## 4 Discussion
Moreover, using simulations and digital twins may be very useful in sports applications because it is possible to simulate load or fatigue conditions that could cause injury during performance. In this way, using this information, coaches may prepare balanced training programs to prevent injury during the sport's season. Alternatively, using these models, experimenters could modify specific biomechanical variables to assess whether movements or athletic gestures improve. This approach would enable coaches to design targeted training programs aimed at altering these biomechanical variables, either to optimize movement execution or to facilitate  
Figure 4: A schematic of how standard muscle synergy analysis could be enhanced with recent findings. Standard synergy analysis is usually performed with the NMF algorithm (mainly the spatial or temporal model); the green path of the scheme follows the main lines of research applied so far in sports; it is not comprehensive for all the models available and it provides mainly a picture of the motor control of athletes at a given stage. The proposed approaches (blue path) suggest separating phasic and tonic components and extracting kinematic–muscular synergies, muscle synergies with negative components, and functional synergies that might be crucial for the better application of synergistic approaches in sports, such as to create rehabilitation protocols or suitable training programs customized for athletes. rehabilitation following an injury. The limitations of this instrument are the time required to simulate a lot of conditions and the need to have experts who know how to use these simulation models and interpret the results obtained. _Expanding the range of applications, especially to sports not analyzed yet._ Even though there is already some literature available, the use of muscle synergies in sports is still quite marginal, especially when considering the practical impact achieved on generating novel training protocols, the improvement of technical gestures, injury prevention, and recovery after injuries. However, considering the limitations of standard approaches based on muscle synergies, many useful applications have already been identified, and several more might be added. Our screening for this contribution revealed that athletes from many popular sports including soccer, volleyball, tennis, and some martial arts, have never been analyzed in the framework of muscle synergies, missing the opportunity to increase the range of applications of this method. Another sporting environment underrepresented in the field of muscle synergies is the Paralympic sport where the application of muscle synergy methods to study motor control is still limited. In Paralympic athletes, EMG was measured to evaluate the start of muscle activation and to calculate the co-activation index in order to analyze how muscles work together to stabilize the joints and promote motor control [41]. To achieve a more accurate view of how muscles are effectively activated to perform a movement and to obtain more information about the central organization of motor strategies, it could be helpful to extract muscle synergies also for studying an athlete's Paralympic movement, for example, when orthoses are used. Motor control information travels from the CNS to the musculoskeletal system through the spinal cord and muscle synergies are some primitive modules that allow the spinal cord to send the information needed for coordinated muscle activation [35; 42]. A spinal cord injury alters the conduction of motor and sensory signals across the sites of the lesion, modifying the performance of movements. Therefore, in athletes with spinal cord injury, motor control is impaired, and it would be interesting to shed light on how muscle synergies are modified so that multi-muscle movements can still be performed effectively [42].