BACKGROUND: Weekly training volumes for triathlete are typically higher and may cause fatigue and musculoskeletal injury risk. High-intensity interval training (HIT) is a potent time-efficient strategy to induce adaptations normally associated with traditional endurance training. Therefore, the aim of this study was to investigate the effect of two weeks of in-season HIT on exercise capacity and hormonal responses in young triathletes. METHODS: Twelve adolescent triathletes performed 18 sessions of HIT over 2 weeks including swim, cycle, and run events. The 6-day training blocks were separated by 1 day of recovery. Pre- and post-training, peak oxygen uptake (VO2peak) and exercise performance were assessed, and blood samples were obtained to detect changes in hormone and metabolite levels. RESULTS: VO2peak was significantly higher (P=0.02) post-training (56.4±8.1 mL7min·kg) versus pre-training (55.1±7.5 mL/min·kg). Mean power and total work during 6×10 s repeated-sprint tests significantly increased (P=0.03) after HIT. Additionally, 750 m swim time (pre- vs. post: 689.7±102.5 s vs. 662.0±75 s, P=0.01) and 20 km cycling time (pre- vs. post: 1856.6±274.8 s vs. 1705.4±266.8 s, P=0.02) were significantly lower post-training compared to pre-training, but there was no significant difference in 5 km run time after HIT (pre- vs. post: 1315.8±81.3 s vs. 1292.0±112.9 s, P=0.31). In contrast to pre-training, ammonia concentration was significantly increased (P<0.01) and creatine kinase concentration was significantly decreased (P=0.02) post-training. CONCLUSIONS: These findings suggest that two weeks of HIT using HRpeak as a monitor of physiological intensity improved VO2peak, sprint performance, and triathlon-specific performance in adolescent triathletes and attenuated levels of muscle damage.
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