International Journal of Exercise Science 13(2): 480-487, 2020. The aim was to investigate individual responses on VO2maxand performance to a block of high intensity aerobic interval training (HIIT) in national-level endurance athletes. METHODS: National-level cyclists and triathletes (five men and two women, 31 ± 3.3 years, VO2max65.1 ± 3.3 ml·kg-1·min-1) conducted 14 HIIT sessions (4x4 min uphill running at 90-95% maximal heart rate) in nine days during preseason. VO2maxin running and cycling, lactate threshold (LT) in cycling, oxygen cost of cycling (CC), and a cycling time-trial (TT) were tested two days pre and seven days post intervention. Feasibility was determined using attendance rates, adherence (defined as completing all sessions), and reported adverse events. RESULTS: The results showed that adherence was 100% with 100% attendance rate. No adverse events were reported. TT (-75.6 ± 50.8 seconds, p< 0.0001)but not VO2maxcycling (-0.2 ± 4.6 ml×kg-1×min-1, p= 0.53) or running (0.2 ± 1.2ml×kg-1×min-1, p= 0.85) was improved on group level. The individual responses varied from -8.2% to +14.5% change in VO2max, and -7.5% to +0.8% in TT. CONCLUSIONS: The large inter-individual differences in responses call for tailor-making HIIT blocks, mapping of biomarkers to avoid overtraining, and studying the effects of such blocks with longer follow-up than seven days.

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