Although it is widely acknowledged that young athletes can safely and effectively engage in physical training, provided that any such training is appropriately structured and supervised (Faigenbaum, 2000; Faigenbaum et al., 2009; Matos and Winsley, 2007), responsible coaches need to be aware of the specific demands of working with elite child athletes (Oliver et al., 2011). It has been established that physiologically, children are not miniature adults (Armstrong and Welsman, 1997) and that any training should consider the unique way in which children will respond and adapt to that specific training (Faigenbaum et al., 2009; Mountjoy et al., 2008).
Globally, the current practice of developing elite child athletes is heavily influenced by three assertions; that it takes 10,000 hours to achieve mastery, that practice time must be focused on learning a particular skill by "highly structured activity" (Ericsson K et al., 1993), and by the model of long-term athlete development (LTAD) proposed by Balyi and Hamilton (2004). The LTAD model outlines a framework of stages for the development of motor-skills and physical skills of young athletes. Athletes who miss certain stages of training development might not reach their full potential due to over emphasis on competition and immediate results instead of training (Balyi and Hamilton, 2004).
Recent studies in adult elite cross-country (XC) skiers indicated that XC skiing performance is characterized not only by aerobic and anaerobic capacity, but also by maximal skiing speeds, explosive- or maximum-force, and muscular endurance power (Osteras et al., 2002; Stoggl et al., 2007a; 2007b, Mikkola et al., 2010; Andersson et al., 2010; Holmberg et al., 2005; Carlsson et al., 2012; 2014). Furthermore, over the past decades, the double-poling (DP) technique has been used to a greater extent, and today is a decisive technique for success in classical XC ski races (Holmberg et al., 2005; Sandbakk and Holmberg, 2014; Stoggl and Holmberg, 2011; Stoggl and Holmberg, 2016). In recent years several elite skiers have successfully employed the DP technique throughout an entire race, eliminating the need for the kick wax that was traditionally applied in connection with the classic diagonal stride technique (Stoggl and Holmberg, 2011; 2016). In addition, attributed to altered demands in XC skiing (e.g. higher skiing speeds, changes in skiing technique with greater emphasis on explosive strength and upper body capacity, etc.), the optimal age of high performance XC skiers was shown to slightly decrease within the past years (Stoggl and Stoggl, 2013). According to Stoggl et al. (2015), increases in skiing speeds and changes in skiing techniques emphasize the importance of speed, strength and coordinative aspects in children and youth XC skiers and, more than ever, stress the importance of practicing high quality training and testing processes within this age group.
While there are several studies that deal predominantly with male elite level XC skiers, almost no research was found relevant to the testing and training of younger athletes (10-17 yrs). Gaskill et al. (1999) examined upper body power (DP ergometer) and race velocity including 124 16-yr old male (n = 55) and female (n = 69) high school XC skiers and 34 adult XC skiers. Furthermore upper body power of the high school XC skiers was compared to that of high school runners (n = 37). A strong sex-independent relationship (r = 0.89) to race velocity was found. Further, youth runners achieved only 46% of the mean upper body power of youth XC skiers. They strongly recommended that XC skiers should focus a large portion of their training on the development of upper body power.
Furthermore, only limited data about elite female skiers and female youth skiers, in particular, exists. To the best of our knowledge, our research group study is the only one that includes validation of motor ability tests and anthropometrics in connection with youth XC skiing performance (girls and boys 12.5 to 14 yrs.) (Stoggl et al., 2015). Our groups prior findings demonstrated clear correlations between measures of general strength, speed and coordination to XC skiing performance. Boys' overall performance was related to upper-body and trunk power (medicine ball throw, push-ups and pull-ups), maximal speed and agility (20-m sprint, and hurdle boomerang run) and jumping power (standing long and triple jump), while girls performance was related to 3000-m run, pushups and maximal speed. Maturity was a major confounding variable in boys, but not in girls. However, while strength abilities definitely are more apparent in boys, there is still potential for improving the XC skiing performance of girls by increasing their overall strength levels (Stoggl et al., 2015).
Corresponding research about specific XC skiing test concepts in youth age is missing. The development and altered demands in adult elite XC skiing should not be applied to youth training and the respective lack of research about young male and female XC skiers emphasizes the need for specific performance diagnostics to detect the strengths and weaknesses of youths related to XC skiing performance ([P.sub.XC]), which could improve the training processes. Therefore, the specific aims of the current study were: 1) Determine if XC skiing-specific test concepts including maximal roller skiing speed, DP performance and gross kinematics are determinants of XC skiing performance in youth athletes; 2) Establish how maturational status effects XC skiing performance in youth athletes and analyse if there are sex specific differences; 3) Establish non-invasive diagnostics.
A total of 49 male and female youth XC skiers (33 males and 16 females) volunteered to take part in this study. The pre-selection of participants was based on a) attending a special school for XC skiing (after passing a qualifying motor skills test) or being a member of a regional XC skiing team, and b) practicing and competing in XC skiing for more than three years. Participants were regional top athletes, as well as podium winners in the Austrian National Championships. Participants' characteristics are presented in Table 1. All participants volunteered to take part and were fully acquainted with the nature of the study; in addition, informed, written consent was given by the parents. The study was conducted in accordance with the Declaration of Helsinki and approved by the local Ethics Committee of the University of Salzburg.
Overall study design
For determination of specific peak velocity and doublepoling performance in flat and uphill terrain, the following test concepts were used: a 350 m DP on flat terrain, a 225 m uphill DP (both outdoor) and three 50 m maximal speed tests on an indoor track using DP, V2 and leg skating. For all tests, roller skiing time, cycle rate, cycle length and anthropometric data were measured. Time (t350, t225 and t50) was measured by fixed light sensors (ALGE-TIMING, Lustenau, Austria).
All participants were familiar with both the indoor and outdoor roller skiing tests.
The outdoor and indoor tests were performed on two separate days one month before the start of the competition season. Furthermore, all participants had to compete in six selected XC skiing competitions (3 classic and 3 skating).
The selected maximal speed roller skiing tests were reported to show high test-retest reliability (r > 0.98) and validity compared with simulated sprint test performance in elite junior and adult XC skiers (1000-m DP and 1100-m classical sprint: r > 0.85) (Stoggl et al., 2006). Comparable tests were used previously in different international test concepts with respect to adult XC skiers prior to the current study (Andersson et al., 2010; Mikkola et al., 2010; Sandbakk et al., 2011 Stoggl et al., 2006; 2007b).
All tests were performed on roller skis (START 71, Lahti, Finland), with every subject using one of 10 roller skis with equal rolling resistance selected out of 15 almost new roller...