Modeling muscle work during upper limb flexion in the shoulder joint

Background. The endoprosthetic procedure is often used for fractures or massive ruptures of the rotator cuff muscles, when conservative treatment is ineffective. In cases where it is impossible to use anatomical shoulder replacement, reversible arthroplasty is widely used. When replacing the shoulder joint, it is important to consider the contribution of the muscles responsible for flexion and abduction of the shoulder, since their function significantly affects the stability of the prosthesis, range of motion and overall recovery efficiency. Objective: to assess the force contribution of individual muscle groups during flexion of the upper limb in the shoulder joint for further use in biomechanics and clinical practice. Materials and methods. The structure of the model includes 6 joints: acromioclavicular (articulatio acromioclavicularis), sternoclavicular (articulatio sternoclavicularis), humeral (articulatio glenohumeralis), humeroulnar (articulatio humeroulnaris), humeroradial (articulatio humeroradialis) and radiocarpal (articulatio radiocarpalis). We considered the movement of the upper limb without load (passive) and when bending with a weight of 2 kg. Results. During passive movements, the rotator cuff muscles perform a predominantly passive role of stabilizers and do not increase the stabilization force with an angle of up to 30–40°. However, when bending the arm with a weight of 2 kg, an increase in muscle activation is noted, and the muscles located above (m. supraspinatus) are activated noticeably less, about 2-fold, unlike the lateral muscles (m. subscapularis, m. infraspinatus and m. teres minor), whose strength increases threefold. This is due to the fact that the upper muscle (m. supraspinatus) does not change the vector of force action when bending the arm; moreover, as the bending angle increases, its length decreases, which reduces the required force. That is, its main role in maintaining joint stability is holding the shoulder head. The muscles located on the side of the scapula, when changing the angle of flexion, in addition to changing the force vectors, increase their length, although not significantly, but due to the action of gravitational forces, when bending the arm with weight, to maintain the stability of the shoulder, the muscles increase their strength. Flexion of the arm to 40–60° occurs mainly by movements in the shoulder joint (glenoid), which is provided by the anterior portion of the deltoid muscle, the pectoralis major muscle and directly the biceps. Further movement requires the involvement of the muscles responsible for the movement of the scapula (m. serratus anterior, m. trapezius (pars descendens), mm. rhomboidei). In shoulder replacement, if possible, it is desirable to preserve the rotator cuff muscles and the deltoid muscle or try to damage them minimally. The teres minor muscle as part of the rotator cuff, together with the infraspinatus muscle holds the joint, preventing dislocation of the prosthesis. The deltoid muscle and the cuff (including the teres minor) provide flexion, rotation, and abduction. Damage leads to weakness, loss of external rotation, and complications (e.g., instabi­lity). Conclusions. The modeling results confirm that the rotator cuff muscles mainly play a role in stabilizing the shoulder joint. In the passive mode (without weight), they show minimal activity up to a flexion angle of 30–40°, performing a predominantly passive stabilization function. In contrast, when the arm is flexed with additional weight, there is a significant increase in the strength of the stabilizer muscles, which indicates their active participation in counteracting the gravitational load and holding the humeral head. The data obtained are valuable in planning shoulder arthroplasty. Preservation of the rotator cuff muscles is crucial for ensuring prosthesis stability, joint mobility, and preventing postoperative complications.