## 2 Materials and Methods
### Pilot Test of the Two-Way Propulsion System  
To evaluate the feasibility of the proposed two-way propulsion system, a prototype was fabricated through metal casting (Figure 9). The prototype included all key components of the system and was used in a pilot test to assess whether the system could function as intended under real-world manual wheelchair use. For this study, the two-way propulsion system was mounted onto the wheels of an active manual wheelchair (NA-430, Nissin Medical Industries CO., Ltd., Nagoya, Japan). The wheelchair weighed a total of 19 kg, comprising an 11 kg frame and two wheels weighing 4 kg each. The wheel equipped with the two-way propulsion system was considered somewhat heavy to be lifted and carried by individuals with SCI who have limited upper limb strength or difficulty using their arms. Therefore, the pilot test in this study focused on confirming whether the individuals with SCI could propel the manual wheelchair equipped with the two-way propulsion system using both pushing and pulling methods and whether they could switch between the two propulsion methods using the mode-switching lever. Prior to the pilot test, it was verified that the two-way propulsion system could be attached to and detached from a manual wheelchair using quick-release axes, and the manual wheelchair used in the test was equipped with the system using these axes.  
Figure 8: Structure of the mode-switching lever.  
For the pilot test, one elderly female participant with SCI, who currently uses a manual wheelchair, was recruited (Table 2). It was assumed that if an elderly female with SCI and weak muscle strength could use the system, then most individuals with SCI would also be able to use it. For this reason, a single participant was selected to represent a lower functional threshold of potential users in this preliminary investigation. While the sample size was limited, this pilot test was conducted to assess the basic operability and functional stability of the system in real-world conditions.  
The pilot test was conducted by having the participant use the manual wheelchair equipped with the two-way propulsion system and experience both propulsion methods (Figure 10). Before the test, the participant was given a 3 min practice session to become familiar with the wheelchair and the pulling method. The participant then propelled the wheelchair along a straight 10 m path, completing five round trips for each propulsion method, for a total of ten round trips. The participant independently switched between the two propulsion methods by operating the lever on the two-way propulsion system.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Age**} & \begin{tabular}{c} **Neurological Level** \\ **(ASIA Classification)** \\ \end{tabular} & \
\begin{tabular}{c} **Duration of Manual** \\ **Wheelchair Use (Years)** \\ \end{tabular} &
\begin{tabular}{c} **Grip Strength (kg)** \\ \end{tabular} \\ \hline
66 & C6 (D) & 4 & 9.25 (L) & 14.5 (R) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the participant in the pilot test.  
Figure 10: Participant performing propulsion tasks during the pilot test.  
Figure 9: Prototype of the two-way propulsion system.