## 3 Internal Load Monitoring
### Rating of Perceived Exertion (RPE)  
The RPE score is a commonly used and cost-effective scale for monitoring internal load demands during training, supported by solid scientific evidence showing correlations with exercise intensity, HR, and lactate concentrations [7]. The RPE scale involves the practitioner pointing to the descriptor (perceived effort intensity) and then the corresponding number. Initially proposed in 1974 by [PERSON] [39], the scores of efforts ranged from 6 to 20 points, which was the former index to rate the post-exercise level of exertion perceptually from a \"very, very light--6\" to a \"very, very hard and maximal--19 or 20\" [40]. Over the years, other scales, such as the category ratio (CR-10) and the session RPE (sRPE), have also been introduced [41, 42, 43]. The RPE scale proposed by [PERSON] reflects the exerciser's subjective intensity of effort during or immediately after exercise. This RPE scale was initially based on HR during running, with 6 corresponding to resting HR (60 beats per minute) and 20 to maximum effort, which would be (200 beats per minute) [39]. This correspondence was investigated in sedentary and athletic individuals, either to discriminate the exercise intensity or the demand upon the cardiorespiratory system. For example, the 6-20 RPE scale was considered a reliable tool for prescribing and self-regulating high-intensity interval training (HIIT) in the sedentary population. This was an assumption postulated after no significant differences were shown in HR responses to HIIT sessions prescribed and regulated by the HR reserve method and RPE at low-intensity interval planning (~50% of HR reserve or 9-11 points on the RPE scale: 135 \(\pm\) 15 bpm vs. 138 \(\pm\) 20 bpm), and at high-intensity intervals (~85% of HR reserve or 15-17 points on the RPE scale: 168 \(\pm\) 15 bpm vs. 170 \(\pm\) 18 bpm) [44]. The application of the RPE scale in sports and fitness has also been used in the CR-10 [39,45]. The CR-10 scale has a narrower numeric range, with 0 representing rest and 10 representing maximum effort. In contrast to the 6-20 scale, the CR-10 features twice the intensity range of the previous scale, but its application is similar to that of the 6-20 RPE scale [5,41,46]. In 2001, [PERSON] and colleagues introduced the sRPE scale. Unlike the other RPE scales (6-20 and CR-10), which assess the perceived intensity of effort during or immediately after exercise, the sRPE scale evaluates the perceived intensity of effort over the entire training session [5]. Originally, this scale was meant to be applied only 30 min after the conclusion of the training session. To use this scale, the individual should be familiarized with it and select the descriptor followed by the corresponding number representing the perceived effort intensity during the training session, where 0 represents rest, and 10 maximum effort [43]. The use of sRPE to monitor training is a reasonably accurate measure of internal load in different exercises, regardless of mode and intensity, such as resistance training, high-intensity interval training, or pyometric training [13]. Indeed, the literature has evidenced that applying the sRPE scale 10 min after the conclusion of the training session does not significantly differ from applying it after 30 min [47], despite the initial recommendation for a longer waiting period. The sRPE, developed by [PERSON], has proven reliability concerning HR zones, with strong correlations reported between HR zones and sRPE (r = 0.75 to r = 0.90) [38]. Moreover, another study involving soccer players also demonstrated a strong correlation between sRPE and HR zones (r = 0.54 to r = 0.85) [5,42]. Typically, sRPE is combined with other factors such as exercise duration, HR, or blood lactate levels to provide a better understanding of the internal load experienced by the individual [7,13].