## 4 Discussion
This evidence supports the potential benefits of the pulling method when used in conjunction with the pushing method. Furthermore, our focus group interviews (FGIs) indicated that in certain situations, such as propelling uphill, the pulling method may be less practical; in such cases, the pushing method is more efficient, meaning a user who relies solely on pulling would face limitations in those scenarios. As a result, technologies like lever or reverse propulsion have so far been explored mainly in sports and recreational contexts rather than being widely adopted for daily mobility. The two-way propulsion system developed in this study provides both conventional pushing propulsion and the new pulling propulsion. By incorporating both methods, the system enables users to distribute muscle activation more evenly across the upper body, helping prevent the overuse of specific muscles. Users can choose the most suitable propulsion mode for the situation (for example, using pushing on uphill terrain). The pulling mode specifically targets the back muscles, which may help strengthen them and alleviate shoulder imbalance and pain. Unlike prior approaches that focused solely on reducing propulsion effort or engaging back muscles (and still involved unidirectional movement), our two-way system integrates both pushing and pulling. In doing so, it addresses propulsion efficiency needs while promoting balanced muscle activation, potentially overcoming the limitations of earlier one-dimensional designs. Moreover, because the two-way propulsion system is designed to be attachable and detachable, users can retrofit their existing wheelchairs without permanent modifications, enhancing practicality and usability. Therefore, this system has the potential to transform a manual wheelchair from merely a mobility device into a combined mobility and exercise tool for individuals with SCI. Findings from the pilot test confirmed that the two-way propulsion system functioned as intended under real usage conditions. The successful implementation of both pushing and pulling modes without malfunction indicates that the system is mechanically reliable. In addition, the participant's ability to independently operate the mode-switching lever shows that the system is user-friendly and easy to operate [34]. These results suggest that the system can feasibly be applied in daily use by individuals with limited upper-body strength, offering both functional stability and simplicity of operation. However, this study has some limitations as an early-phase development. First, the gear ratio of the reverse (pulling) drive was set to 1:2 in the prototype, whereas the forward drive was 1:1. This meant that the pushrim had to be rotated twice as much in pulling mode to cover the same distance, potentially increasing fatigue. This observation (based on one user's feedback) suggests that a 1:1 gear ratio for both modes would be preferable; accordingly, the system will be re-tested in the future with a revised 1:1 reverse gear ratio. Second, the weight of the two-way propulsion system may add to the effort required for manual propulsion. Each unit weighs approximately 2 kg, so installing the system on both wheels adds \(\sim\) 4 kg to the wheelchair. Although the pilot participant did not report difficulty due to this added weight, reducing the overall system weight may be necessary to accommodate a broader range of users. This additional mass is comparable to other two-way wheel systems, which roughly double the weight of standard wheels (e.g., about 7 kg vs 3.5 kg per pair) [34]. While such weight increments do not substantially increase energy cost on flat terrain, they can impact maneuverability on slopes. Accordingly, the use of lighter materials (for instance, nylon gears) has been proposed to reduce the weight of planetary gear trains without sacrificing strength [34].