Low-intensity training (Zone 1/2 using a 5- or 7-zone intensity model) is a cornerstone of endurance training. It allows athletes to stimulate aerobic adaptations while imposing substantially less systemic stress than high-intensity training, making it possible to accumulate a large training volume to support physiological adaptations.
An interesting question can be: which principle should we follow to maximize training adaptation in each scientific exercise intensity domain:
low intensity zone (below 1st threshold)
medium intensity zone (between 1st and 2nd threshold)
high intensity (above the 2nd threshold)
In the context of high-intensity training, we have seen that the key determinant of aerobic adaptations is maximising the time spent at a high fraction of VO₂max.
However, the same principle does not appear to apply to medium-intensity training. As seen in a previous KIW Research Note, a study has reported that the average fraction of VO₂max reached during medium-intensity workouts is not related to the magnitude of aerobic adaptations.
But what about low-intensity training below/at the first threshold?
It is common to hear that you should always train as close as possible to the first threshold (the upper end of zone 2/low intensity domain) to maximise aerobic gains, with the argument that fatigue is essentially the same because you are still below the threshold.
But is this actually true? Does training right at the first threshold produce greater physiological and performance adaptations than training 10–15% below it, while causing a similar amount of fatigue when the duration of the session is the same?
A hot-off-the-press (2026) study published by Guimarães and Colleagues (Federal University of São Paulo, Brazil) on Journal of Strength and Conditioning Research tried to answer this question. (1)
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