High-intensity intervals: is less even more than we thought?

Andrew Hamilton looks at research suggesting that four and half minutes a week of 15-second intervals can produce significant gains in endurance performance

A number of previous studies have demonstrated that high-intensity interval training (HIIT) allows you to achieve significant fitness gains, in a short time frame and in a way that doesn’t leave you physically and mentally exhausted. But can HIIT really benefit athletes interested in building endurance rather than sprint performance? The answer to that question is an emphatic yes! Scientists have discovered that even short sessions of HIIT can significantly change the biochemistry of muscle cells in quite profound ways; exactly how these biochemical and structural changes occur is still unclear but in very simple terms, it seems that the repeated and intense bursts of exercise during HIIT stimulate ‘endurance genes’ in your muscles, boosting their activity, enabling you to perform faster for longer.

15-second intervals

Some studies have demonstrated the benefits of HIIT using very short intervals of just 30 seconds, but there’s also evidence that even shorter intervals can still produce excellent results.In one study, Canadian researchers examined the effects of reducing the duration of the work-interval duration when doing sprint interval training(1). In particular, they wanted to see whether reducing the interval duration from 30 seconds to just 15 seconds affected the gains in maximal and sub-maximal performance over a 4-week training period. To do this, 36 healthy subjects were assigned to one of three training groups:

  1. Endurance training, consisting of 60 minutes stationary cycling, 3 sessions per week for weeks 1-2, then increasing to 75 minutes per session for weeks 3-4;
  2. Sprint interval training on a stationary bike, consisting of repeated 30-second all-out intervals starting with 4 intervals per session for weeks 1-2, then increasing to 6 intervals per session for weeks 3-4;
  3. Sprint intervals as above but with intervals of just 15 seconds – ie half the interval duration.

Before and after the 4-week training period, the subjects were fitness tested, which included measures of lactate threshold (how much power can be sustained before fatiguing lactate starts to accumulate in the muscles), critical power (the highest power you can sustain for a particular period of time regardless of lactate accumulation) and peak power achieved in a test to exhaustion. After 4 weeks of training, the key finding was that all three groups made the same gains in all three measures of fitness – ie slashing the volume of interval work by 50% still produced the same benefits!

Implications for endurance athletes

The first thing to say is it seems remarkable that just 3 to 4.5 minutes of work intervals per week produced the same fitness gains as 180-225 minutes a week, which is what the endurance group clocked up! Some caution is required here because these although these subjects were healthy, they weren’t trained athletes. That said, this study provides yet more evidence that some HIIT can be a very powerful training tool, especially if time is short. There’s also support for these findings from other studies. For example, a 2010 study found that performing 7 x 30-second sprint intervals was just as effective at increasing markers of aerobic fitness as 3 x 20-minute hard efforts despite the fact that during long intervals, the total work performed was 8 times greater and the exercise duration 17 times longer than during the sprint intervals(2). Meanwhile, another Canadian study found that a HIIT programme consisting of 6 intervals of just 10 seconds sprinting produced significant gains in measures of aerobic fitness(3).  More recently, researchers found that a programme including  very high-intensity 30-second sprint intervals induced greater improvements in maximal oxygen uptake (a key indicator of endurance performance) in already well-trained cyclists than did simply continuing with their normal cycling programme(4).

In terms of your own training, if time is tight, or you’re dipping your toes in the ‘interval training waters’ for the first time, consider using short duration, very high-intensity intervals; they’re relatively easy on the body, quick to perform and yet are effective for building speed and endurance. The current consensus is that intervals should be performed 2-3 times a week; if you elect for 30-second intervals, six intervals at 90% of your peak power has been shown to be effective. For even shorter intervals (10-15 seconds), up to ten intervals at maximum power is recommended. Remember though – while high intensity intervals are a very effective training tool when time is limited, you’ll still need to do some regular longer sessions to properly develop your distance capability!


  1. Eur J Appl Physiol. 2014 Nov;114(11):2427-36
  2. Eur J Appl Physiol. 2010 Oct;110(3):597-606
  3. Eur J Appl Physiol. 2010 Sep;110(1):153-60
  4. Eur J Appl Physiol. 2016 Aug;116(8):1495-502

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