The aim of the study was to quantify and compare the effects of repeated-sprint training (RST) vs. short-bout high-intensity interval training (HIIT) on hamstring architecture and physical fitness in rugby league players. A parallel, 2-group, pretest posttest design was used, whereby 24 representative U20 players were assigned to either RST or short-bout HIIT for a 6-week intervention delivered alongside usual training. Assessments of biceps femoris long-head (BFlh) muscle architecture, countermovement jump (CMJ), eccentric hamstring strength, sprint force-velocity power (FVP) profiles, and 1,200 m shuttle run time (i.e., aerobic fitness) were performed. Compared with baseline, the RST group was associated with moderate improvements in aerobic fitness (change ±90% confidence interval [CI]: -4 ± 5 seconds) and maximal theoretical velocity (0.11 ± 0.10 m·s-1), as well as a moderate increase in BFlh fascicle length (1.05 ± 0.46 cm), a small increase in BFlh muscle thickness (0.12 ± 0.16 cm), and a moderate reduction in the BFlh pennation angle (-0.85 ± 0.64°). The short-bout HIIT group was associated with a moderate improvement in aerobic fitness (-10 ± 4 seconds) and a small improvement in CMJ peak power (1.4 ± 1.4 W·kg-1), as well as a large increase in BFlh fascicle length (0.99 ± 0.44 cm), a moderate increase in BFlh muscle thickness (0.15 ± 0.13 cm), and a small reduction in the BFlh pennation angle (-0.67 ± 0.50°). Changes in aerobic fitness were greater for short-bout HIIT when compared to RST, and this difference was moderate (effect size ±90% CI: 0.89 ± 0.82; pMET 0.108). Conversely, changes in 10-m sprint (0.82 ± 0.66; pMET 0.062), 20-m sprint (0.95 ± 0.71; pMET 0.043), 30-m sprint (0.104 ± 0.73; pMET 0.031) times, and certain FVP characteristics, were greater for RST, and these differences were moderate. Both groups increased fascicle length, but RST was more effective at improving sprint speed, while short-bout HIIT was more effective for improving aerobic fitness. Collectively, our findings demonstrate the potential benefits of these conditioning methods, which may cause morphological, physiological, and neuromuscular adaptations in athletes when applied alongside usual training practice.
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