Context-Dependent Differences in Muscle Architecture Following Fatigue in Ultramarathon Athletes: A Comparison...

Background/Objectives: Understanding how different fatigue contexts influence muscle architecture is essential for optimizing training and recovery strategies in endurance athletes. Ultramarathon running involves prolonged mechanical load and high eccentric demands, which may elicit different acute responses compared to controlled laboratory protocols. This study aimed to examine the effects of time, condition (laboratory vs. race), and muscle on ultrasound-derived muscle architecture in ultratrail runners. Methods: A repeated-measures within-subject design was employed. Forty ultratrail runners completed two fatigue conditions: (1) a standardized laboratory downhill running protocol and (2) an ultramarathon race (CSP 2025; 106 km, +5600 m elevation gain). Muscle thickness and pennation angle of the rectus femoris, vastus lateralis, and medial gastrocnemius were assessed using ultrasound before and after each condition. Linear mixed models were used to evaluate the effects of time, condition, muscle, and their interactions. Results: Forty participants were recruited; 29 completed all assessments. No significant effects of time or condition were observed for muscle thickness, and no interaction effects were detected, indicating that muscle size remained stable across conditions and time points. A significant main effect of muscle was identified (p < 0.001), reflecting inherent morphological differences, with greater thickness in the vastus lateralis compared to the rectus femoris and medial gastrocnemius. In contrast, pennation angle showed a significant main effect of condition (p = 0.031) and a significant condition × muscle interaction (p = 0.005), indicating muscle-specific differences between laboratory and race contexts. No significant effect of time was observed for pennation angle. Conclusions: Muscle thickness appears to remain stable following acute fatigue, regardless of the assessment context. In contrast, pennation angle may be more sensitive to condition-specific and muscle-dependent factors. These findings suggest that ultrasound-derived architectural changes observed immediately after exercise likely reflect acute physiological responses rather than true structural adaptations. Therefore, the interpretation of muscle architecture should consider both contextual factors and methodological constraints.