Progressive high-intensity interval training (HIIT) is not superior to unmodified non-progressive HIIT in an uncontrolled setting

BACKGROUND: High-intensity interval training (HIIT) is an important training component to improve aerobic and anaerobic exercise capacity. Higher HIIT workloads in general may generate additional effects on the improvement of exercise capacity, while missing adherence to more strenuous training regimes may affect training success. This study investigated if higher training workload generated by progressive HIIT (proHIIT) is superior to HIIT when used in an uncontrolled setting. METHODS: Thirty-four moderately trained females and males performed a 4-week training intervention with three exercise sessions per week. Participants were randomized into two HIIT groups using the individual lactate threshold at baseline: Group 1 (N.=17), HIIT, four runs at maximal speed (all-out) with 30 s active recovery (total: 48 runs), Group 2 (N.=17), proHIIT, 4 runs at maximal speed (all-out) with 30-second active recovery with one extra repetition every week (up to seven runs, for a total of 66 runs). An incremental field test protocol with standard blood lactate (LA) diagnostic and heart rate monitoring was used to access changes in exercise capacity. RESULTS: Overall, power output (running speed) at LA threshold (baseline LA+1.5 mmol/L) increased by +3.6% (P=0.004, effect size [ES] 0.38) after 4 weeks of HIIT. However, no significant between-group differences pre- vs post-intervention were detected. CONCLUSIONS: Our data suggest that proHIIT does not provide additional improvement of running speed at individual lactate threshold over HIIT in an uncontrolled setting.