Modeling of Scapular and Clavicular Muscle Function During Upper Limb Abduction

Abstract

Introduction. Modeling combined movements, such as upper limb abduction involving multiple joints and muscle groups, requires advanced mathematical tools; therefore, it is often represented as a sequence of motions in the glenohumeral, sternoclavicular, and acromioclavicular joints. Although this approach does not fully replicate anatomical movement, it provides valuable insight into the magnitude of muscle forces generated during limb abduction.

Objective. This study aimed to analyze the muscle forces responsible for scapular and clavicular movement during shoulder abduction.

Materials and Methods. Modeling was performed using the OpenSim software package based on the DAS3 model. The model included six joints: acromioclavicular, sternoclavicular, glenohumeral, humeroulnar, humeroradial, and radiocarpal. A total of 138 muscles were included. Movements in the sternoclavicular and acromioclavicular joints were simulated. The abduction of the upper limb was analyzed within a range of 0° to 90°.

Results. The sternoclavicular joint becomes active when shoulder abduction reaches approximately 30°. The force of the rhomboid muscles increases to 400 N at a clavicular elevation angle of approximately 20–25°. Subsequently, scapular rotation affects the rhomboid muscles, reaching a maximum force of 700 N at full shoulder abduction, with the torque increasing to 32 N·m. At the onset of scapular movement, the force vector of the upper fibers of the serratus anterior is directed toward the clavicle, causing a decrease in total force up to an elevation angle of 10°. With further clavicular movement, serratus anterior force increases to 370 N at an elevation angle of 20–25°, while the generated torque decreases from 30 N·m to 15 N·m. During clavicular elevation, the lower portion of the serratus anterior generates forces ranging from 475 N to 535 N at a maximum angle of 20° to stabilize the scapula. The corresponding torque increases to 50 N·m after 10° of clavicular elevation. The onset of the clavicular movement is accompanied by an increase in the total trapezius muscle force to 1150 N at a clavicular rotation angle of 10°. With further elevation, the muscle force decreases to 970 N. During scapular rotation, trapezius muscle force increases again to 1150 N at 90° of shoulder abduction. The levator scapulae muscle reaches a force of 200 N only at the initial stage of clavicular movement, while scapular rotation requires an increase in muscle force up to 160 N.

Conclusions. Modeling the function of muscles responsible for scapular and clavicular movement made it possible to determine the sequence of involvement of joints and corresponding muscles required to achieve shoulder abduction up to 90°. The obtained data allowed identification of potential functional disorders of the shoulder girdle in cases of clavicular or scapular injury, or combined trauma.