## 4 Discussion  
To the best of the authors' knowledge, this is the first study designed to investigate the effects of a volume-based (i.e., sets per exercise or muscle group) resistance training protocol on performance in well-trained endurance athletes. The primary findings of this study demonstrate that elite youth cross-country skiers improve their double-poling aerobic capacity and performance after a 10-week training intervention independent of loading, provided that sets are equated and carried out with a close proximity to momentary concentric muscle failure. Both HL and LL increased maximal dynamic strength and power, as well as aerobic capacity and power, during double-poling ergometry; however, only HL increased significantly in average power during the 15 s sprint. Group differences in traditional determinants of endurance performance were only found for changes in peak power during the double-poling peak oxygen uptake test. Figure 5: Smallest worthwhile change: individual responses for pre- to post-test changes summarized in two categories: high-responder (dark gray), >0.5”STDpre; and low-responder (light gray), <0.5”STDpre. Significant upper-body strength increases of 9-17% were observed in both intervention groups, aligning with previous studies showing 6-24% strength gains after 6-12 weeks resistance training in cross-country skiers [7]. HL showed superior strength gains, supporting a strength-endurance continuum and a dose-response effect of load on dynamic strength [47; 54; 55]. This observation warrants caution due to the presence of an outlier (a decrease in performance) in both LL and control groups, which could influence the overall interpretation of the data. The cause of this variance remains uncertain, whether it is natural variability, issues with test-retest reliability, fatigue, or learning effects. Strength gains likely resulted from both neural adaptations and hypertrophy [56; 57; 58; 59; 60], with studies indicating similar hypertrophy changes in HL and LL conditions when effort (proximity to failure) and volume (number of working sets) are equated [47; 54; 61], emphasizing the significance of load in maximizing dynamic strength in experienced individuals [47]. The study observed moderate to large correlations between changes in dynamic strength and changes in peak power during double-poling oxygen uptake tests. This highlights the critical role of the neuromuscular system in generating high-propelling forces for optimal double-poling performance, which aligns with prior research [4; 5; 6; 62]. The results highlight the importance of maximum strength in improving mean power output for HL during the double-poling sprint test, with a significant increase of 12.2% (relative and absolute). In contrast, LL and CON conditions showed non-significant improvements of 7.7% (absolute) and 6.1% (relative), and 4% (absolute) and 2.2% (relative), respectively. Further investigation is warranted due to the low delta-correlations (8.5-9% explained variance) observed between sprint power and strength measures. This may indicate a need for enhanced technical efficiency through sprint-specific training to facilitate improved force production transfer into sport-specific double-poling performance. The study observed large improvements in VO2 peak-DP for both absolute terms (HL: 8.5%, LL: 9.2%) and relative terms (HL: 9.4%, LL: 9.4%), with no significant changes observed in the control group. It is worth noting that there were no alterations across the groups, aligning with prior research [7]. The change in the ratio between VO2 max and VO2 peak-DP highlights the distinct effect of resistance training on aerobic performance during DP, necessitating further exploration of the underlying mechanisms. The findings align with resistance training research, highlighting hypertrophic effects across muscle fiber types and capillary adaptations in response to training duration and intensity [47; 54; 63; 64; 65; 66].