INTRODUCTION: Elite cross-country (XC) skiers undergo standardized testing to monitor their performance development and to calibrate training prescriptions. However, the long-term progression of relevant performance indicators, such as maximal oxygen uptake (VO2max), performance at the second lactate threshold (LT2), and time-trial performance over an athlete's career remains unclear, with previous research focusing on the junior to U23 level [1]. This study aimed to investigate the longitudinal data from > 15 years of exercise testing with the Swiss XC ski national team to examine the trajectory of key performance indicators across the careers of male and female skiers. METHODS: The longitudinal development of VO2max, LT2, and 24-minute double poling performance (24-min DP), described in further detail elsewhere [2], was analyzed in 72 male and 49 female current and former Swiss XC-ski national team athletes. The performance trajectories were modeled using mixed-effects approaches, with a linear random intercept for VO2max, a linear random slope for LT2, and a third-degree polynomial for 24-min DP. Fixed effects included age, sex, and their interaction, with random effects at the individual level. The study included 357 observations (n = 27 females, 48 males) for VO2max, 911 (n = 46 females, 65 males) for LT2, and 601 (n = 49 females, 70 males) for 24-min DP. RESULTS/DISCUSSION: VO2max in females showed a non-significant tendency to increase by 0.16 mL/kg/min per year (p=0.085), with no increase in males (0.03 ml/kg/min per year, p=0.627). Although males had a higher baseline VO2max at age 18 (70.8 vs. 58.2 mL/kg/min, 21.6% difference), the rate of change with age was not significantly different between the sexes (p=0.259). LT2 improved significantly with age for both sexes, with females gaining 0.09 test stages per year (p<0.001) and males showing a stronger age effect with 0.12 test stages per year (p<0.001). At age 18, males had a 28.9% higher baseline LT2 (5.95 test stages vs. 4.62 for females). In the 24-min DP test, both males and females showed a significant linear improvement with age during the first years (p<0.001). However, there was a notable decrease in the rate of improvement in males, as indicated by the quadratic age term (p<0.001). On the other hand, females showed a consistent linear performance trajectory throughout their careers, with no significant quadratic age term (p=0.430). Female skiers showed a baseline performance of 3820 m at age 18, while this value was at 4788 m in males, with males outperforming females by 968 m (25.3%). CONCLUSION: VO2max, LT2, and 24-min DP showed distinct developmental trajectories across age and sex in elite cross-country skiers. While VO2max remained unchanged, LT2 continuously increased throughout their career. On the other hand, 24-min DP followed a non-linear progression only in males, with a slowdown in performance improvement as they transitioned from the U23 to the senior age category. These results highlight the importance of high initial VO2max at the junior level and the greater trainability of LT2 and aerobic time-trial performance with the accumulation of specific training as athletes progress in their careers.
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