## 2 Muscle Synergies in Sports So Far
### Study of Muscle Synergies to Improve Rehabilitation Strategies  
The high incidence rates and prevalence of injuries in professional and elite athletes show the need to find new rehabilitation strategies to achieve faster and more effective recovery. Several studies analyzed muscle synergies to investigate muscle coordination in order to understand how to improve the rehabilitation process following sports-related injuries [5; 16; 17]. [PERSON] et al. compared two groups of swimmers: swimmers without injury (control group) and swimmers with shoulder pain (swimmer's shoulder group). They discovered that the active synergy in the early pull phase showed a higher activation of the pectoralis major (PM) and a lower contribution of the rectus femoris (RF) in the control group than in the swimmer's shoulder groups. Therefore, while the control group performed the pull motions using the PM, the swimmer's shoulder group compensated for the lack of traction of the PM, gaining propulsion by using lower limb movements. Moreover, they noted that the synergy responsible for the late pull showed a larger contribution of the lower trapezius (LT) for the control group with respect to the swimmer's shoulder group. Indeed, the LT can depress the scapula, allowing the performance of the pull movement, which is avoided in those with swimmer's shoulder. Finally, the synergy that was activated before and after the hand entered the water shows higher upper trapezius (UT) weight for the swimmer's shoulder group than the control group, and this may be precisely the cause of swimmer's shoulder. Thus, each phase of movement requires specific and coordinated muscle activation. Indeed, the proper propulsive force given by linkage between the upper and lower limbs is necessary during the early pull phase, while a high contribution of the LT and a reduction in the involvement of the UT are required in the late pull and at hand entry, respectively. Therefore, it was found that each phase of the rehabilitation process needs the rehabilitation of different target muscles [5]. However, muscle synergies are not only used to identify target muscles to rehabilitate but they have also been applied to develop personalized therapeutic strategies for athletes affected by the yips [17]. The yips is a psycho-neuromuscular disorder that is characterized by involuntary movements interfering with the automatic execution of fine movements. Muscle coordination is necessary for sports movements and the mechanisms of impairment in athletes with the yips remain unknown. [PERSON] et al. tried to assess the spatiotemporal muscle coordination in baseball players while throwing, comparing healthy baseball athletes and athletes with the yips. They discovered that only the athletes presenting dystonic symptoms during the throwing execution exhibited specific spatiotemporal patterns of muscle synergies that differed from those in the control players, while when symptoms were not reproduced, muscle synergies were not impaired. Therefore, muscle synergy analysis can help to identify the characteristics of muscle coordination in players who show dystonic symptoms and can be useful for developing personalized therapeutic treatments, depending on the individual characteristics of the yips symptoms in order to try to resolve athlete-specific situations [17]. Instead, [PERSON] et al. employed a synergy analysis to validate a recent rehabilitation protocol for hamstring injury to verify if it was more effective than traditional treatments [16]. Since hamstring injuries are very frequent, they tried to characterize the neuromechanical profile of the exercises of an alternative eccentric training protocol for hamstring muscle injuries (L-protocol) defined by [PERSON] et al. [18]. This protocol consists of three eccentric exercises that load the hamstring muscle during extensive lengthening. In this study, muscle synergies are used to derive an easily understandable index of functional co-activation of the four muscles.