## 1 Introduction  
Significant adaptations are observed in maximal oxygen uptake (\(\text{VO}_{\text{2 max}}\)) and aerobic performance after a few short sessions of high-intensity interval training (HIIT) performed at speeds close to \(\text{VO}_{\text{2 max}}\) (\(\text{vVO}_{\text{2 max}}\)) [1, 2] and/or above maximum speed [3, 4]. There is substantial evidence supporting the benefits of HIIT in competitive runners [5], highlighting that this high-intensity protocol, executed in brief periods, elicits physiological responses comparable to those achieved by continuous programs of prolonged exercise [6, 7, 8].  
These adaptations establish a favorable relationship between training time and effectiveness [9, 10, 11] for the HIIT training model, regardless of the sport in focus, athlete's experience, and fitness level [4]. In addition to the intensification proposed by HIIT, the variation of training stimuli emerges as a potential strategy for metabolic and neuromuscular enhancements [12], beyond adaptations related to running performance.  
In a competitive running scenario, physiological and mechanical changes resulting from alterations in terrain (horizontal vs. inclined vs. declined) influence the dynamics of the stretching-shortening cycle and, consequently, the energy cost [13,14]. In this sense, uphill running, for example, demonstrates an increase in neuromuscular [15] and metabolic [16] overload compared to horizontal running, resulting in a higher perception of effort. Studies indicate an increase in muscle activation and a predominance of concentric overload during uphill running [17], possibly explaining the increase in energy cost and the reduction in acute performance [18].  
Currently, there is substantial evidence supporting the effectiveness of different training programs for runners, aiming to minimize disparities caused by variations in inclination and optimize results [19]. Traditional approaches, such as continuous training at different intensities and resistance training regimens, have been widely studied and associated with improvements in endurance, metabolic efficiency, and aerobic performance in runners [20]. However, there is a current gap in specific evidence regarding the effects of HIIT on runners, especially concerning different inclinations. While HIIT is recognized for its general benefits, the specific application of these principles in varied uphill contexts in running still seems to lack investigations to establish clear and direct evidence on acute responses in variables of interest of aerobic performance [21].  
Although previous studies have addressed the effects of HIIT in different sports contexts, there is a scarcity of research specifically focused on runners and their response to HIIT at different slope percentages [22,23]. In addition to methodological limitations, available studies have not adequately explored the complexity of the relationship between HIIT and uphill running. Furthermore, the lack of experimental research hinders a comprehensive understanding of adaptations over time [24]. This study aimed to investigate the impact of six sessions of HIIT running on 1% and 10% slopes on key physiological parameters, including VO2 max, peak velocity (VPeak), heart rate (HR), rate of perceived exertion (RPE), and time to exhaustion (TLim). Given the higher neuromuscular demand associated with a 10% slope, our hypotheses were as follows: (a) running on a 10% slope will result in more substantial adaptive improvements compared to a 1% slope (H1); (b) HR and RPE will show equal adaptation patterns over time; and (c) training on a 10% slope will lead to significant performance gains for both TLim1% and TLim10% (H3--adaptive transfer hypotheses), while training on a 1% slope will generate specific improvements, primarily in flat terrain performance.