Wheelchair racing is one of the major competitive parasports for athletes with disabilities. Intensive training is necessary to acquire a unique wheelchair propulsive strategy that can be regarded as maximizing the residual function for athletes. We sought to elucidate the mechanisms underlying the unique modality of wheelchair racing performance based on a comprehensive biomechanical analysis. We collected data consisting of whole-body kinematics, electromyography of upper-limb and trunk muscles, and wheel torque during wheelchair propulsion from 20 athletes with different classifications and competition levels (13 males, seven females, aged 13-64 years). A classification-based comparison revealed that the peak torque angle of the T54 athletes (130 ± 18°) was deeper than that of the T52 athletes (106 ± 26°, p = 0.020) and the T53 athletes (87 ± 9°, p = 0.008). Regarding the competition levels, the elite athletes demonstrated larger torque generation at deeper handrim positions (124°-210°, t*>3.207). These results suggest that torque generation at the deeper handrim position is a key factor in optimizing wheelchair propulsion with relevance to both residual function and acquired skill. In order to identify mechanisms underlying wheelchair racing performance, the kinematic-muscular synergy analysis based on muscle activation and joint kinematics was conducted. Detected synergies could be reasonably interpreted as the four distinct wheelchair propulsion phases: Contact, Push, Release, and Recovery. These functional components and their interaction with muscle recruitment and joint movement patterns reflect a common strategy of wheelchair propulsion across different classifications and competition levels. The present results contribute to updating our understanding of the biomechanical mechanisms underlying wheelchair racing performance.
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