This thesis followed young athletes attending a sports high school by evaluating various monitoring methods and athlete development. An eight-week longitudinal research period was implemented to examine changes in frequent variables of athlete monitoring (e.g. submaximal tests, nocturnal heart rate variability (HRV), cortisol, orthostatic test etc.) (I), as well as validate several monitoring outcome parameters of heart rate (HR) and HRV (II). Secondly, assessment of performance related outcome changes and associations between subjective stress, sleep and performance-related tests were examined to determine which factors have the greatest influence on performance (III). Finally, a continuous one-year training period was studied to provide developmental and performance-related details that coaches and athletes may encounter (IV). A total of 37 young athletes participated in the studies. Laboratory-based performance-related measures consisted of submaximal running tests, explosive jump tests, in cremental maximal tests, and ski-specific double pole tests. A contactless ballis tocardiography (BCG)-based sleep device was used to collect nocturnal HR indices and sleep data. Training load was reported in individual training diaries. Blood levels of ferritin, vitamin D and hemoglobin were monitored while subjective levels of perceived stress were assessed with a monthly questionnaire. Nocturnal HRV and salivary levels of cortisol were inversely related (r = -0.552, p = 0.001). Nocturnal HR and HRV, which were collected under real-life conditions with a BCG device, revealed to have a good relationship with morning values derived from orthostatic tests. At a group level, diminished sleep duration exerted a negative effect on perceived stress scores (PSS), with females displaying significantly higher PSS values. One-year analysis revealed a significant im provement in ski-specific double pole performance (DPP), but no other significant changes were observed. There were no significant associations between the changes in DPP and any other variable. Thus, the present results suggest that measures of nocturnal HRV indices, sleep duration, and perceived stress levels appear to be appropriate monitoring tools that may facilitate training and perfor mance in young athletes, and one-year of endurance training induced significant improvements in ski-specific tests, but additional changes were minimal.
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