Academic stress and mental fatigue predict subjective but not objective internal load in adolescent soccer players-a prospective cohort study.

Introduction: Adolescent soccer players aspiring to reach the elite level are usually involved in a dual career. Moreover, they experience rapid physical and psychological development with unique social environments. Non-training stressors, such as educational responsibilities, can induce stress and mental fatigue, potentially negatively affecting training load. The relationship between non-training stressors and training load has received little investigation. Therefore, this study aimed to investigate the relationship between academic stress, mental fatigue and training load; specifically, the extent to which academic stress and mental fatigue predict internal load. Methods: Using a prospective cohort study design, 35 players from an Under-16 and Under-18 youth academy team were monitored daily for half a season. Measures for mental fatigue, academic stress, external load and internal load were collected before, during or after on-field training sessions. Using multilevel modelling, academic stress, mental fatigue and external load indicators served as independent variables to predict the rating of perceived exertion and the heart-rate-derived training impulse (TRIMP). Results: Final multilevel models indicated that academic stress and mental fatigue were positively associated with rating of perceived exertion, but academic stress and mental fatigue did not predict TRIMP after controlling for external load indicators. Conclusion: We conclude that academic stress and mental fatigue could lead to unwanted alterations in the rating of perceived exertion. Coaches should therefore remain alert to deviations from normal scores and relatively high levels of academic stress or mental fatigue. Given the multifaceted nature of non-training stressors, a holistic monitoring approach is recommended.