How does HIIT make you fitter 1

Interval exercise typically involves repeated bouts of intense exercise followed by short periods of recovery. This method is commonly divided into high-intensity interval training (HIIT; ‘near maximal’ efforts of 85-95% max heart rate) and sprint interval training (SIT; ‘supramaximal’ all-out efforts). Both forms of interval training induce similar adaptations to moderate-intensity endurance training such as increased aerobic capacity (VO2max) and mitochondrial content.

But what exactly is the role of intensity on the beneficial adaptations to aerobic capacity, and how intense do the intervals need to be? These are questions that senior researchers at McMaster University in Canada set out to answer in a paper reviewing the state of the art in our understanding.

How does interval training work?

Improvements in aerobic capacity come from two main sources:

  1. Increased muscle mitochondria content and capillary density
  2. Increased maximal stroke volume and cardiac output from the heart, combined with increased blood volume

Increased mitochondria density of the muscle not only allows higher aerobic capacity, but allows proportionally more fat to be used as fuel. This is good news also for lactate production allowing athletes to exercise for longer at higher % of their VO2max.

This mitochondrial content responds rapidly to training, and a single session is enough to activate increases. This activation stops after about 5 days of training at the same intensity, however if the intensity is increased it continues to rise for several weeks. There is also good evidence that higher intensities produce greater signals (mRNA for PGC-1a if you’re really interested) to create more mitochondria than lower intensities.

The authors also carried out studies to show that the on-off intermittent nature of interval training increases the adaptations compared to continuous workouts. They have demonstrated that three weekly sessions of Sprint Interval Training (1 min SIT per session) produced similar increases in signalling chemicals to 150 mins of weekly steady state exercise (that’s 98% off for the bargain hunters amongst you).

Higher exercise intensities also require more muscle fiber types to be recruited. Whereas moderate intensity exercise can rely almost exclusively on type I (red) muscle fibers, high intensities are needed to increase the mitochondrial content of type II (white) fibers.

With respect to the capillary density of the muscle, there is no clear evidence from research that exercise intensity rather than volume affects the relatively long timescale over which this increases.

A key measure of aerobic capacity is of course VO2max, and several studies over the past 25 years have concluded that HIT is more effective in increasing VO2max than the same amount of work performed in medium intensity training, both in healthy and unfit people.

These authors report that duration and frequency of interval training have much less impact than intensity, but still we need guidance on what structures of training do have the biggest bang per buck. Renowned exercise physiologist Stephen Seiler has looked at this in detail in skiers, runners and cyclists, and concluded that 8 min intervals produce bigger fitness gains in more athletes:

Seiler S, Jøranson K, Olessen BV, Hetlelid KJ. Adaptations to Aerobic Interval Training: Interactive Effects of Exercise Intensity And Total Work Duration. Scand. J. Med Sci Sports 23, 74-83, 2013.


Dr Seiler also gives a useful prescription list for coaches and self coached athletes using HIIT:

  • Work bout duration and total accumulated work duration are two critical variables in the prescription that push (or pull) athlete into desired intensity zone.
  • Rest interval duration has less relative impact, once you get above 90 seconds rest, but adding work intensity to rest periods will bring down work bout intensity quickly (Fartlek training).
  • Prescribe to athletes that they should maintain constant power throughout session, maximal session effort.
  • Let heart rate vary within the parameters of the prescription, do not use heart rate recovery to determine rest duration, just fix the rest duration at about 120 seconds.
  • HIT intensity vs accumulated duration? – lower intensity and higher AWD gives equal or greater gain for less “pain”!

This final point is an important one. Performing intervals is not usually regarded as ‘fun’, so it’s good to find a format that maximises the benefits whilst minimising the suffering!

As we have reported in a previous blog post performing HIT on good heart rate variability (HRV days), HIT zone on the ithlete Pro Training Guide, delivers consistently improved results, and also feels better than doing them when you are under recovered and fatigued.


Ref 1: J Physiol 595.9 (2017) pp 2915–2930 2915 SYMPOSIUM REVIEW

Physiological adaptations to interval training and the role of exercise intensity

Martin J. MacInnis and Martin J. Gibala

Department of Kinesiology, McMaster University, Hamilton, Ontario, Canada


Ref 2: Many short or a few long HIT work bouts – does the HIT prescription matter?

Ole Wåle and Andreas M. Pedersen Masters Research project, University of Agder Kristiansand, Norway (Supervisor Dr Stephen Seiler).