It’s about the long game, not epic workouts: unpacking HIIT for endurance athletes

High-intensity interval training (HIIT) prescriptions manipulate intensity, duration, and recovery variables in multiple combinations. Researchers often compare different HIIT variable combinations and treat HIIT prescription as a “maximization problem”, seeking to identify the prescription(s) that induce the largest acute VO 2 /HR/RPE response. However, studies connecting the magnitude of specific acute HIIT response variables like work time >90% of VO 2 max and resulting cellular signalling and/or translation to protein upregulation and performance enhancement are lacking. This is also not how successful endurance athletes train. First, HIIT training cannot be seen in isolation. Successful endurance athletes perform most of their training volume below the first lactate turn point (<LT1), with “threshold training” and HIIT as integrated parts of a synergistic combination of training intensities and durations. Second, molecular signalling research reveals multiple, “overlapping” signalling pathways driving peripheral adaptations, with those pathways most sensitive to work intensity showing substantial feedback inhibition. This makes current training content and longer-term training history critical modulators of HIIT adaptive responses. Third, long term maximization of endurance capacity extends over years. Successful endurance athletes balance low-intensity and high-intensity, low systemic stress, and high systemic stress training sessions over time. The endurance training process is therefore an “optimization problem”. Effective HIIT sessions generate both cellular signal and systemic stress that each individual athlete responds to and recovers from over weeks, months, and even years of training. It is not “epic” HIIT sessions but effective integration of intensity, duration, and frequency of all training stimuli over time that drives endurance performance success.