## 4 Discussion  
The main finding of the present study is that footwear (CON vs. AFT) did not affect performance or related physiological parameters during the simulated race or when analyzing each segment of the race separately (i.e., uphill, downhill, and mixed) (Tables 1 and 2). However, changes in running biomechanics (i.e., step frequency and vertical oscillation of the center of gravity) were observed when analyzing the data of the entire race and in the uphill and downhill segments, but not in the mixed segment. Runners reported similar levels of comfort for both shoe types. However, the AFT shoes were perceived as less flexible than the CON shoes. The time trials order resulted in a strong influence of fatigue (i.e., Time Trial 1 vs. Time Trial 2) on most parameters analyzed (mainly on performance and physiological variables during the uphill segment). Additionally, an interaction between footwear and fatigue was observed on biomechanical and comfort variables (e.g., step frequency, vertical stiffness, vertical oscillation of the center of gravity and flexibility comfort), mainly in the uphill and downhill segments. The type of shoe (CON vs. AFT) did not affect performance or physiological variables in the simulated mountain run, whether analyzing the entire circuit (Table 1) or individual segments (Table 2). This contrasts with previous studies on road running, where AFT shoes significantly improved performance [20, 44, 45]. AFT shoes are primarily designed for flat road surfaces and lack effective adaptation for hilly, mountainous terrains. Features like higher stiffness, the 'Teeter-totter effect', lower elastic hysteresis, and increased cush-ioning [5, 6, 8] may not benefit this discipline. On the contrary, these characteristics seem detrimental during downhill segments, where both the fastest kilometer and maximum power decreased with AFT shoes (Table 2, Figure 4). Additionally, the lower flexibility of AFT shoes--known to affect metatarsophalangeal joint flexion--reduced comfort ratings among mountain runners compared to conventional shoes (Table 1). The shoes (CON vs. AFT) influenced mountain running biomechanics by reducing step frequency, while step amplitude and contact time showed non-significant increases (Table 1). The vertical oscillation of the center of gravity increased during uphill and downhill segments but remained unchanged in the mixed segment (Table 2). These changes align with recent findings in older runners using AFT in road races at similar running speeds [46]. However, they differ from observations in younger runners, where both step amplitude and contact time increased significantly [17]. The contrasting results between the current study (lower frequency and higher amplitude at the same running speed) and the previous study (similar frequency and higher amplitude at faster speeds) may be explained by the improved performance observed with AFT in the latter. In the current study, contact time increased due to a decrease in step frequency, unlike in the study by [PERSON] et al. [17], where contact time increased despite a consistent step frequency. This may partly explain why running economy improved in that study, as a longer contact time can reduce motor unit recruitment to produce the same force, as shown in previous research on high-level long-distance runners [47]. However, the decrease in step frequency observed with AFT in our study suggests that the slight increase in contact time was insufficient to enhance running economy or performance. This aligns with recent findings indicating that a higher step frequency improves running economy when comparing different types of footwear at the same speed [48]. Conversely, a lower step frequency at the same speed has been linked to a higher risk of stress injuries in long-distance runners [49]. Additionally, fatigue during mountain running has been shown to reduce step frequency in uphill and downhill segments [50].