## 4 Discussion
However, considering that individuals with shoulder pain have greater kinematic variables in the RP [32], it seems more important to stabilize the joints, including the shoulder girdle. The LD stabilizes the humeral head during shoulder joint extension movements [33], and the MD and PD maintain the afferent position by pressing the humeral head against the glenoid fossa [34]. The IS, one of the rotator cuff muscles, also contributes to the stabilization of the glenohumeral joint [35]. The UT is involved in stabilizing the scapula along with the lower trapezius [36]. From the above, we considered that the LD, MD, PD, IS, and UT are the muscles that stabilize the scapula and glenohumeral joint and that Synergy 4 contributed to the stabilization of the shoulder girdle during the RP. The findings of this study indicate the presence of four muscle synergies in wheelchair propulsion motion: Synergy 1 reflects propulsion achieved through shoulder flexion and elbow flexion; Synergy 2 involves propulsion through shoulder flexion and elbow extension; Synergy 3 describes the action of shoulder extension returning the upper limb, which has moved forward during the push, back to its original position; and Synergy 4 pertains to the stabilization of the shoulder girdle during the RP. Muscle synergy analysis allows us to perceive muscles that act simultaneously as a single group rather than being interpreted individually, which may be meaningful because it makes the interpretation of the propulsion motion easier. In the present case, the ten muscles work in coordination with each other to form four groups, and the balance of each muscle activity within a group may be related to injuries and performance. These data can be applied in future studies as foundational information for devising training programs aimed at improving performance and preventing shoulder joint injuries in WB athletes. A previous study reported that in wheelchair propulsion motion, a decrease in PM muscle strength increased AD activity, and a decrease in LD muscle strength increased MD activity [37]. Therefore, WB athletes with impaired trunk movement, such as low pointers, would be expected to alter their muscle synergy patterns by the compensatory activity of the upper limb muscles to offset impaired trunk muscle function. Owing to the higher incidence of shoulder joint injuries in WB athletes who cannot control trunk movements compared with other athletes [38], it is possible that these compensatory changes in muscle synergies are involved in the mechanism of shoulder joint injuries. Further studies are necessary to elucidate the relationship between various muscle synergy patterns and shoulder joint injuries. This study represents the first investigation into the muscle synergy analysis of periarticular shoulder muscles during WB wheelchair propulsion motion, thus serving as the first step toward understanding this relationship. A limitation of this study was the small sample size and the fact that the participants had no previous experience driving a wheelchair for WB. Because of the lack of familiarity with the propulsion motion, demonstrating the validity and reliability of the muscle synergies extracted in this study remains a topic for future studies. Furthermore, there are limited data on kinematic variables, such as joint angles and moments during wheelchair propulsion motion; therefore, this study was limited to a discussion of muscle activity. If we could clarify the relationship between the muscle synergy patterns and kinematic variables, we could obtain more useful information that would assist in understanding wheelchair propulsion motion. In the future, we would like to broaden the demographics of the participants to include healthy individuals with wheelchair propulsion experience and WB athletes. This study not only examined the validity and reliability of these results but also investigated the differences in muscle synergy between healthy and WB athletes to contribute to performance improvement and injury prevention in WB athletes.