateur and recreational athletes – not just elite athletes. Indeed, recreational athletes who introduce intervals to a training schedule stand to gain even more than more advanced level athletes (who will almost certainly already be utilizing interval training in their training programs)⁽⁵⁾.

Intervals: how hard?

When interval training first came to the fore as a training strategy, sessions were typically modelled around interval lengths 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⁽⁶⁾. 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’).

However, a landmark 1997 study by a Japanese professor called Izumi Tabata showed that just 8 x repeats of very high-intensity, shorter intervals of 20 seconds’ duration could achieve excellent gains in endurance fitness⁽⁷⁾. In the years following, subsequent research on a wide range of athletes has indeed confirmed that high-intensity interval training (HIIT) using short intervals can be a very effective method of sharpening endurance fitness in athletes^(8-10), which to a large extent explains the huge growth in the popularity in these short but intense interval sessions.

Duration matters too

The option of performing very short, very intense intervals instead of longer-duration intervals at a lesser intensity was something of a revelation for athletes short of time or an inherent love for interval sessions! However, the intensity of interval session isn’t the sole determinant of the fitness benefits that accrue. It turns out that what you get out of intervals depends on both the intensity and the total time spent working, not just one or the other⁽¹¹⁾. This explains why endurance athletes such as cyclists often mix in longer-duration more moderate-intensity intervals (MIIT) into their training programs^(12,13). Indeed, research shows that these longer MIIT (although still challenging) sessions performed for four weeks or more can match the benefits gained from in terms of improving lactate threshold (the intensity at which fatiguing lactate begins to accumulate in muscles) and maximum power outputs when working near to maximum oxygen uptake (power output at VO2max)⁽¹⁴⁾.

Because HIIT is a very effective and time-efficient method of training, there’s been very little recent research into the use of moderate-intensity interval training in athletes. Earlier this year however, the reported on new research on cyclists, which compared a 1-week block of MIIT and an equivalent 1-week block of their regular training in the build-up phase to a race season⁽¹⁵⁾. In this study, researchers recruited 30 well-trained cyclists and compared:

·        An MIIT block involving six moderate-intensity interval sessions over seven days, each with five to seven repeats of 10-14 minute work intervals (ie long intervals), performed at a perceived exertion (RPE) of 14-15 on the Borg 6-20 scale (‘somewhat hard’ to ‘hard’).

·        A time-matched regular training block, during which the cyclists performed their regular preparatory-phase training routine, which primarily involved low-intensity exercise. However, they were also specifically instructed to perform either two MIIT sessions (as described above) per week, or one MIIT session and one high-intensity interval session per week.

The rather surprising finding was that the six moderate-intensity interval sessions performed over seven days, followed by a six-day active recovery period produced superior improvements in endurance performance compared to the same volume/time spent performing a regular training period.

HIIT vs. MIIT

Dozens of studies have compared a block (from one to several weeks) of HIIT to an equivalent-duration block of continuous steady-state training and found that HIIT sessions are superior for raising maximum aerobic capacity – especially in already well-trained athletes. In a similar vein, there have been (albeit fewer) studies comparing a block of MIIT to an equivalent block of continuous steady-state training; in these studies, the MIIT seems to excel for raising lactate thresholds, allowing athletes to maintain a higher power output before hitting their threshold. What there’s not been to date however, is a study directly comparing a block of HIIT to an equivalent block of MIIT in well-trained endurance athletes. However, published just three weeks ago, a new study by a team of Norwegian scientists has investigated this very question.