A symbiosis of training forms

Everyone knows the feeling; you’ve been working out but you soon realise that this training will not break any records. The data on your Garmin watch only confirms this feeling. You’re just not feeling too fit today …

Fitness, it often turns out to be some vague term used to describe how you assess your physical form at given time.
However, this vague term can be transformed into a more scientific term, namely VO2 max. This measure stands for the oxygen absorption capacity of your cardiovascular system, in other words; the amount of oxygen your body absorbs during exercise.
And oxygen is exactly what your muscles desperately need during your running sessions.

Studies have shown that your VO2 max is strongly dependent on the strength of your heart muscle on the one hand (the stronger your heart muscle, the more red blood cells it can pump around and the more oxygen it will carry to your organs) and on your red blood cell count on the other hand (the more red blood cells, the more oxygen it will carry to your organs).

The effect of interval training on the oxygen absorption capacity of your body can be explained on the basis of the Fick principle. The heart pumps oxygen-rich blood into the body, where organs that need oxygen to function will extract their necessary oxygen from the blood. Consequently, only oxygen-poor blood should enter the heart. Which is not the case…

The fact that oxygen-rich blood still arrives at the heart indicates that your organs have not absorbed all the oxygen present in the bloodstream. So there are other factors that play a role in oxygen uptake. The density of the capillaries (blood vessels where the actual oxygen exchange takes place) and the amount of mitochondria, along with their efficiency, appear to be such factors. The greater the density of capillary blood vessels, the more oxygen exchange can take place in the muscles. Interval training will have such effect on these factors …

Interval training can consist of HIIT (High-Intensity Interval Training), intense exercise intervals of up to 5 minutes, or SIT (Sprint Interval Training), all-out sprints of up to 30 seconds alternated with a few minutes of rest.
Recent studies have shown that both forms of interval training induce both central and peripheral adaptations, but the degree to which these adaptations occur differs.
For example, HIIT will optimize your heart muscle function so that more blood can be pumped around in the long run, thus transporting more oxygen to your organs.
SIT, on the other hand, will induce more peripheral adaptations; such workouts will increase your capillary density so that more oxygen can be taken up by your organs, for example your muscles.

To optimise your fitness level, it is therefore strongly recommended to combine your endurance training with interval training. Also alternate between HIIT workouts and the SIT workouts in terms of interval training to achieve both central and peripheral adaptations.



  • A.P. Bacon, R.E. Carter, E.A. Ogle, M.J. Joyner. VO2 max Trainability and High Intensity Interval Training in Humans: A Meta-Analysis. September 2013. PLOS ONE.
  • S.G. de Oliveira-Nunes, A. Castro, A.V. Sardeli, C.R. Cavaglieri, M.P.T. Chacon-Mikahil. HIIT vs. SIT: What Is the Better to Improve VO2 max? A Systematic Review and Meta-Analysis. December 2021. International Journal of Environmental Research and Public Health.
  • M.A. Rozenblad, A.S. Perotta, S.G. Thomas. Effect of High-Intensity Interval Training Versus Sprint Interval Training on Time-Trial Performance: A Systematic Review and Meta-Analysis. February 2020. Sports Medicine.