A Bottom-Up Classification of Wheelchair Users Based on Propulsion Kinematics beyond Injury Level: Exploratory Findings

Wheelchair propulsion is critical for the health and quality of life of wheelchair users. However, previous studies have primarily focused on quantitative comparisons of propulsion patterns among individuals with different injury levels without considering other factors. This study aims to explore distinct wheelchair propulsion strategies among individuals with spinal cord injury using a bottom-up approach based on kinematic patterns, and to investigate how these strategies relate to injury level, physical activity (PA) level, and coordination. Twenty-eight wheelchair users performed propulsion as fast as possible on an ergometer. Joint kinematics were analyzed using principal component analysis (PCA) to cluster participants. Muscle synergy analysis was performed using non-negative matrix factorization of electromyography data from six upper limb muscles. Three distinct clusters were identified, with differences primarily in PA levels (p = 0.02) and wheelchair use experience (p = 0.04) rather than injury level. Cluster 1 (n=4), characterized by longer wheelchair use experience and higher PA levels, demonstrated higher hand velocity (p = 0.01) and longer push phase percentage (p < 0.01), with distinct joint coordination patterns consistent with relatively efficient propulsion strategies. Clusters 3 (n=12) with less physical activity levels showed conservative propulsion patterns, and exhibited significantly lower hand velocity. Muscle synergy analysis revealed differences in neuromuscular control, particularly in the timing and coordination during push phase. Our findings suggest that wheelchair propulsion strategies are influenced more by experience and PA level than by the injury level alone. This highlights the importance of promoting PA and systematic propulsion training in rehabilitation programs to enhance wheelchair users' mobility and independence.