1 13 GENERAL INTRODUCTION The frequency, duration, and intensity of physical activity in daily life or elite sports can be expressed in physical performance measures, such as the number of steps in a day, the covered distance at different running intensities in a soccer match, or the perceived exertion of a training session in elite runners. An intriguing question is whether physical activity or performance can be predicted based on earlier physical activity or physical performance, such as predicting the total number of steps on a day based on the number of steps in the morning or predicting the covered distance at the end of a soccer match based on the covered distance at the beginning of a match. Also, the potential of physical activity or physical performance measures to predict other outcomes, such as the risk of sustaining injuries [14]–[19] or the rate of improvement in the performance of running a marathon [20], is of interest. The combination of monitoring physical activity and physical performance, along with predictive analytics of physical activity, performance, and injuries, presents an opportunity to intervene in physical activity patterns throughout the day, during training sessions, races, or matches. This intervention can help optimize physical performance and reduce the risk of injuries. MONITORING AND ANALYTICS In the last decade, options for measuring physical activity and physical performance have dramatically increased [21], [22]. This increase is mainly based on technological developments (but not limited to), such as wearable sensor devices, automatic tracking systems, video-based motion analysis, and Global Positioning Systems (GPS) [21], [22]. These systems increase the ability to provide insight into physical activity patterns in daily life and sports [23]. For example, wearable sensor devices, such as the activity trackers of Fitbit or Garmin, can monitor physical activity in daily life. The global shipment volume of wearable sensor devices was 266.3 million in 2020 and is expected to reach 776.23 million units by 2026 [24]. In the last decade, researchers have taken advantage of Fitbit’s public appeal, prominence, and relatively low cost by incorporating these devices into their studies on physical activity [25]. In sports, computer-aided tracking technology has developed substantially to monitor athletes’ physical activity and physical performance during training and match play. Monitoring systems are used in all kinds of team sports, such as soccer [26], Australian football [27], basketball [28], hockey [29], or individual sports like speed skating [30] and running [31]. Monitoring systems using tracking technology have evolved considerably. For example, Van Gool et al. were the first to track a soccer match in the eighties filming at 5hz and processing it afterwards [32]. Nowadays, technology can quickly record and process the data of all athletes’ physical activity throughout an entire match or training session [33]. These monitoring systems have become commonplace in professional
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