Introduction: Youth rugby league athletes are increasingly engaging in rigorous resistance training to meet the demands of their sport. As part of periodisation, resistance training workload volume is typically altered in a wave-like progression (i.e. mesocycles). While exposure to an unfamiliar stimulus is desirable during periodisation, youth athletes may initially encounter elevated levels of exercise-induced muscle damage (EIMD), including delayed onset muscle soreness (DOMS), creatine kinase (CK), and loss of muscle function. Developing a greater comprehension of the acute responses to variations in resistance training workload volume could aid practitioners in managing symptoms of EIMD, ensuring training quality, and minimizing injury risks. Therefore, this study aimed to investigate the impact of transitioning between different mesocycles on indirect markers of muscle damage and muscle performance metrics in resistance-trained youth rugby league athletes. Methods: Resistance-trained youth male rugby league athletes (n=18) volunteered for this six-week study. Participants underwent a four-week mesocycle aimed at maximal strength development, followed by one maximal strength exercise session in Week 5 and one hypertrophy exercise session in Week 6. Outcome measures included CK, DOMS, lower limb explosive strength (drop box jump) and upper body explosive strength (plyometric push-up). All measures were evaluated before, at 24 hours (T24), and at 48 hours (T48) after the maximal strength training session in Week 5. Additionally, muscle damage and performance measures were assessed before, at T24, and at T48 following the hypertrophy session (i.e., same strength exercises conducted in Week 5 but with reduced intensity and increased volume) in Week 6. Comparisons of variables between strength and hypertrophy sessions over time were examined via 2-way ANOVA and post-hoc pairwise comparisons with Bonferroni correction. Results: Measures of DOMS and CK was significantly increased, while drop jump performance was significantly lower 24-hours and 48-hours after the hypertrophy modality in Week 6, although these trends were not identified after the strength modality in Week 5. Furthermore, the changes in DOMS, CK and drop jump performance were significantly more pronounced after the hypertrophy modality in Week 6 than the strength modality in Week 5. However, there were no differences in plyometric push-up performance after strength and hypertrophy modalities, nor between the strength and hypertrophy modalities. Discussion: Raising workload volume during a mesocycle transition led to heightened muscle damage and hindered lower body explosive strength measures for up to 48-hours following several weeks of maximal strength training. Coaches should closely monitor acute strength responses amid workload volume fluctuations and implement strategies to enhance training quality and performance in athletes. Impact/Application to the field:Whilst a heightened physiological stimulus is desirable at the commencement of a new training phase; coaches should monitor acute responses and incorporate recovery interventions to enhance training quality and performance for their athletes.
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