## 4 Discussion
Considering these findings, the reduction in step frequency observed with AFT in the current study may represent a negative alteration in running technique. An interaction between time trials order (Time Trial 1 vs. Time Trial 2) and footwear (CON vs. AFT) was observed in step frequency, vertical stiffness, vertical oscillation of the center of gravity, and the perception of forefoot flexibility. These effects were noted both overall (Figure 3) and in specific segments, such as uphill and downhill (Figure 4). During Time Trial 2, step frequency decreased with AFT shoes, while it remained stable or slightly increased with CON shoes. This suggests that CON shoes may be better suited for improving running economy, delaying fatigue, and preventing overuse injuries [48, 49, 50]. In contrast, the vertical oscillation of the center of gravity showed the opposite trend. During Time Trial 2, it remained consistent with AFT shoes but decreased with CON shoes. This oscillation increased as step frequency decreased and vice versa (Table 1), aligning with findings from a previous study [51]. Overall, comfort and perceived effort were not influenced by the type of footwear, aligning with the absence of performance differences (Table 1). Previous studies on mountain running have reported a decline in comfort only after prolonged efforts (e.g., from the third set, 44 min, or 7.8 km) rather than during the initial phases [52], which supports our findings. Specific comfort, particularly for AFT, is difficult to compare with previous research, as no study to date has analyzed comfort in different areas of the foot. In this study, participants reported improved forefoot comfort during Time Trial 2. However, the progression of comfort over time remains unexplored [52]. Future studies should investigate this further to determine whether these results are influenced by the protocol design or reflect actual differences in footwear performance. Runners reported that AFT shoes felt less flexible than conventional ones, likely due to the carbon plate, the midfoot or forefoot strike pattern required for uphill terrain, and the increased mechanical effort in this section of the circuit [24]. Furthermore, current AFT models are perceived as less soft compared to newer, innovative designs [53]. This aspect, given its potential impact on performance, should be carefully considered by footwear manufacturers [22]. Notably, greater performance declines and changes in physiological variables were observed in the uphill segment between Time Trials 1 and 2 (Table 2). These may be linked to fatigue or discomfort when transitioning from conventional to AFT shoes. Additionally, the loss of grip during Time Trial 2 could be attributed to light rain and humid conditions during earlier runs. This highlights the need for consistent terrain conditions during evaluations, as such factors significantly influence both comfort and performance. The main limitation of this study was the completion of two time trials in a single assessment day. This introduced fatigue as a factor (e.g., reduced performance in the second set compared to the first) and involved different testing orders for the shoe conditions, which may have affected the statistical power of the findings. However, this protocol was specifically designed to account for the highly variable terrain and weather conditions that can significantly impact mountain running performance across different days. To evaluate the effects of footwear on performance, physiological, biomechanical, and comfort parameters, the trial order was randomized according to the runners' performance levels. Another limitation was the use of different brands for CON and AFT footwear. However, this approach ensured a fair comparison of AFT and CON shoes with similar characteristics (e.g., mass and drop) without depending on a single model that could become outdated over time.