## 2 External Load Monitoring
### Time-Motion Analysis
#### 2.1.2 Global Positioning Systems (GPS)  
GPS is widely used for measuring external load, especially in team sports [22]. These devices are integrated with satellites, providing information about distance, speed, and acceleration. For example, GPS measurements of player displacement during a specific test on an Australian football field were compared with data from computer-based tracking (CBT) analysis and a trundle wheel pedometer. The results showed a high correlation between GPS and CBT data (r = 0.99), and both methods were strongly correlated with the pedometer (r = 0.99) despite a slight overestimation of the distances traveled compared to the actual values [23]. Therefore, when combined with accelerometers, GPS can offer a comprehensive range of data about player movement during sports performance [24; 25; 26; 27].  
An important aspect of using GPS in team sports is the ability of this device to quantify specific demands according to players' positions, which is essential information to adjust training in line with player needs. Additionally, this instrument aids in categorizing athletes by better individualizing the workload within the training program [28]. The literature has investigated the validity and reliability of GPS in various team sports such as rugby, soccer, and American football [28]. For example, in soccer, which is characterized by technical-tactical complexity that defines match performance, the external load is usually measured using GPS for tracking positioning and understanding factors contributing to successful performance, such as high-speed running, player load, accelerations, and decelerations during matches [29]. In rugby, GPS devices can provide information on the specific match demands according to the player's position, thereby enabling better planning of training tasks aligned with the physical demands of official match requirements [30].  
However, the validity and reliability of GPS can be influenced by factors such as duration, speed, type of activity, and different devices used. For example, studies have shown that GPS accuracy can decline at higher speeds or in complex environments (e.g., the standard error for the estimate of the velocity is 0.7% during walking, but can attain 5.6% during running, as well as the coefficient of variation for the measurement of the velocity tending to be lower (<5.3%) with a higher sampling rate) [22; 24; 31]. Furthermore, while GPS provides reports with various data points, many coaches and athletes may still struggle with interpreting or relating these parameters effectively for monitoring external load (e.g., the control of the velocity--ranging from 2 to 20 km/h--correlated with r = 0.99 with the stopwatch record, as well as distance measures improving with longer duration, showing the coefficient of variation being reduced from ~32 to 4% in distances ranging from 10 to 140 m) [22; 31]. In addition, the use of inertial devices to analyze training load in different sports modalities has increased over the past decade, with GPS being the most commonly used technology [26; 27; 31].  
Although GPS provides a significant amount of information for monitoring external load, one of its limitations is the use of speed zones for athletes [17]. Consequently, the combination of GPS with accelerometers has become common in team sports, not only to complement the information limited by GPS but also because accelerometers provide additional complementary data [27; 28; 31]. The combined use of GPS and accelerometers enables coaches to better interpret performance, conditioning, and physical fitness, especially in team sports, allowing for more effective monitoring of external load throughout the training program [28; 31].