## 1 Introduction  
The factors affecting competitive long-distance road running performance (e.g., physiological, biomechanical, environmental, etc.) have been widely studied in the literature [1]. However, in recent years, technological advances in footwear have been included. In 2017, the American company Nike(r) (Nike Inc., Beaverton, OR, USA) introduced a revolutionary shoe model (Nike ZoomX Vaporfly NEXT?), which [PERSON] used to break, for the first time, the 2 h barrier in a marathon, although the conditions in which this feat was achieved prevented the record from being official [2]. Since then, records in men's and women's road races have been more frequently broken [3], to the extent that the current official men's marathon record is only 36 s away from the 2 h barrier ([PERSON]; 2:00:35; Chicago Marathon, 2023).  
The advantages of this new generation of footwear, known as 'advanced footwear technology' (AFT), are primarily related to the combination of highly rigid, curved carbon fiber plates with lightweight, reactive midsole foams that are also higher than conventional midsoles [4,5]. The stiffness of the carbon-fiber plate reduces the amount of energy lost at the metatarsophalangeal joint [6], while its curved shape favors the 'Teeter-totter effect' [5]. As a result, World Athletics regulated running shoes designs to allow for only one rigid carbon-fiber plate or material with similar properties [7]. The new midsole foams (e.g., thermoplastic polyurethane and polyether block amide), in addition to being lighter than traditional compounds (i.e., ethylene-vinyl acetate), also provide better cushioning and more energy return while running (i.e., the elastic hysteresis decreases). However, these advantages disappear after approximately 450 km of use [8,9]. The greater lightness of these new foams allows for higher midsoles without increasing the shoe's mass, and, although World Athletics has limited their height to 40 mm for middle- and long-distance road races [10], the effect of a higher midsole has not yet been investigated.  
Preliminary research has shown an improvement in running economy of \(\sim\)4% with the use of AFT [8,11], while more recent studies have reported lower performance improvements between 0.8 and 1.7% for long-distance flat road running [12], as well as for uphill [13,14] and downhill running [14]. However, other studies have shown no improvements in running economy for long-distance flat [15,16], uphill [16], or downhill running [13]. For elite runners, improvements are approximately \(\sim\)1%, with an individual response identified (i.e., \(\sim\)25% of runners did not benefit from its use) and potentially greater gains for women than for men [17,18].  
Furthermore, previous studies have highlighted the benefits of AFT on comfort and perceived exertion during running, suggesting a potential link between increased comfort and improved running economy [19, 20, 21, 22]. Moreover, changes in biomechanics, such as increased contact time, step length, and leg stiffness, have been observed during high-intensity running when using this footwear [20]. Despite these findings, there is a lack of research on AFT's impact in specific contexts like mountain running, where terrain and profile variations (e.g., uphill and downhill) may influence these benefits [23, 24, 25, 26, 27, 28, 29]. This gap in the literature underscores the need to explore whether the advantages observed in road running extend to mountain running scenarios.  
Thus, our study aimed to evaluate the effects of AFT on performance, biomechanics, and perceived comfort during a simulated mountain running event. By addressing this unexplored area, we seek to determine how the unique characteristics of mountain running may modulate the previously established benefits of AFT, aligning with the hypothesis that AFT might alter biomechanics and perceived comfort without significantly affecting performance.