Manual wheelchair users, especially those with spinal cord injuries, often suffer from shoulder overuse and pain. Understanding the specific role of the muscles involved in wheelchair propulsion is essential to prevent these complications. Yet, there is uncertainty about which muscles are primarily recruited, and their contribution to propulsion power. For example, some studies have suggested that the rotator cuff muscles are the primary contributors to wheelchair propulsion. Several studies have explored muscle activation patterns using musculoskeletal models, but often omit glenohumeral stability and key muscles like the trapezius, serratus anterior, and rhomboideus. Investigating muscle work provides insights into the actual mechanical output, offering a more accurate assessment of efficiency by considering factors like muscle force and movement distance.
The aim of this study was to examine individual muscle contributions to mechanical work during the push and recovery phases of wheelchair propulsion using a musculoskeletal model and to identify the role of rotator cuff muscles. The thoracoscapular shoulder model featuring accurate scapula kinematics, inclusive of the rhomboideus, serratus anterior, and trapezius muscles, was employed. In a previous experiment electromyography, kinematic and pushrim forces of 5 persons with paraplegia were collected. The filtered pushrim forces and the kinematic data as well as the scaled musculoskeletal model were inputs to the rapid muscle redundancy solver, that enforced glenohumeral joint stability while estimating muscle activations and muscle power. Muscle work was integrated from the estimated muscle power and normalized by the total mechanical work. During the push phase, significant contributions came from the pectoralis major, anterior deltoid, infraspinatus, serratus anterior, triceps brachii, and biceps brachii. In contrast, the recovery phase primarily involved the teres major, subscapularis, trapezius, posterior deltoid, middle deltoid, and rhomboideus. The contribution of rotator cuff muscles to propulsion was noted but to a lesser extent than previous reports, with no contribution from the supraspinatus. Surprisingly, the teres major showed high work values, possibly due to insufficient activation of the latissimus dorsi. Our findings support the importance of incorporating muscles like the serratus anterior, trapezius, and rhomboids into musculoskeletal models. Furthermore, the contribution of reserve actuators to total work generation was below the threshold of 5%. The comparison between estimated muscle activations and measured electromyographic activations generally showed excellent to good magnitude matching. This study's outcomes can aid in designing ergonomic wheelchairs and guide the development of tailored rehabilitation and training methods to decrease upper extremity stress in wheelchair users.