## 2 Materials and Methods
### Preliminary Interviews for the Two-Way Propulsion System
#### 2.2.1 Focus Group Interviews  
FGLs were conducted twice with nine individuals with SCI to assess the necessity of the two-way propulsion system (Figure 1). The first interview included three participants  
Figure 1: Focus group interviews on conceptualizing the two-way wheelchair propulsion system.  
with a T12-level injury. The second interview included six participants: four with a C5-level injury, one with a T9-level injury, and one with a T12-level injury. These FGIs aimed to determine whether the participants had experienced musculoskeletal issues while using a manual wheelchair. In addition, participants' opinions were collected regarding the necessity of the two-way propulsion system, including their expectations for its exercise effects on back muscles and their willingness to use the system.  
To investigate the willingness of individuals with SCI to use a two-way propulsion system if it were to be practically implemented, two FGIs were conducted with a total of nine individuals with SCI (Table 1). All participants were long-term manual wheelchair users following the onset of their injuries. A structured interview guide was developed to facilitate effective discussions, and a trained moderator led the sessions. Prior to each FGI, participants were informed about the purpose and procedures of the interview, and a comfortable atmosphere was created through informal discussions about their wheelchair use history and post-injury physical activity experiences. The formal interviews began with questions exploring participants' experiences of shoulder pain and other physical issues associated with prolonged manual wheelchair use, as well as the strategies they had employed to alleviate such problems. The concept of the two-way propulsion system was then introduced, and participants were asked to share their opinions on whether the proposed system might help prevent or reduce shoulder-related issues and whether they would be willing to use it if it became commercially available. All discussions and responses were recorded by the moderator for subsequent analysis [22].  
The following insights were gathered from participants regarding the two-way propulsion system. Most participants reported experiencing shoulder pain due to prolonged use of a manual wheelchair and stated that strengthening their back muscles through exercise had helped alleviate the pain. Based on these experiences, they expected that the movements required for the pulling method would provide exercise benefits for the back muscles and contribute to shoulder pain relief. Additionally, they suggested that both the pushing and pulling methods should be available in a manual wheelchair, as the pulling method requires considerable effort when used on a ramp. Regarding the design, they preferred an alternative shape for the two-way propulsion system, as the bar-shaped design might interfere with propulsion.  
As a result, most participants indicated a willingness to use the two-way propulsion system if it became commercially available. They believed that utilizing a manual wheelchair as both a mode of transportation and a means of exercise would allow them to incorporate physical activity into their daily routines in a time-efficient manner, thereby supporting better health management without the need for additional workout sessions. Some also noted that if the system were detachable, it would be more accessible, as there would be no need to purchase a new wheelchair. However, concerns were raised about the  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Round of FGI** & **Neurological Level (ASIA Classification)** \\ \hline First round & T12 (A) \\  & T12 (D) \\  & T12 (A) \\ \hline  & T12 (A) \\  & C5 (C) \\  & T9 (D) \\  & C5 (D) \\  & C5 (D) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Participants’ neurological levels and ASIA impairment scale by FGI round.  
potential increase in the physical effort required for propulsion due to the added weight of the system.