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

Background: 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. Methods: 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 (VO2max) and ski speed reached, blood lactate concentration, respiratory gas exchange data, and body weight. Results: Maximal speed at VO2max increased significantly from 13.5 ± 2.16 to 14.8 ± 1.7 km/h (p = 0.0196; Cohen’s d = 1.06). VO2max itself was retained (p > 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 (p = 0.021), indicating enhanced metabolic efficiency. Body weight decreased modestly but statistically significantly by 0.54 kg (p = 0.016). Conclusions: The dynamic HIIT protocol improved maximal performance (speed at VO2max by 32.9%) and metabolic efficiency in trained skiers without altering VO2max. These findings support the usefulness of flexible, individualized HIIT models to enhance aerobic endurance, especially for athletes at risk of performance plateaus.