Muscle Synergy of the Periatricularis Shoulder Muscles during a Wheelchair Propulsion Motion for Wheelchair Basketball  
[PERSON]  
1 Graduate School of Biomedical and Health Sciences, Hiroshima University, 1-2-3, Kasumi, Minami-ku, Hiroshima 734-8553, Japan; [EMAIL_ADDRESS] (T.T. ); [EMAIL_ADDRESS] (T.T. ); [EMAIL_ADDRESS] (S.A.); [EMAIL_ADDRESS] (R.M. ); [EMAIL_ADDRESS] (Y.U.) [PERSON]  
1 Graduate School of Biomedical and Health Sciences, Hiroshima University, 1-2-3, Kasumi, Minami-ku, Hiroshima 734-8553, Japan; [EMAIL_ADDRESS] (T.T. ); [EMAIL_ADDRESS] (T.T. ); [EMAIL_ADDRESS] (S.A.); [EMAIL_ADDRESS] (R.M. ); [EMAIL_ADDRESS] (Y.U.) [PERSON]  
2 Institute for Human Movement and Medical Sciences, Niigata University of Health and Welfare, 1398, Shimami-cho, Kita-ku, Niigata 950-3012, Japan; [EMAIL_ADDRESS] (Division of Rehabilitation, Department of Clinical Practice and Support, Hiroshima University Hospital, 1-2-3, Kasumi, Minami-ku, Hiroshima 734-8553, Japan; [EMAIL_ADDRESS] (Department of Orthopedic Surgery, Graduate School of Medical and Dental Sciences, Kagoshima University, 8-35-1, Sakuragaoka, Kagoshima 890-8544, Japan; [EMAIL_ADDRESS] (Correspondence: [EMAIL_ADDRESS] 2020 2021 2021  
in WB, with the shoulder joint specifically representing the largest percentage at 22.2% [4]. Athletes participating in WB perform repeated overhead movements, which increases their risk of developing shoulder joint injuries involving the rotator cuff and labrum, compared to other wheelchair sports [5; 6]. In fact, between 14.0% and 75.0% of WB players report experiencing shoulder pain, suggesting a significant tendency for shoulder joint injuries among these athletes [7]. In WB, repetitive upper limb movements, such as passing, shooting, and wheelchair propulsion, contribute to the increased incidence of shoulder joint injuries [8]. Particularly, the wheelchair propulsion motion is crucial for achieving better performance in WB, which requires quick movement around the court in a wheelchair [9]. Generally, wheelchair propulsion motion is divided into two phases: the push phase (PP), during which the hands are in contact with the hand rim, and the recovery phase (RP), during which the hands leave the hand rim and last until the hands contact the hand rim again [10]. A study investigating muscle activity during daily wheelchair propulsion reported increased muscle activity of the anterior deltoid (AD), biceps brachii (BB), and pectoralis major (PM) during the PP but increased muscle activity of the middle deltoid (MD), posterior deltoid (PD), and upper trapezius (UT) during the RP [11]. Additionally, the infraspinatus (IS) was active in both the PP and RP phases, and the triceps brachii (TB) began to be active in the latter half of the PP and showed high activity in the RP [11]. Thus, the muscles with high activity in each phase of the propulsion motion differ, with shoulder flexors mainly active in the PP and shoulder extensors mainly active in the RP. However, previous studies have focused only on examining the activity of individual muscles, and there have been no investigations into the coordinated activity of muscles acting simultaneously. Coordinated muscle activity can be assessed through muscle synergy analysis, which evaluates the functional modules of motion by the central nervous system [12]. Since the modules coordinate the activation of muscle groups with different weights under the control of the nervous system during the accomplishment of certain movements [13], understanding the functional modules in movement can provide insight into basic motor control science and motor coordination strategies [14]. In most cases, the muscle synergies are extracted using the non-negative matrix factorization (NMF) analysis, an algorithm proposed by [PERSON] and [PERSON] [15].