Innovative Detachable Two-Way Wheelchair Propulsion System: Enhancing Mobility and Exercise for Spinal Cord Injury Users

Abstract

Background: Prolonged manual wheelchair usage often leads to musculoskeletal disorders in the upper body of individuals with spinal cord injury (SCI) due to repetitive, unidirectional movements. To mitigate these issues, targeted exercise of the back muscles—particularly those involving pulling movements of the arms and shoulders—is recommended. Therefore, this study aimed to develop a detachable, two-way propulsion system for manual wheelchairs, enabling propulsion through both pushing forward and pulling backward on the wheelchair pushrims. Methods: The propulsion system was engineered using a planetary gear train to facilitate dual-direction propulsion. Specifically, the planetary gear reverses the rotational direction, allowing the wheelchair to advance forward even when users pull the pushrims backward. Thus, the wheelchair can move forward through either pushing forward or pulling backward actions. Results: A prototype of the proposed system was fabricated using 3D printing technology and its functionality was verified. The prototype successfully demonstrated the two-way propulsion capability and the operation of the attachment mechanism. Additionally, the pilot test confirmed that an individual with SCI was able to propel a manual wheelchair equipped with the two-way propulsion system using both propulsion methods and switch between the methods independently while maintaining stability and safety throughout the test. Conclusion: The developed detachable two-way propulsion system shows significant promise as both a mobility aid and an exercise device, potentially reducing musculoskeletal complications among individuals with SCI who regularly utilize manual wheelchairs.

1. Introduction

Spinal cord injury (SCI) is a disability caused by traumatic factors, such as accidents or falls, or non-traumatic factors, such as tumors or congenital deformities [1,2]. Individuals with SCI experience significant functional impairments due to the complete or incomplete loss of sensory and motor functions following the injury [2,3]. These impairments often result in walking difficulties, and, as a result, individuals with SCI primarily use a manual wheelchair for mobility in daily life [3,4,5,6]. A manual wheelchair is an assistive device that relies on user propulsion via pushrims. However, prolonged use leads to repetitive, unidirectional pushing motions that predominantly engage the anterior muscles of the upper body (e.g., anterior deltoid, pectoralis major). Over time, continuous overactivation of these muscles can cause overuse injuries, with shoulder imbalance and pain being the most common symptoms [7,8,9,10,11]. To prevent these issues, strengthening the opposing muscle groups (the back muscles, including latissimus dorsi, trapezius, and posterior deltoid) is essential [12,13,14]. In practice, however, individuals with SCI often struggle to maintain a consistent exercise routine. It is therefore important to find ways for them to engage in exercise as a natural part of daily life. Because many individuals with SCI rely on manual wheelchairs for long hours each day, the wheelchair’s propulsion method itself can be targeted as a means of exercise and injury prevention. Notably, pulling the arms backward is a common exercise to strengthen the back muscles. This inspired the concept of a propulsion method involving pulling back on the pushrims, in contrast to the conventional pushing motion [6,15,16]. If both propulsion methods—pushing and pulling—are incorporated into a single manual wheelchair, the frequency of repetitive unidirectional movements could be reduced, thereby preventing excessive strain on specific muscle groups. In response to these challenges, researchers have explored alternative wheelchair propulsion strategies to reduce upper-limb strain. For example, lever-driven wheelchairs have been proposed as a less demanding propulsion mode, reducing the activation of certain shoulder muscles during use [17]. Likewise, a reverse pushrim propulsion system (exemplified by the ROWHEELS device) uses a planetary gear mechanism to allow users to pull the handrims backward to move forward, engaging the stronger posterior shoulder muscles and promoting a more balanced muscular effort [18]. In fact, reverse propulsion has been shown to redirect shoulder loading and reduce the risk of shoulder impingement by engaging posterior muscle groups [18]. Moreover, a recently developed bimodal wheel with an epicyclic gear train allows both pushrim and push–pull lever propulsion, significantly reducing peak propulsive forces compared to standard pushrim use while introducing a trade-off of increased system weight and friction [19]. Altogether, these limitations underscore the need for an integrated solution that combines both pushing and pulling propulsion options in a single wheelchair design. The present study therefore proposes a new propulsion system for a manual wheelchair to strengthen the back muscles of SCI users. We developed a detachable two-way propulsion system that enables both the traditional pushing motion and a novel pulling motion on the pushrims, using a planetary gear train to reverse the drive when pulling. A prototype of the system was manufactured and tested to verify that both propulsion modes function as intended and that the device can be attached to and detached from a standard wheelchair.

2. Materials and Methods

2.1. Conceptualization of the Detachable Two-Way Propulsion System for a Manual Wheelchair

To propel an existing manual wheelchair, users must push the pushrims forward, primarily engaging the muscles in the front of the upper body. The anterior deltoid contributes to generating propulsion force by facilitating arm flexion, while the pectoralis major, which is responsible for shoulder internal rotation, is activated to support the shoulder movement associated with pushing the pushrims forward. Prolonged use of a manual wheelchair may lead to excessive activation of these muscles, potentially resulting in shoulder imbalance and pain. To alleviate and prevent these issues, strengthening the back muscles—such as the latissimus dorsi, trapezius, and posterior deltoid—is essential. The latissimus dorsi plays a key role in shoulder extension, the trapezius helps maintain shoulder stability, and the posterior deltoid is responsible for pulling the arms backward and facilitating shoulder external rotation. Exercises that incorporate movements aligned with these functions can effectively enhance back muscle strength.

However, individuals with SCI often face difficulties in maintaining a consistent and regular exercise routine. Thus, a solution is needed to minimize the cost and time required for exercise while still providing exercise benefits. If a manual wheelchair could be propelled in a way that strengthens the back muscles, rather than the existing propulsion method (pushing method), it could help address shoulder imbalance and pain. At the same time, it could function not only as a means of mobility but also as a means of exercise for individuals with SCI. When pulling the pushrims backward, the arms move in a backward motion with shoulder external rotation, engaging the back muscles such as the latissimus dorsi, trapezius, and posterior deltoid, as previously described. This suggests that incorporating the pulling method, as opposed to the existing propulsion method (pushing method), could provide exercise benefits for the back muscles. However, relying solely on one propulsion method could still lead to repetitive and unidirectional movements, potentially causing musculoskeletal issues. Based on this consideration, the concept of a two-way propulsion manual wheelchair was devised, integrating both the existing propulsion method (pushing method) and the newly proposed propulsion method (pulling method). Instead of designing an entirely new manual wheelchair, it was considered more efficient to develop a propulsion assistive device that can be utilized for existing manual wheelchairs. This approach was expected to simplify the design and manufacturing process while also potentially lowering the barrier to adoption for individuals with SCI and other manual wheelchair users. Consequently, the concept of a detachable two-way propulsion system for manual wheelchairs was formulated (Figure 1), enabling users to switch between the pushing and pulling propulsion methods as needed.

2.2. Preliminary Interviews for the Two-Way Propulsion System

As part of an iterative, user-centered design approach, preliminary interviews involving individuals with SCI and wheelchair experts were conducted before initiating the design process to gather user requirements and assess the initial necessity and feasibility of the two-way propulsion system concept.

A focus group interview (FGI) was conducted with individuals with SCI. A focus group interview encourages diverse communication among participants and facilitates the exchange of ideas and experiences, which might remain underdeveloped in individual interviews, as participants reflect on each other’s thoughts [20]. To explore the perspectives of manual wheelchair users, a focus group interview was conducted as part of this study. In addition, an expert interview is a more efficient and targeted method of data collection compared to participatory observation or systematic quantitative surveys. Conducting expert interviews can help streamline the data collection process and provide practical insider knowledge [21]. To gain insights from specialists, expert interviews were conducted as part of this study.

2.2.1. Focus Group Interviews

FGIs 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 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. Participants’ neurological levels and ASIA impairment scale by FGI round.

Round of FGI	Neurological Level (ASIA Classification)
First round	T12 (A)
	T12 (D)
	T12 (A)
Second round	T12 (A)
	C5 (C)
	T9 (D)
	C5 (D)
	C5 (D)
	C5 (D)

). 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 potential increase in the physical effort required for propulsion due to the added weight of the system.

2.2.2. Expert Interviews

Expert interviews were conducted three times with two professors in mechanical engineering and one specialist in wheelchair manufacturing to assess the feasibility of the two-way propulsion system (Figure 2). The first and second interviews involved one professor each. The third interview involved the specialist. These expert interviews aimed to determine whether the two-way propulsion system could be practically implemented and gather opinions on the most suitable mechanical structure for its realization.

Experts explained that a wheel can rotate in the opposite direction of the applied rotational force. However, they noted that implementing a mechanism in which the wheel consistently rotates forward, even when receiving either a forward or backward rotational force—an essential function of the two-way propulsion system—could be relatively challenging. Various power transmission systems, including a four-bar linkage, gears, ratchet gears, and a planetary gear train, were considered to control wheel rotation for the implementation of the two-way propulsion system. Considering the characteristics of each system, a planetary gear train was expected to maximize the feasibility of the two-way propulsion system.

Findings from the expert interviews suggested that implementing the two-way propulsion system could be relatively challenging due to the complexity of designing a mechanism that consistently converts both forward and backward rotational forces into forward wheel movement. However, within the field of mechanical engineering, its practical implementation was deemed feasible, and the most suitable power transmission system was identified. Drawing from the interview findings, a planetary gear train was determined to be the optimal mechanism for designing the two-way propulsion system, as it allows for efficient torque transmission and directional control while maintaining mechanical stability. Experts also noted that, particularly for individuals with high-level spinal cord injuries who have limited trunk strength, supplementary chest pads or backrest supports should be considered [23,24]. Adding these additional support devices may make it more feasible to perform pulling motions in a stable manner.

Therefore, the two-way propulsion system was designed utilizing a planetary gear train to enable efficient two-way propulsion while maintaining mechanical stability.

2.3. Planetary Gear Train

A planet gear refers to a gear that transmits power by revolving around a fixed gear. A planetary gear train, which includes multiple planet gears, consists of a sun gear, a ring gear, planet gears, and a planet carrier, as shown in Figure 3: (1) sun gear: a gear with external teeth that serves as the central axis of the planetary gear train; (2) ring gear: a gear with internal teeth that is co-axial with the sun gear; (3) planet gears: gears with external teeth that simultaneously mesh with both the sun gear and the ring gear; (4) planet carrier: a component that supports the planet gears and is co-axial with both the sun gear and the ring gear [25]. Planet gears rotate on their own axis while also orbiting around the sun gear, driven by the movement of the planet carrier.

A planetary gear train is compact and lightweight, making it widely used in various fields [26]. In addition, it can transmit power in seven different ways depending on the input, output, and fixation settings of the sun gear, ring gear, and planet carrier, which are co-axial. Among these, the driving shaft and driven shaft rotate in the same direction when either the ring gear or the sun gear is fixed. In contrast, when the planet carrier is fixed, the driving and driven shafts enter a reversing state, where they rotate in opposite directions [27]. When the planet carrier is fixed, the planet gears cannot orbit around the sun gear and can only rotate on their own axes. At this point, if rotational force is transmitted to the ring gear, the ring gear begins to rotate. The planet gears, which are meshed with the ring gear, also rotate in the same direction as the ring gear. However, since the planet carrier is fixed, the planet gears cannot revolve around the sun gear and can only rotate on their own axes. As a result, the sun gear, which is meshed with the planet gears, rotates in the opposite direction to the planet gears. Consequently, the initial rotational force applied to the ring gear results in the sun gear rotating in the opposite direction. By utilizing this reversing mechanism of the planetary gear train, it is expected that a manual wheelchair can be designed to move forward when the pushrims are either pushed or pulled.

2.4. 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 transmitted to the wheel through 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.

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). Unlike a conventional planetary gear train, it does not include a planet carrier; instead, the planet gears are fixed inside the reverse drive gear, preventing them from orbiting around the sun gear. When the rotary component engages with the reverse drive gear, its ring gear is driven by the pushrims, causing it to rotate in the same direction as the pushrims. The planet gears meshed with the ring gear also rotate in the same direction. However, since the planet gears are fixed, they can only spin on their own axes without orbiting around the sun gear. As a result, the sun gear, which is meshed with the planet gears, rotates in the opposite direction to the planet gears. The sun gear is connected to the main shaft and transmits the rotational force to the wheels. Consequently, engaging the rotary component with the reverse drive gear activates the new propulsion method (pulling method), where pulling the pushrims backward results in the wheels moving forward. The reverse drive gear was designed with a gear ratio of 1:2, resulting in greater torque output at the wheel during the pulling motion. Although this configuration reduces the distance covered per stroke, it helps compensate for the increased physical demand of pulling by requiring less force from the user.

When the two-way propulsion system is attached to the wheels, its main shaft is positioned at the center of each wheel, thereby serving as the wheels’ central axis. As a result, when the main shaft rotates due to the gears in the propulsion system, the wheels rotate in the same direction.

The mode-switching lever is attached to the front of the propulsion unit and is integrated with a plate (Figure 8). When the lever is lowered, it pivots around its axis, causing a protruding section to press against the plate and apply pressure. This pressure pushes the rotary component inward, engaging it with the reverse drive gear and switching to the pulling condition. Conversely, when the lever is raised, the pressure on the plate is released, allowing the rotary component to move outward and engage with the forward drive gear, switching to the pushing condition.

2.5. 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 axles, and the manual wheelchair used in the test was equipped with the system using these axles.

For the pilot test, one elderly female participant with SCI, who currently uses a manual wheelchair, was recruited (

Table 2. Characteristics of the participant in the pilot test.

Age	Neurological Level (ASIA Classification)	Duration of Manual Wheelchair Use (Years)	Grip Strength (kg)
66	C6 (D)	4	9.25 (L)	14.5 (R)

). 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.

3. Results

In the pilot test, the manual wheelchair equipped with the two-way propulsion system successfully implemented both propulsion methods—pushing and pulling—without malfunctioning. The participant completed five round trips using each propulsion method, resulting in a total of ten round trips along a 10 m straight path. The system operated stably throughout the task and no irregularities were observed during propulsion or switching between methods.

The participant independently operated the mode switching lever to change the propulsion method. The lever, which is activated by a simple up-and-down tilting motion, required only minimal force and was easily manipulated. This confirmed that the switching mechanism was intuitive and accessible. These results suggest that the two-way propulsion system is functionally stable and can be effectively used even by individuals with weak upper-body strength. However, the participant remarked that the pulling method was more fatiguing than the pushing method. This was because the 1:2 gear ratio of the reverse drive gear required more pushrim rotations to travel the same distance.

Additionally, it was confirmed that the two-way propulsion system could be attached to and detached from the wheelchair wheel using quick-release axles. When the skewer of the quick-release axle was positioned at the center of both the two-way propulsion system and the wheel, pressing the button on the axle’s head enabled the system to be attached to the wheel. Conversely, when the system was already attached, pressing the button again allowed it to be detached from the wheel.

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 the attachment 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]. 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 ~ 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]. Moreover, friction losses in the gear train appear minimal; one study observed no significant difference in gross mechanical efficiency between a lever-driven push–pull system and a conventional pushrim system [33]. In addition, reducing the overall weight of the two-way propulsion system is expected to improve ease of attachment and detachment. Third, a comprehensive usability evaluation is needed. We cannot generalize the system’s usability and effectiveness because the pilot test was conducted with only one participant. Further studies with a larger group of SCI users are required to determine whether diverse users can use the system comfortably and easily. In addition, it will be necessary to quantitatively examine whether the pulling mode of the system effectively activates the intended back muscles (e.g., via electromyography and fatigue analysis in future trials). Fourth, this study did not address the needs of users with high-level cervical SCI who have limited or no trunk control. For such individuals, performing a pulling motion may be challenging without additional postural support since, unlike pushing (where the backrest supports the user’s trunk), pulling provides no inherent trunk stabilization. Prior research on wheelchair-adapted rowing exercise indicates that individuals with tetraplegia often require torso support (vests or chest pads) to safely generate pulling force [35]. Future iterations of the two-way propulsion system should consider optional trunk stabilization accessories (e.g., chest harnesses or supportive seating adaptations) to accommodate users with compromised trunk muscle function [35].

5. Conclusions

This study successfully demonstrated the feasibility of a detachable two-way propulsion system for manual wheelchairs, enabling users to propel the wheelchair by both pushing and pulling on the pushrims. A prototype was fabricated and tested, confirming the system’s ability to support both propulsion methods while maintaining compatibility with existing wheelchair frames. The two-way propulsion system introduces a novel approach whereby a manual wheelchair can function as a mobility device and a tool for back muscle exercise. By promoting the use of posterior muscle groups through pulling movements, the system has the potential to reduce or prevent the shoulder imbalance and pain commonly experienced by long-term manual wheelchair users. It may also lead to a more balanced upper-body workload, helping to prevent chronic musculoskeletal complications in individuals with SCI. Additionally, because the system is attachable and detachable, it can be implemented on existing wheelchairs without structural modifications, thereby enhancing its practical usability. Future work will focus on refining the system (e.g., optimizing gear ratios and weight) and conducting larger-scale user evaluations to further validate the benefits of mobility and exercise.