## 1 Introduction  
Cross-country skiing is a winter endurance sport, performed in varying terrain, necessitating constant transitions between sub-techniques, with propulsion derived from varying degrees of upper- and lower-body involvement. In recent decades, the average skiing speed in World Cup competitions has increased by 5-8%, with the addition of mass starts and sprint competitions that demand higher power outputs. This shift underscores the growing importance of anaerobic capacity, maximal strength, and speed, which now play a more significant role alongside aerobic metabolism in determining competitive success [1, 2]. Double poling (DP), a sub-technique within the classical style with the highest degree of upper-body involvement, has therefore become increasingly important [3, 4, 5, 6]. Research has focused on strength and power development, especially in the upper body, due to double poling's significance in cross-country skiing [7]. Double-poling performance is linked to upper-body strength and power, resulting in higher peak forces, shorter contact times, increased cycle length, and reduced poling frequency at both maximal and sub-maximal speeds [1; 6; 8; 9; 10]. The significance of having superior upper-body endurance capacity is emphasized by strong correlations between peak oxygen uptake in double-poling (VO2 peak-DP) and race performance [11; 12; 13; 14]. Several studies have investigated the impact of adding heavy resistance training (within the three to twelve repetition range) to the regular endurance training regime of cross-country skiers on double-poling performance, with varying results. Some studies indicated positive effects of resistance training on double-poling performance [15; 16; 17; 18]. However, other studies reported positive effects in experimental as well as control groups (both active and passive), with no changes between groups [10; 18; 19; 20], or in the control group only [21]. While certain studies have reported improvements in exercise economy during double poling at submaximal intensities [15; 16; 17], others did not report a significant reduction in oxygen cost [19]. The effects of resistance training on VO2 peak-DP are inconsistent, too, underscoring the need for a detailed analysis of potentially contributing factors. Notably, only a few studies reported VO2 peak-DP increases following resistance training among junior cross-country skiers, suggesting a nuanced relationship between resistance training and physiological adaptations in this athletic population. [19; 20]. However, when normalized for body mass, no significant changes were observed in any of these studies. Other studies have reported no significant effects of resistance training on VO2 peak-DP, although it could be expected that better double-poling performance would allow a greater utilization of VO2 max in the double-poling technique [15; 16; 17; 18]. In this context, previous research suggests that peak oxygen uptake (VO2 peak) is considerably influenced by the active muscle mass during upper-body dominant modes [22; 23; 24]. In accordance, various investigations in cross-country skiers reported a decrease in oxygen uptake when using techniques with decreasing muscle-mass recruitment [25; 26; 27]. For example, [PERSON] et al. [28] reported a decrease in VO2 peak-DP by 15% and 22% when power output during DP was only generated by the upper body and a further decrease by 23% and 25% when the arms worked in isolation compared to whole body DP for men and women, respectively. However, previous literature has mainly focused on resistance training methods aimed to provide minimal muscle hypertrophy [29; 30; 31; 32], and no study has so far aimed to investigate the potential effects of increasing muscle mass in the upper body on VO2 peak and DP performance, even though significant correlations between fat-free mass in the upper body and DP performance have already been reported [33; 34].