Longitudinal Physiological Monitoring and Evidence-Based Training Periodization in Junior Cross-Country Skiers

Purpose: This longitudinal case study examined the efficacy of systematic physiological monitoring in guiding individualized training periodization for junior cross-country skiers across three consecutive competitive seasons. Methods: Six male junior cross-country skiers (age 15.3-18.7 years) from a regional sports academy underwent quarterly laboratory testing on a cycle ergometer, including determination of anaerobic threshold (AnT) via ventilatory breakpoint, maximal alactic muscular power (MAM) via 6-second sprint, stroke volume (SV) estimation via HR-power extrapolation, and daily heart rate variability (HRV) monitoring. Training zones were individually prescribed and dynamically adjusted based on test results. Results: Over three seasons, mean AnT power increased 16.9% (225 +/- 18 to 263 +/- 22 W; Cohen's d = 1.87), MAM increased 30.8% (650 +/- 45 to 851 +/- 62 W; d = 3.70), and estimated VO2max improved 14.8% (58.2 +/- 3.1 to 66.8 +/- 2.9 mL/kg/min; d = 2.78). Ventilatory threshold showed strong agreement with blood lactate measurements (r = 0.91, mean difference = 6.2 W). A targeted SV training protocol produced measurable SV increases in four of six athletes. HRV monitoring enabled early detection of functional overreaching in two athletes, prompting training modifications that prevented progression to non-functional overreaching. Individual response patterns varied substantially, underscoring the necessity of personalized training approaches. Conclusions: Systematic physiological monitoring integrated into a coaching feedback loop can guide effective individualized training periodization in developing endurance athletes. The ventilatory threshold method provides a practical, non-invasive alternative to blood lactate testing for training zone determination.