Modeling the Function of Shoulder Joint Muscles During the Abduction of the Upper Limb

Introduction. The functioning of the shoulder muscles during shoulder joint arthroplasty cannot be overestimated. Current strategies for muscle preservation during arthroplasty include: 1) minimally invasive approaches, 2) precise positioning of endoprosthesis components, and 3) early mobilization and postoperative rehabilitation. Objective. This study aimed to evaluate the force contribution of individual muscles of the shoulder girdle during upper limb abduction in the shoulder joint. Materials and Methods. The model comprises six joints: acromioclavicular (articulatio acromioclavicularis - ac), sternoclavicular (articulatio sternoclavicularis - sc), glenohumeral (articulatio glenohumeralis - gh), humeroulnar (articulatio humeroulnaris - hu), humeroradial (articulatio humeroradialis - hr), and radiocarpal (articulatio radiocarpalis - rc). Force generation during upper limb abduction was assessed both without load and with a 2 kg weight. Results. The analysis revealed that the main muscle responsible for the abduction of the upper limb is m. deltoideus (acromial part). This portion of the deltoid muscle develops maximal force during the abduction of the arm. Other muscles mainly serve as stabilizers, maintaining the humeral head within the glenoid cavity, with their force directed opposite to the movement. It should be noted that the activity of the muscles studied in this work was most prominent within the abduction range of 30° to 50°, depending on their functional role during the movement. Beyond 50°, the scapula and its associated musculature become actively involved. Conclusions. The maximum force during abduction of the upper limb is generated by m. deltoideus (acromial part). As the abduction angle increases beyond 30°–50°, there is a decrease in rotator cuff muscle force or a transition to a “support mode.” These findings improve our understanding of the shoulder girdle muscle biomechanics and are significant for movement physiology.