INTRODUCTION: The high training loads of endurance athletes requires an adequate balance between training and recovery to maximize adaptations and mitigate the risk of side-effects. This case study determined the main factors associated with unexpected underperformance and prospectively described the holistic process of returning to sustainable world-class level in a male cross-country skier. METHODS: Longitudinal training data was retrospectively analyzed across nine seasons and categorized into training forms, intensities, and modes. Following two seasons of unexpected underperformance, the participant was prospectively followed in the process of returning to sustainable world-class level. Day-to-day training data were analyzed and interviews with the participant and the head coach conducted. RESULTS: Longitudinal training data across seven seasons demonstrated a non-linear 30% increase in total training volume (from 772 h to 1002 h), mainly caused by increased volume of ski-specific endurance training. Coincidingly, the participant gradually reached a world-class performance level. After two seasons of unexpected underperformance with relatively similar training volumes and intensity distributions as in the preceding seasons, the possible contributing factors were identified: lack of training periodization, limited monitoring and intensity control, particularly in connection with a "extreme" regime of training with low carbohydrate availability and days including two MIT sessions, as well as lack of systematic technique training and follow-up by coaches on a daily basis. Consequently, the return to world-class level included the introduction of a clear micro-cycle periodization, more systematic physiological monitoring and testing, more accurate intensity control, increased carbohydrate intake during and between sessions, as well as increased emphasize on technique training and an assistant coach present during day-to-day training. DISCUSSION/CONCLUSION: These longitudinal data describe the main factors leading to unexpected underperformance, in addition to providing unique insights into the corresponding process of returning to sustainable world-class level in a male cross-country skier. The holistic approach described in this case study may serve as a theoretical framework for future studies and practical work with underperforming endurance athletes.
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