In 2015, Zoran Milanović and his team at the University of Niš published in Sports Medicine a meta-analysis of 46 randomized controlled trials examining the effect of HIIT (High-Intensity Interval Training) on VO2max. With more than 700 total participants, it is one of the most comprehensive syntheses available on the subject. Here is what it actually says.
Why VO2max?
VO2max, the maximum rate of oxygen consumption, is the reference marker for cardiorespiratory capacity. It predicts all-cause mortality better than most other available scientific indicators. In sedentary individuals, every 1-MET (metabolic equivalent) increase is associated with roughly a 13% reduction in cardiovascular mortality.
HIIT improves VO2max in a statistically significant way across all studied populations, with gains ranging from +8% in already-trained populations to +25% in sedentary or reconditioning subjects. The effect is greater than that of continuous cardio of equivalent volume.
The key results
Why the range is so wide
The variation between +8% and +25% is not a sign of imprecise science, it reflects real differences between populations and protocols. Several factors explain this dispersion:
- Initial fitness level: the strongest predictor. A sedentary subject with a VO2max of 25–30 ml/kg/min can gain 6–8 ml/kg/min in 8 weeks. An athlete at 55 ml/kg/min will progress 2–4 ml/kg/min at most.
- Interval intensity and duration: protocols at 85–95% of HRmax with 1–4 minute intervals produce the largest gains.
- Weekly frequency: 3 sessions per week appears optimal. Beyond that, marginal gains diminish and overtraining risk rises.
- Program duration: effects appear from 4 weeks but generally plateau between 8 and 12 weeks for a given protocol.
HIIT vs. continuous cardio: what the literature says
Milanović et al. confirm a robust finding: for an equivalent total training volume, HIIT produces greater improvements in VO2max than moderate continuous cardio (MICT). This superiority is particularly pronounced in untrained subjects and in short protocols (4–8 weeks).
The distinction matters for time-constrained individuals. A 20–25 minute HIIT protocol can produce cardiovascular adaptations comparable to 45–60 minutes of moderate cardio, according to the literature. This is one of the reasons HIIT has become a standard in cardiorespiratory reconditioning protocols.
Our infrared HIIT protocol combines the documented cardiovascular effects of HIIT with the thermal benefits of an infrared chamber: increased energy expenditure, active sweating, facilitated muscle recovery. The protocol is adapted to each client's cardiorespiratory profile, measured beforehand.