## 4 Discussion  
This study predicted muscle forces using two widely recognized biomechanical modeling software programs, OpenSim and AnyBody Modeling SystemTM, for two clinical tests for the AC joint OA: the CBA and the HBB test. The results obtained from this study highlighted observable agreements and discrepancies in muscle force predictions between the two software platforms. While the magnitude of the muscle force predictions differed between OpenSim and AnyBody Modeling SystemTM, the muscle activation patterns showed qualitative similarities. This presents the complexities associated with musculoskeletal modeling, its limitations, and its implications for clinical orthopedic practice and biomechanical research. Understanding the individual function of each muscle's ability to  
Figure 6: Muscle force predictions for the hand-behind-the-back test using OpenSim and AnyBody Modeling SystemTM. strain the AC joint can reliably dictate its influence on a positive test result to identify AC joint OA. The deltoid has three heads, anterior, middle, and posterior, that flex and medially rotate, abduct, or extend and laterally rotate the arm. The anterior head of the deltoid plays more of a role in the CBA test, while the posterior head plays more of a role in the HBB test to position the arm behind the patient. While the coracobrachialis, pectoralis major, and latissimus dorsi do not cross the AC joint, they are strong muscle forces in arm positioning and glenohumeral motion that aid in forces placed across the AC joint. Additional accessory muscles that do not directly cross the AC joint and assist in stabilizing the shoulder girdle or arm/forearm positioning during the clinic tests (CBA/HBB) include the pectoralis minor, brachialis, biceps short head, serratus, sternocleidomastoid, and triceps. Of note, the rotator cuff muscles (supraspinatus, infraspinatus, trees minor, and subscapularis) are shoulder-stabilizing muscles as well that allow rotation of the glenohumeral joint to assist in arm positioning. However, they do not directly cross the AC joint articulation. The muscle force predictions by the AnyBody Modeling SystemTM and OpenSim during the CBA test offer helpful insights into the muscles potentially involved in this motion. Notably, both software platforms showed a close match in predicting the involvement of the deltoid clavicular muscle, suggesting its important role in performing the CBA test. This finding aligns with studies on shoulder biomechanics, which underscore the deltoid clavicular muscle's role in shoulder elevation, shoulder rotation, and shoulder adduction due to its anatomical origin crossing the AC joint [26; 27; 28]. Clinically, the forces generated by the deltoid muscle are significant, as they correlate with stress across the AC joint, a factor known to contribute to symptomatic pain and a positive test for AC joint OA. Research demonstrated that heightened activation of this muscle correlates with increased joint stress, intensifying symptoms in patients with AC joint OA [29; 30]. This understanding reinforces the importance of the deltoid clavicular muscle in diagnosing and managing AC joint OA. However, during the CBA test, there were observable differences in the predictions of other muscle forces between OpenSim and AnyBody Modeling SystemTM, such as the biceps short head, brachialis, coracobrachialis, pectoralis major, and triceps, as shown in Figure 5. AnyBody Modeling SystemTM and OpenSim might employ different assumptions regarding muscle activation patterns, recruitment strategies, and joint mechanics, which could lead to substantial differences in predicted forces. Further, research has pointed out that differences in anthropometric and anatomical definitions defined by each simulation tool can significantly affect muscle force predictions [31]. These discrepancies are critical as they emphasize the necessity for precise and accurate modeling approaches to accurately accommodate the complexities of the human system and movement and predict the required kinetics [32].