## 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.