## 4 Discussion  
We investigated the muscle synergies of the periarticularis shoulder muscles in 12 healthy adult men during the propulsion motion of a wheelchair for WB and found that the motion was performed using four muscle synergies. The results of this study, conducted  
Figure 4: The muscle synergy of wheelchair propulsion. The graph on the right visualizes the contribution of each muscle to the corresponding synergy, where 0.3 or more was considered a muscle with a high contribution. The graph on the left shows the activation timing of each synergy, and the vertical dashed line in the graph indicates the timing of the transition from the push phase to the recovery phase (44%). LD, latissimus dorsi; AD, anterior deltoid; MD, middle deltoid; PD, posterior deltoid; BB, biceps brachii; TB, triceps brachii; IS, infraspinatus; PM, pectoralis major; UT, upper trapezius; SA, serratus anterior. on healthy participants, may be useful as standard muscle synergy data during wheelchair propulsion without the influence of disability or pain. Participants had no experience driving a wheelchair for WB, so they practiced for 10 min before the measurement, which may not have been sufficient to familiarize them with the wheelchair propulsion motion. In this study, the percentage of time that the PP occupied in one cycle was 44%, which is similar to 42% in a previous study that performed motion analysis of wheelchair propulsion motion [24]. Therefore, although this study was conducted with healthy participants who were not accustomed to wheelchair propulsion motion, we proceeded with the discussion based on the assumption that the percentage of time the PP occupied in one wheelchair propulsion cycle was a standard result. Synergy 1 exhibited the highest activity at 32% of the propulsion cycle, and the contributions of the shoulder flexor muscle groups, including the AD, BB, PM, and SA, were high. In the PP, forward propulsion is achieved primarily by the movement of the shoulder joint from extension to flexion. As the shoulder flexion moment and power are maximal at the PP, the activity of the shoulder flexor muscle group is necessary to generate forward propulsion [28]. That is, Synergy 1 can be thought of as a synergy in which the muscles involved in the shoulder flexion movement coordinate their activities to generate forward propulsion. Synergy 2 exhibited a peak in activity during the transition from the PP to the RP (46% of the cycle), with high contributions from the AD, TB, PM, and SA. Push muscles were active in the range of 13-58% of the propulsion cycle and continued their activity for a short time after the end of the PP [29]. In the present study, the peak activity of each muscle was also after the end of the PP: 44% in the AD, 46% in the PM, and 53% in the SA. Additionally, muscular exertion during the push is shifted from the elbow flexors to the elbow extensors to generate propulsion [30]. The results of the muscle synergy analysis indicated that the shoulder flexors AD, PM, and SA and the elbow extensor TB contributed to forward propulsion in the latter half of the PP by cooperating with each other. Synergy 3 exhibited the highest activity of 70% during the propulsion cycle (RP). The LD, MD, PD, TB, IS, and UT activities peaked during the RP, of which Synergy 3 had a higher contribution from shoulder extensors, such as the PD and TB. In the RP, the upper limbs must be moved to the starting position of the next PP by shoulder extensor muscle activity. The results of the muscle synergy analysis showed a low contribution of the LD, which also acts on shoulder joint extension. These results suggest that the participants in this study were dependent on the upper limbs, and their motions were not coordinated with the trunk muscles. Synergy 4 reached its peak activity at 71% of the cycle (RP), with the LD, MD, PD, IS, and UT contributions > 0.3. Because the upper limb motion during the RP does not need to follow the hand rim, it is believed that the upper limb joints have a relatively high degree of freedom [31].