Based on the current research, it appears that high intensity interval training (HIIT) is effective at stimulating central adaptations and increasing metabolic stress, while moderate intensity continuous training (MICT) may be more effective at targeting local adaptations such as improving muscle efficiency, oxygen extraction and lactate clearance.

Studies examined in this article suggest that central adaptations in younger people occur more quickly but there is a greater scope or ceiling for local adaptations. This is one of the reasons athletes train at a lower intensity to target local adaptations.

Scope for central adaptations in older adults appears to be more limited. It could be argued therefore that higher doses of HIIT are more relevant to a healthy, younger, more inactive group with diminishing returns after a year of continuous training. For example, a study that included only 12 participants under 30, who had not participated in training showed that MICT resulted in greater mitochondrial adaptations and lower blood lactate at the same relative work rate, while HIIT led to greater increases in V̇O2max and peak power.

It is difficult however to make definite conclusions based on scientific literature due to the lack of comparison studies, the variety of training protocols that have been used, the differences in samples and the power of the studies.

Additionally, it is unclear how the effects of HIIT and MICT may differ over a longer period of time, as most studies only last for a few weeks. Athletes and people that train regularly understand the intensity related recovery requirements.

Furthermore, training at higher intensity is associated with higher risk of injury. More research, including longer-term studies, is needed to determine the safety and effectiveness of long-term HIIT. Therefore, it is not yet justified to claim that HIIT is the superior form of exercise, as suggested by a retracted meta-analysis from 2019 calling it a 'magic bullet'.

When we look at athletes training programs, we can see that HIIT makes up a small component of their training programmers while MICT forms the main component. Coaches have established effective training protocols through trial and error. It can be argued that these protocols are superior to the scientific literature that have extremely limited methodologies. Extrapolating data from these studies, as some meta-analyses have done, provides inconclusive evidence for informing training protocols.

Iñigo San Millán on the subject of comparing the two said on twitter ‘All I can say in my humble opinion after 26 years working with world champions and Olympic medalists in many sports as well as TdF winners is that HIIT is a very very small % of their workload. And I insist, a very very small percentage.’

However, Iñigo did say on a podcast that his athletes have already maximised the central adaptations. Their hearts are already the best human pumps with the highest stroke volume and cardiac output. This is an interesting point, as we have looked at the literature which suggests that cardiac remodeling has a ceiling for adaptations, and these adaptations are generally limited to younger adults.

Therefore the major gains are made in the muscle and mitochondria. Having said that, he still recommends a heavily weighted zone 2 training program with a smaller percentage in the other zones.