Research papers

A 2005 study of 38 elite cyclists, published in the Journal of Strength & Conditioning Research, showed that interval training leads to improved respiratory function, including better blood flow through the lungs and oxygen-carbon dioxide exchange. In the study, HIT increased performance by improving ventilatory threshold and VO2 max, boosting the body’s ability to use oxygen to generate energy.

Another study, published in the Journal of Physiology in July 2006, found improved adaptations in muscle cells after interval training in contrast to traditional steady-state endurance training. The study compared two groups of active young men over two weeks. One group engaged in traditional long-duration training for 90–120 minutes, while the other did four to six sets of sprint intervals (30 seconds all-out and four-minute rest intervals). It revealed superior adaptations in muscle tissue of the HIT group.

HIT led to improved respiratory function

Literature review

AUT University’s review of 45 published journal articles, carried out on behalf of Les Mills, focused on 24 papers that compared HIT with steady-state moderate intensity exercise. The authors focused on moderately trained recreational athletes through to those with general metabolic syndrome (including obesity and hypertension) as more representative of the general population. The training studies ranged from two to 20 weeks, with typically three to five sessions a week.

Some studies used supra-maximal intensity (up to a reported 170 per cent of VO2 max). But most used ‘work’ phases of around 90 per cent VO2 max – also described as 15-17 out of 20 on a self-rating scale of perceived exertion (hard to very hard). Sessions generally lasted around 40 minutes, including work and recovery phases: work phases were typically 30 seconds to two minutes; with recovery mostly one to four minutes of light to moderate exercise (70 per cent HR max).

In several studies HIT produced 5-10 per cent greater increases in VO2 max, often in less time. In some cases even greater differences were observed. Where the steady state group did experience significantly greater increases in VO2 max “it was patently owing to very big differences in training volume”.

In all cases, HIT produced greater improvements in anaerobic fitness, insulin sensitivity, endothelin function and body fat levels, with significantly greater reductions also observed in systolic and diastolic pressure in several studies.

Although often anecdotally quoted as a benefit of HIT, only one study specifically examined EPOC (excess post-exercise oxygen consumption) and found no significant difference compared with steady-state moderate intensity exercise.

Several of the papers commented anecdotally that subjects typically reported their enjoyment of interval-style training more than steady-state training, leading to good adherence.

The authors of the review found no evidence that repeat high intensity exercise bouts had a harmful effect on any of the populations. They also found no evidence that exercise intensity alone has a negative effect on resting hormone levels (testosterone, cortisol etc).