## 1 Introduction  
Monitoring training loads is essential for optimizing exercise dosage according to individual characteristics and goals while minimizing risks such as overtraining [1, 2, 3]. Evidence shows that monitoring training loads and receiving real-time feedback can significantly improve exercise performance and training outcomes [4, 5]. Advances in sports technology have greatly enhanced training monitoring capabilities, offering deeper insights into training processes and facilitating the individualization of both preventive and rehabilitative programs [6, 7, 8, 9]. Although an increasing number of exercise parameters can now be tracked and quantified through sensors, many health and sports performance technologies still lack independent validation [10]. Consequently, it is essential to evaluate the validity of these methods for specific applications, and consumers should prioritize evidence-based products with proven efficacy. Isoinertial training systems, commonly known as flywheel systems, have gained increasing attention for their effectiveness in promoting adaptations in strength, power, and speed [11; 12]. These systems utilize the inertia of a rotating flywheel and offer distinct advantages over traditional gravity-based methods, including the potential for eccentric overload, which may lead to superior specific adaptations compared to other training methods [13; 14]. The benefits of isoinertial training are broad, impacting both high-performance athletes and the general population. Notable outcomes include rapid and substantial increases in muscle mass and strength [15; 16]. Among older adults, isoinertial training has been linked to significant improvements in strength, muscle mass, and balance, contributing to the prevention of sarcopenia and dynapenia [17; 18; 19]. In athletes, isoinertial training programs have demonstrated improvements in explosive and reactive strength, sprint capacity, change in direction, and jump performance [20; 21; 22]. Additionally, high-intensity isoinertial sessions have been associated with enhanced post-activation performance enhancement (PAPE) [23]. Moreover, isoinertial training has been shown to improve eccentric strength and functional strength ratios, thereby reducing the risk of injuries, particularly to the hannstrings and anterior cruciate ligament [20]. Despite these benefits, accurately quantifying load during isoinertial exercises is more complex than in traditional weight training systems. In isoinertial exercises, determining a one-repetition maximum (1-RM) is not feasible because there is no maximal load to lift. Instead, the user accelerates the inertia (flywheel discs) during the concentric phase, which then returns stored energy during the eccentric phase [24]. The gold standard for monitoring these exercises typically combines kinetic systems (e.g., force platforms) and kinematic systems (e.g., linear encoders), with force and power measurements being the most commonly used control parameters [25; 26; 27]. Although these systems provide precise measurements, they are often impractical for daily training due to their high cost, complex setup, and lack of portability [28]. In contrast, rotary encoders, which estimate kinetic parameters based on flywheel velocity, offer a more practical and accessible solution for athletes and coaches [25]. However, these devices may introduce errors in force and power estimations, which must be rigorously validated before they can be reliably used in professional practice [27; 29]. Isoinertial training systems are typically classified based on the type of shaft they use, which can be either cylindrical or conical [26]. Each type has distinct characteristics that make it more suitable for specific applications [21; 27; 30], with certain systems designed to target specific muscle groups [16; 31]. Cylindrical shaft devices generally produce high resistance and are mainly used in lower-body exercises performed in vertical or diagonal-ascending directions [15].