Muscle work during manual wheelchair propulsion in people with a spinal cord injury: a model-based approach

Manual wheelchair users are at high risk of developing shoulder pathologies due to the repetitive demands of propulsion. Understanding individual muscle contributions is essential to improve rehabilitation strategies and prevent overuse injuries. This study applied a modeling approach to evaluate the individual muscle contributions, particularly the rotator cuff muscles, to mechanical work during the push and recovery phases of wheelchair propulsion in individuals with a spinal cord injury. We used a thoracoscapular shoulder model in OpenSim, which incorporated accurate scapular kinematics and muscles often omitted in previous models, such as the serratus anterior, trapezius, and rhomboids. Experimental data from five individuals with paraplegia were analysed to estimate muscle work. During the push phase, significant contributors were the anterior deltoid, pectoralis major, infraspinatus, and serratus anterior. The recovery phase mainly involved the subscapularis, teres major, trapezius, posterior deltoid, middle deltoid and rhomboids. The supraspinatus showed no mechanical work, and overall rotator cuff contributions were lower than previously reported. Model outputs were verified using reserve actuator contributions and power ratios, and validated by comparing computed with measured muscle activations. Moreover, our findings support the importance of incorporating scapula-driving muscles like the serratus anterior, trapezius, and rhomboids into upper-extremity musculoskeletal models.