## 4 Discussion
Moreover, the absence of serratus, sternocleidomastoid, and trapezius muscles in OpenSim's model indicates a potential incompleteness of the model and questions its use in clinical tests like the CBA and HBB tests. Most significantly, the pectoralis major and trapezius have vector forces acting on the clavicle and across the AC joint based on their insertions. These vectors are crucial for shoulder adduction, elevation, and CBA motion, as the muscle forces stress the AC joint to delineate symptomatic pain while testing for AC osteoarthritis. The sternocleidomastoid, to a lesser extent, has a stabilizing effect on the clavicle's medial end. The HBB test results also highlighted agreements and discrepancies in muscle force predictions between the AnyBody Modeling SystemTM and OpenSim, as shown in Figure 6. The observable consistency in the brachialis and deltoid scapular muscle, along with similar activation patterns in the biceps long head, infraspinatus, pronator quadratus, pronator trees, supraspinatus, and triceps long head in each software, indicates their potential role during the HBB test. These repeated observations across both software assure researchersand clinicians that the deltoid muscle, a key contributor, is being accurately assessed by these models. Additionally, the accessory muscles that aid shoulder stabilization and arm positioning can be reliably predicted. However, the notable variation in force predictions for the supinator, trees minor, pectoralis minor, latissimus dorsi, pectoralis major clavicular head, biceps long head, infraspinatus, pronator trees, supraspinatus, and triceps long head underscores the potential differences in the algorithms used by the AnyBody Modeling SystemTM and OpenSim to solve the muscle redundancy problem. The OpenSim model does not include the levator scapulae and trapezius clavicular muscle, the latter being a more significant omission due to its role in AC joint stabilization. Another key aspect is the role of the accessory muscles during these movements. For instance, the pectoralis minor, serratus anterior, trapezius, and levator scapulae play a crucial role in stabilizing the shoulder girdle and supporting joint mechanics during diagnostic tests like the CBA and HBB tests. These muscles, while not directly crossing the AC joint, assist in maintaining scapulothoracic and clavicular stability, which is essential for reducing abnormal joint loading and ensuring proper force transmission through the shoulder complex. For example, the serratus anterior prevents scapular winging, and the trapezius aids in scapular retraction and elevation, both of which are vital for functional shoulder movements [19]. Weakness in these muscles can exacerbate joint misalignment, increasing stress on the AC joint and worsening OA symptoms. Strengthening accessory muscles through targeted rehabilitation exercises, such as scapular stabilization and isometric holds, can improve shoulder stability, alleviate pain, and restore functional balance. This aligns with previous findings emphasizing the role of periarticular muscle strengthening in reducing joint degeneration and enhancing outcomes in OA patients [33]. By incorporating accessory muscle-focused strategies into rehabilitation protocols, clinicians can optimize patient recovery and prevent further deterioration of the AC joint. Future studies using electromyography (EMG) can provide deeper insights into the contributions of these muscles to shoulder biomechanics and refine therapeutic approaches. Additionally, differences in how each software models the path and geometry of muscles could have resulted in varied force predictions. Further, the variation in predicted forces for the same muscle across different software may reflect the complex interplay of muscle synergies-groups within the software to produce movement and influence the force-length-velocity relationship of muscles [34; 35]. This hinders relying on one software to identify the potential muscles involved during the CBA and HBB tests.