## 2 Muscle Synergies in Sports So Far
### Muscle Synergy Optimization to Improve Athlete Performance  
In the context of sports, relevant information for athletes and coaches is showing the difference in muscle synergies between elite and beginner athletes and is being used to plan training strategies and optimize performance enhancement programs [3; 8]. Indeed, several studies have investigated how the characteristics of motor modules, including the number of synergies, the spatial structure, and temporal coefficients, change according to the athlete's skill levels [3]. This relationship between skill levels and muscle synergies has been investigated in various disciplines, such as badminton [8], Japanese archery [9], swimming [10], and wrestling [3]. [PERSON] et al. discovered differences in muscle synergies extracted by comparing advanced and novice badminton players during a smash shot. Indeed, three and two synergies were extracted from the advanced and novice players, respectively. The synergy responsible for the rotation and inclination of the trunk to the nondominant side to hit a shuttlecock at a higher point was common between both groups, while a second synergy was different for each subject due to the uncontrolled flying of the shuttlecock. The advanced players also showed an additional synergy that maintained control over the body while hitting a strong shot. This synergy was not present in the beginners because their coaches mainly trained the upper limbs to speed up the movement of the arm to hit the ball while neglecting training the coordination of the trunk muscles. Therefore, to improve the performance level of badminton players, it is suggested not only to train the coordination of the forearm muscles but also the trunk muscles [8]. Moreover, [PERSON] et al. investigated motor control in Japanese archery athletes, comparing the modules between an elite and a novice group. Extracting two modules from each of the groups, the first module, which mainly reflects the bilateral erector spinae activity during the first half of the shooting sequence, was shared between the groups. The second module, instead, showed a significant difference between the two groups because the elite group engaged mainly the internal oblique/transversus abdominis to fix the trunk during shooting sequences, while the novice group compensated for the low activity of the internal oblique/transversus abdominis with the strong activation of the muscle placed between the costal margin of the ribs and iliac crest. Indeed, the competitive skills correlated positively with improved posture maintenance. Thus, to enhance the movements and optimize performance, the coaches could implement more trunk stabilization exercises [9]. Instead, to improve breaststroke swimmers' performance, [PERSON] et al. suggested focusing training sessions on optimizing the temporal aspects of inter-limb coordination. When comparing the muscle coordination of the upper and lower limbs between beginners and elite breaststroke swimmers, although the number and the weight coefficients of the muscle synergies were similar between the two populations, there was a significant backward shift in the second synergy of the beginners compared to that of the elite. This synergy involved the upper limb muscles and was responsible for arm-to-leg coordination differences, so the inter-limb coordination was probably the key for improving the breaststroke performance [10; 11]. Finally, another example in which muscle synergies were investigated to improve sports performance was the assessment of the wrestling performance during the execution of the Double-Leg Attack maneuver, comparing elite and sub-elite athletes [3]. Indeed, [PERSON] et al. discovered that elite wrestlers showed a greater degree of intra-group similarity across all three temporal activations with respect to the sub-elite athletes.