## 2 Materials and Methods
### Instruments and Variables  
Mental Load. To assess the mental load perceived after each training session, the Questionnaire to quantify the Mental Load in Team Sports was used [19]. The 4-item questionnaire requires players to self-report their (i) rate of perceived exertion (RPE; how demanding would you quantify the physical effort of this session?); (ii) cognitive load (how demanding would you quantify the cognitive effort of this session?); (iii) affective load (how demanding would you quantify the effort made in this session to manage emotional relationships with the other participants?); and (iv) emotional load (how demanding would you quantify the effort made to manage your emotions during this session?). This instrument uses a Likert scale format with a response range from 0 (none at all) to 10 (maximal) for each question. This questionnaire has demonstrated reliability and validity for the assessment of mental load in team sports and it has been previously used in soccer studies [20; 21].  
Motivation. A Visual Analogue Scale (VAS-100 mm) was used to assess motivation. This scale has already been used in previous soccer studies to measure motivation [22; 23; 24]. Participants marked their degree of motivation along a line from 0 (minimum) to 100 (maximum). The specific question given to the players was: \"How motivating does you quantify the session on a line from 0 to 100?\". Original units of the VAS-100 mm for motivation were transformed to their equivalent on a scale from 0 to 10 to be consistent with the format of the other scales obtained.  
Mental Fatigue (MF). To quantify the subjective feelings of MF after each training session, a Visual Analogue Scale (VAS-100 mm) was used. Athletes were asked to indicate on a single horizontal line from 0 (none at all) to 100 (maximal) how mentally fatigued they felt after the training session. During analysis, the original units of the VAS-100 mm for MF were transformed to their equivalent on a 0-10 scale (e.g., 66 mm was transformed to 6.6). This transformation allowed us to show all the response options in a range from 0 to 10 to align with the format of other obtained scales. This instrument has been used in previous studies exploring MF in soccer [22; 23; 24].  
Vector S7. To examine the external load of the training sessions, Vector S7 was used. This Global Position System (GPS) device was manufactured by Catapult Sports (Catapult Sports, Melbourne, Australia) and includes a tri-axial accelerometer, tri-axial gyroscope, and tri-axial magnetometer; each provided sampling rates of 100 Hz, whereas the GPS provided a sampling rate of 10 Hz. For a better performance, Vector S7 devices were positioned on each player in the center of their upper back, with each player using the same device during the entire experimental period. Catapult devices have previously been found to have high rates of reliability [25]. The variables considered for further analysis were the following: distance covered above 19 km\(\cdot\)h\({}^{-1}\) (TD \(>\) 19); distance covered above 25 km\(\cdot\)h\({}^{-1}\) (TD \(>\) 25); sprint number performed by soccer players; the maximum velocity achieved during sessions (max. speed); total high-intensity accelerations (\(>\)3 m\(\cdot\)s\({}^{-2}\); ACC); total high-intensity decelerations (\(>\)3 m\(\cdot\)s\({}^{-2}\); DEC); and player load (PLoad), consideredas the accelerometer-derived measurements of total body load in its 3 axes (i.e., vertical, anterior-posterior and medial-lateral).