Purpose: High-intensity interval training (HIIT) has emerged as a time-efficient alternative to traditional endurance training. This study investigated the molecular, systemic, and physiological adaptations induced by an 8-week HIIT program in national-level flatwater kayak athletes. Methods: Six trained male kayakers completed an 8-week HIIT intervention. Skeletal muscle biopsies and venous blood samples were collected before and after training to assess markers related to endocrine function, growth and remodeling, angiogenesis, and inflammation. Physiological and performance measures were evaluated using a maximal oxygen uptake (VO₂max) test and kayak ergometer trials. Results: HIIT elicited significant improvements across molecular, systemic, and performance parameters. Post-training analyses showed increased expression of IGF-1R, MMP-4, MMP-9, and TNF-α (p < 0.05), along with elevated serum testosterone concentrations (p < 0.05). Notable performance gains were observed in paddling speed at the second ventilatory threshold (PSVT2; p < 0.05) and in 1000 m (p < 0.01) and 200 m (p < 0.001) time-trial performances. Conclusions: An 8-week HIIT program effectively enhanced molecular signaling, systemic adaptation, and sport-specific performance in elite flatwater kayak athletes. The concurrent upregulation of anabolic, remodeling, and inflammatory pathways suggests that HIIT facilitates coordinated muscular and systemic adaptations beneficial for kayak performance.

The purpose of this study was to examine the independent contribution of aerobic physiological variables to 500 m sprint kayak performance in national-level male athletes. Twenty-four male flatwater kayak athletes completed post-intervention physiological and performance assessments following a 6-week training intervention. Sprint performance was assessed using a 500 m kayak ergometer time trial. Pearson correlation and hierarchical multiple regression analyses were performed after accounting for training-group allocation. Power output at VO₂max (POVO₂max) demonstrated the strongest association with 500 m performance time (r = −0.71, p < 0.001). Hierarchical multiple regression analysis showed that training-group allocation explained a small, non-significant proportion of the variance in performance time (R² = 0.082, p = 0.648). The addition of POVO₂max significantly increased the explained variance (R² = 0.708, ΔR² = 0.626, p < 0.001); whereas, VO₂max (ΔR² = 0.012, p = 0.363) and post-exercise blood lactate concentration (ΔR² = 0.004, p = 0.855) did not provide additional explanatory value. These findings indicate that POVO₂max was the strongest independent physiological predictor of 500 m sprint kayak performance after accounting for training-group allocation.