A Novel Two-Week Dynamic HIIT Protocol Improves Roller Skiing Speed and Metabolic Efficiency in Trained Cross-Country Skiers: A Pilot Study

<b>Background</b>: The aim of the study was to investigate the effects of a novel two-week dynamic high-intensity interval training (HIIT) protocol, characterized by fixed-load and variable-time intervals ("two times up to ten minutes"), on performance and metabolic adaptations in well-trained cross-country skiers. <b>Methods</b>: Ten qualified skiers (six males, four females) completed six interval training sessions over two weeks. Pre- and post-intervention tests were performed to assess maximal oxygen consumption (VO_2max) and ski speed reached, blood lactate concentration, respiratory gas exchange data, and body weight. <b>Results</b>: Maximal speed at VO_2max increased significantly from 13.5 ± 2.16 to 14.8 ± 1.7 km/h (<i>p</i> = 0.0196; Cohen's d = 1.06). VO_2max itself was retained (<i>p</i> > 0.05), equivalence testing confirmed stable values within a ±2.8 mL/kg/min margin. Time to reach RER = 1.0 improved significantly across sessions (<i>p</i> = 0.021), indicating enhanced metabolic efficiency. Body weight decreased modestly but statistically significantly by 0.54 kg (<i>p</i> = 0.016). <b>Conclusions</b>: The dynamic HIIT protocol improved maximal performance (speed at VO_2max by 32.9%) and metabolic efficiency in trained skiers without altering VO_2max. These findings support the usefulness of flexible, individualized HIIT models to enhance aerobic endurance, especially for athletes at risk of performance plateaus.