Internal Load and Fatigue Characteristics in Male Cross-Country Skiers During Summer Training

Du, C, Xu, H, Shi, J, Wei, J, Quan, H, and Zhu, T. Internal load and fatigue characteristics in male cross-country skiers during summer training. J Strength Cond Res 40(7): e727-e735, 2026-This study aims to measure the internal load and fatigue responses of cross-country skiers during a 3-week summer training period, comparing differences across various training methods and their combinations. Twelve male cross-country skiers participated in this longitudinal observational study. We recorded the internal load for each training session via Firstbeat sport devices on the basis of the Banister training impulse (bTRIMP) model. The fatigue assessment metrics included the resting heart rate (RHR) and quick recovery test percentage (QRT%). Physiological performance was assessed by measuring maximal oxygen consumption (V̇ o2 max). Statistical significance was considered at p < 0.05. The internal load progressively decreased over the 3-week period. The bTRIMP during the competition session was significantly greater than that during the routine training session ( p < 0.05). Among the different training modalities and their combinations, strength-related sessions produced the lowest bTRIMP values, followed by running and competition, whereas roller skiing presented the highest values. Resting heart rate and QRT% displayed different temporal patterns. No significant differences in RHR were found after different training modalities ( p > 0.05). However, the postcompetition QRT% was significantly lower than the post-training value ( p < 0.05). Neither RHR nor QRT% significantly differed after various combined training sessions ( p > 0.05). A moderate negative correlation was observed between RHR and QRT% ( r = -0.361, p < 0.001). V̇ o2 max did not significantly differ ( p > 0.05). Overall, although there was considerable variation in the internal load across different types of sessions, no significant cumulative signs of fatigue were detected.