Modeling the work of the shoulder muscles during upper limb flexion in the shoulder joint

Background. The scapula and clavicle play a key role in stabilizing and transferring loads from the upper limb to the trunk. Raising the arm involves movement in the glenohumeral and scapulothoracic joints. With combined damage to the structures of these joints, both the integrity of the bony ring of the shoulder girdle and the muscle-ligamentous balance are disrupted. The purpose was to determine the magnitude of the force and torques created by the muscles responsible for stabilizing the scapula and clavicle during arm flexion. Materials and methods. The base model is a three-dimensional model of the shoulder and upper limb for simulating arm dynamics in real time, including independent muscle control of the scapula and clavicle. We have analyzed the force and torque created by the muscles responsible for stabilizing the scapula during flexion (raising the arm) forward by 90°. The work of the following muscles was evaluated: mm. rhomboideus, trapezius scapularis and clavicularis, serratus anterior (upper), levator scapulae. Results. During shoulder flexion to 90°, the scapula performs lateral rotation, adjusting its position to the movement of the humerus. This is critical to preserve the subacromial space, prevent impingement, and ensure smooth movement. Serratus anterior (lower) is the main initiator of scapular lateral rotation du­ring shoulder flexion. Its torque is highest in the first 20–30°, which is critical for “acceleration” of the movement. Up to 60° of flexion, the activity of serratus anterior (lower) is maximal, this is where the greatest torque is generated, as shown in the graph. After 60°: the stabilization function passes to the trapezius, the serratus works less actively (isometrically). The main force pair for stable movement of the scapula during flexion consists of serratus anterior (lower) and trapezius (upper/lower), which provide controlled lateral rotation. It is these muscles that develop the greatest torques when the arm moves forward — with serratus anterior (lower) at the beginning of the movement, trapezius (upper/lower) at an angle close to 90°. Other muscles (rhomboideus, levator scapulae) modulate and limit excessive movements, keeping the scapula stable. Conclusions. According to the modeling data, it was determined that the main muscles that stabilize the scapula during shoulder flexion are serratus anterior (lower) and trapezius. They create maximum torques when the arm moves forward. Based on the anatomical position of these muscles — direct attachment to the most mobile areas of the scapula, with a fracture, they lose support for crea­ting a moment, which leads to a significant limitation of upper limb movements and disruption of scapular stability.