## 4 Discussion  
This study aimed to determine the effect of six HIIT running sessions on a 1% or 10% slope on VO2 max, VPeak, HR, RPE, and TLim performance, as well as the influence of the specific training on performance on the different slopes. The main effects observed were significant increases in VO2 max and VPeak, without any effects of training on TLim1% and TLim10% performance, regardless of the slope used. To the best of our knowledge, this was the first study to compare the effects of interval training on different slopes in a short training period and observe the increase in VO2 max and VPeak. The significant difference between the TLim1% and TLim10% tests was also an important finding, as this directly impacted the impulse training (i.e., TRIMP, volume \(\times\) intensity) between the groups (1186.0 \(\pm\) 367.0 and 767.1 \(\pm\) 234.5, respectively, for 1% and 10%; \(p\) = 0.001). This condition reflected the TRIMP values developed over the sessions, but the ESs of interventions on 1% and 10% slopes showed moderate clinical significance,  
Figure 3: HR and RPE in the face of stimuli in the first vs. the last session. Subtitle: *—significant differences between stimulus; (**A**) adaptive behavior of HR; (**B**) adaptive behavior of RPE. similar to that previously observed for VO2 max and vVO2 max[29]. Additionally, it is worth highlighting that the six sessions were sufficient to generate chronotropic adaptations, at least for GT1%. The RPE responses were also significantly modified for both experimental protocols, suggesting that the different modulation of effort observed was the result of interoceptive (metabolic) or cognitive changes (there is no way to distinguish). Several studies have shown the effects of HIIT on physiological and biochemical markers and performance in six sessions of cycling [3,4] or running on a horizontal plane [29,30]. In contrast, little chronic evidence has been published on uphill running [15]. This study supports the evidence that only a small number of sessions with reduced exercise duration leads to physiological and performance adaptations from the administration of stimulus in vVO2 max. Considering the TLim1% and TLim10% performances, even after metabolic equalization between running protocols (377 \(\pm\) 119 s and 246 \(\pm\) 68 s for 1% and 10% slopes, respectively), the lowest Ttotal was developed by the protocol using a 10% slope. This was an important finding and demonstrated that with less training time, the results after six interval running sessions were similar to VO2 max and VPeak between GT1% and GT10%. Moderate ESs for the GT1% and GT10% groups for these variables support these findings and further suggest that uphill running may be established as a strategy for greater efficiency and time savings compared to the TTotal observed in GT1%. The improvement in VO2 max seemed to be primarily affected by the power developed during running (intensity-dependent), together with the time spent in VO2 max[31]. The application of only 50% of TLim as the duration of the training protocol, although this did not lead the participants to maximum effort and still produced a lower TTotal of running on the 10% incline, did not appear to have been a determining factor for the adaptive responses of VO2 max. The reduction in HR throughout the sessions is an attribute that could explain the intrinsic changes in VO2 max since they are collinear variables. Therefore, we can infer that the reduced HR response for the same workload came from an adaptation related to the volume of blood ejected, which, in turn, altered the capacity to perform work. However, this theoretical rationale would be valid exclusively for the GT1% group and would not explain the effects of training on GT10%, suggesting that other adaptive factors may have been linked to these effects for the 10% slope. The TTotal administered to each stimulus was probably sufficient to achieve VO2 max (-2 min) in both training protocols (GT1% and GT10%).