## 2 Materials and Methods
### Design of the Detachable Two-Way Propulsion System  
The detachable two-way propulsion system for a manual wheelchair is a power transmission system that provides both propulsion methods--pushing and pulling--and enables switching between them (Figure 4). The system is designed to engage both the front and back muscles in the user's upper body during manual wheelchair use, and, therefore, offering only the new propulsion method (i.e., the pulling method) would be insufficient. To address this, the system incorporates two different types of gears, allowing both the existing and new propulsion methods to be utilized. Furthermore, to enable users to switch propulsion methods easily and intuitively, a mechanism was developed that allows them to select the appropriate gear using a lever. The two-way propulsion system consists of a gear slider, a forward drive gear, a reverse drive gear, a main shaft, a mode switching lever, a pushrim, and spokes. Six spokes connect the pushrim to the power transmission unit, which consists of the gear slider, two gears, the main shaft, and the mode-switching lever. When the two-way propulsion system is attached to the wheel, the rotational force applied to the pushrim is transm  
Figure 3: Structure of the planetary gear train. the gears. It is mounted to the wheelchair wheels using standard quick-release axles, which allow for easy attachment and detachment (Figure 5). One system is mounted on each wheel, so two systems are required for a complete wheelchair. The two-way propulsion system weighs approximately 2 kg, and, since two systems are required--one for each wheel--a total of 4 kg is added to the manual wheelchair. For clarity, only one system is depicted in the following figures. The gear slider is integrally connected to the pushrim through spokes and rotates in the same direction when the pushrim is either pulled forward or pushed backward (Figure 6). As it rotates with the pushrim, the gear slider functions to transmit the rotational force to one of the gears. To enable this transmission, it is designed to mechanically engage with the gear through a locking mechanism. This engagement occurs when the ball of the gear slider fits into the groove of a gear, allowing torque to be transferred from the pushrim to the engaged gear. The pushing method is applied when the gear slider engages with the forward drive gear, whereas the pulling method is applied when it engages with the reverse drive gear. The gear slider is not fixed to the central axis, which allows it to move horizontally along the axis. Because it can slide between the two gears, both of which are aligned along the central axis, it is capable of engaging with either gear depending on its position. Figure 4: Wheel equipped with the two-way propulsion system. Figure 5: Standard quick-release axles of the two-way propulsion system. The red circle indicates the quick-release axles as applied in the two-way propulsion system. The forward drive gear, when engaged with the rotary component, receives the rotational force from the pushrims and rotates in the same direction as the pushrims (Figure 7). This rotational force is then transmitted to the main shaft, causing the wheel to rotate. Consequently, when the rotary component engages with the forward drive gear, the two-way propulsion system enables the existing propulsion method (pushing method), where pushing the pushrims forward results in the wheels moving forward. The forward drive gear was configured with a gear ratio of 1:1, meaning that one full rotation of the pushrim results in one full rotation of the wheel. This direct transmission allows for a linear correspondence between user input and wheel movement, preserving the familiar sensation and mechanical behavior of conventional manual wheelchairs. The reverse drive gear is designed using a planetary gear train and consists of a ring gear, planet gears, and a sun gear (Figure 7).