## 4 Discussion  
This study developed a detachable two-way propulsion system for a manual wheelchair and fabricated a prototype to verify the feasibility of two-way propulsion (both pushing and pulling) and its detachable installation. Various previous studies have shown that prolonged manual wheelchair use leads to repetitive unidirectional movements, increasing the likelihood of upper-body musculoskeletal issues, particularly shoulder pain and imbalance. To address this, a number of strategies have been proposed to reduce strain on the upper-body muscles of manual wheelchair users. One approach has been the Pushrim-Activated Power-Assisted Wheelchair (PAPAW), an assistive device that provides motorized assistance to the wheels of a manual wheelchair so that users can propel with less effort [28]. Using PAPAW has been reported to reduce the force required for propulsion and increase propulsion efficiency, potentially helping to prevent upper-body musculoskeletal issues. Nevertheless, relying on external power greatly reduces the physical effort of propulsion, so PAPAW offers little exercise benefit. Strengthening the back muscles remains essential for alleviating shoulder imbalance and pain in long-term wheelchair users. Since PAPAW still uses the standard pushing motion, it does not incorporate pulling movements and, therefore, cannot sufficiently engage the back muscles. As a result, while PAPAW may reduce upper-body strain, its ability to alleviate musculoskeletal issues (through strengthening) is limited. In this study, a 1:2 gear ratio was employed to increase torque during the \"pulling\" motion; however, the pilot test also indicated that the resulting higher number of rotations per travel distance could heighten user fatigue. Moving forward, we plan to apply a 1:1 or other reduction ratio to quantitatively assess mechanical efficiency and user energy expenditure [29]. In addition, further research is needed to systematically evaluate internal gear wear and durability by analyzing the overall dynamic characteristics of the planetary gear structure [26]. In addition, the current prototype of the two-way propulsion system was confirmed to be attachable to and detachable from a manual wheelchair using quick-release axles. However, it is considered difficult for individuals with SCI to perform theattachment and detachment independently. The system was intentionally designed to be detachable to allow individuals with SCI to utilize their existing manual wheelchairs for exercise without the need to purchase a new one. While this was the primary intent, individuals with SCI with adequate upper limb function are expected to be able to attach and detach the system themselves if the overall weight of the system is reduced. This would, in turn, improve the overall usability of the system. Therefore, future studies will aim to reduce the weight of the two-way propulsion system. From this perspective, several studies have investigated manual wheelchairs propelled by pulling levers to facilitate back muscle engagement [17; 29; 30; 31; 32]. In these designs, users pull back on levers attached to the wheels to generate rotation, thereby naturally activating the back muscles. Additionally, a manual wheelchair has been developed in which users pull the pushrims backward instead of using levers [18]. Similarly, this rim-pulling approach engages the back muscles and aims to mitigate upper-body musculoskeletal issues associated with wheelchair use. However, since these approaches rely on a single propulsion method (either only lever-pulling or only rim-pulling), users may still experience repetitive movement in one direction, potentially affecting muscle balance over time. Nevertheless, reverse propulsion (pulling back on the rim) has been shown to strongly engage the posterior shoulder muscles during wheelchair use [18], and push-pull lever systems significantly reduce peak propulsive forces relative to conventional pushrim propulsion [33].