## 4 Discussion  
RPE is a measure of internal load widely used in team sports because it is easy to record and assess, it has a low cost, and reflects the external load [17]. In several team sports, it was demonstrated that training load variables (e.g., total distance, high speed running, and accelerations) influence RPE [8; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. Our study confirms that external loads recorded during the training sessions  
Figure 3: Distribution of discrepancy during soccer training week. The distribution of under-estimation (i.e., the algorithm provided a value lower than the observed one) and over-estimation (i.e., the algorithm provided a value higher than the observed one) in three different RPE and S-RPE class ranges (i.e., \(<\)4, between 4 and 7, and \(>\)7). MD refers to the distance to previous (+) and following (\(-\)) match day. Figure 2: Bland–Altman analysis. Relationship between the mean and the difference among the observed values and the ones predicted by the ordinal regressor. (**a**) and (**b**) refer to RPE and S-RPE, respectively. The black line refers to the mean of the difference (i.e., bias), the black dot line reflects the 1.96 standard deviation to the mean of the difference (i.e., confidence interval), and the red line reflects the relationship between mean and difference (i.e., systematic error). reflect the perceived exertion of the players and their workload (i.e., RPE and S-RPE, respectively). The main result of this study is that we find a discrepancy between the observed RPE and the RPE predicted by the ordinal regressor, especially for extreme RPE values (i.e., \(<\)4 and \(>\)7). Indeed, while RPE values close to the average (i.e., 4-7) the ordinal regressor is accurate and overestimates the perceived exertion at low values of RPE and underestimates it at high values. Despite several previous studies which detected a strong relationship between perceived exertion and external loads [6, 12, 13, 14, 15, 16], the data recorded in this study reported an inconsistent relationship between extreme values of RPE and training workload. Hence, our results demonstrate that not only the external features but also the contextual one (e.g., psychological aspects induced by the distance to the matchday) affect the players' perceived exertion. This aspect is crucial for athletic trainers and coaches to understand the response of their player to external stimuli and to plan the future training session to minimize the effect of training workloads and the stress induced by the match day. Considering that the extensive use of group exercises and the different physical requirements associated with each position could have an impact on both within- and between-player differences in workload training, the quantification of internal and external workload can be a valid method to monitor training sessions. Another exciting result of this study suggested that RPE and S-RPE are strongly affected by the volume of the total training and weakly affected by the training intensity (as also suggested by [PERSON] et al. [12]). Accordingly, a previous study showed that in elite rugby players the RPE is mostly explained by the volume of the training workload (i.e., total distance covered in a session) and by the stress sustained during the physical activity (i.e., a combination of accelerations, decelerations, changes of direction, and impacts) [6]. Moreover, in elite soccer players, the S-RPE is weakly reflected by the high-intensity volume (i.e., high-intensity running) [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31] as well. These cited studies suggest that the session's internal load is reflected by the external load, but the S-RPE provides additional information of the players that the external load could not take into consideration. A previous study showed that it is possible to derive a more robust index than RPE to assess internal load during a training session by multiplying the RPE by the session time [12].