## 3 Internal Load Monitoring
### Physiological Markers  
Physiological markers for monitoring training internal load can be used to detect fatigue levels, physiological stress, and training recovery [2; 3; 5]. Among the physiological markers, HR is the commonly used method by athletes and exercise practitioners. This variable primarily assesses whether an individual can tolerate the training load and the recovery process. The most applied HR analyses for monitoring internal load are HR recovery (HRR) and HR variability (HRV) [2; 3; 59]. Every time an exercise is completed, there is an immediate rapid reduction in HR. Decreases in HRR can indicate fatigue, attaining, an inability to withstand the training load, or undesirable overreaching [60; 61]. HR recovery corresponds to the decline in HR after the end of the exercise. Typically, this recovery interval can vary from 30 to 120 s after the session's conclusion, with 60 s being the commonly used duration [61]. HR recovery can be expressed in two ways: absolute HRR (the number of heartbeats recovered within a certain period) and relative difference (the relative difference between the average HR in the final 30 s of exercise and the rate 60 s after exercise completion) [5; 61; 62]. Similar to HRR, the HRV has been used to obtain insights about an individual's positive or negative adaptations to physical training [63; 64]. HRV involves measuring the intervals between cardiac beats. A reliable method for its measurement is the natural logarithm (Ln rMSSD) of the differences between intervals (R-R), calculated for 10 to 60 s [63]. For HRV monitoring, it is recommended to measure it at least three times a week over a prolonged period to obtain adaptive responses to training. A reduction in HRV may indicate that the individual is experiencing a negative response to training, while an increase in HRV suggests that the individual is experiencing positive adaptations [61; 63]. Monitoring HR through HRR or HRV can provide important information about an individual's training adaptations. However, monitoring should not solely rely on HR and should be complemented by other methods and tools, such as RPE or lactate concentrations, to enhance the reliability of internal load assessments [3; 61; 63]. Saliva samples that monitor hormonal markers can provide information about an individual's health status and prevent overtraining [64; 65]. Hormones measured in saliva, such as testosterone, cortisol, or the testosterone/cortisol ratio, seem to have implications for detecting overreaching or overtraining states [38; 66]. Testosterone is an anabolic hormone that plays a role in growth and protein synthesis, as well as in psychological aspects contributing to performance [65]. Cortisol, on the other hand, is crucial for metabolism as it increases substrate availability to muscles and regulates immune system function. Increased levels of cortisol after physical exercise or competition are expected due to exercise-induced stress [64], which is further supported by a high correlation between cortisol levels and exercise intensity (r = 0.86) [66]. Due to its crucial importance, cortisol is one of the most measured hormones for training monitoring, especially among athletes [47; 65]. The testosterone/cortisol ratio is an interesting tool for monitoring sports since it reflects the balance between anabolism and catabolism states. A high ratio suggests anabolic adaptations, while a low ratio may be detrimental as it indicates a catabolic state [65]. Hormonal monitoring can be valuable for managing training loads and the individual's recovery process [64; 67]. Blood samples are also used for biochemical analyses, providing information to assess an individual's training status. Among the primary biochemical markers for exercise monitoring is creatine kinase.