Patients with shoulder impingement usually go to an orthopedic clinic, where they are usually given oral anti-inflammatory pain medication first, and then local closure therapy if it is not effective, and arthroscopic surgery is recommended to clean it up if it is still not effective. These are effective treatments for acromioclavicular impingement, but some patients, especially young people who love sports and fitness, still experience recurrence of symptoms. The reason for this is that muscle imbalance is overlooked in the treatment. When treating impingement, the Department of Rehabilitation Medicine focuses not only on the pathological changes in the subacromial structures (bursa and supraspinatus tendon): inflammation of the bursa and inflammation of the tendon (see Figure 1), but also on the pathogenesis of impingement from a biomechanical point of view, so that treatment can be tailored accordingly. Figure 1: Lesion sites in acromioclavicular impingement We believe that most patients with acromioclavicular impingement have a muscle imbalance. The main clinical manifestation of acromioclavicular impingement is shoulder pain when the arm is raised to its highest point. This is mainly due to the narrowing of the gap between the acromion and the humeral head during the supination movement, and the impact of the humeral head on the subacromial structures (bursa, supraspinatus tendon), which repeatedly causes inflammation of the bursa and further progression can cause inflammation of the supraspinatus tendon (Figure 1). During the patient’s supination, there are two groups of muscles that produce an upward and downward pull on the humeral head (see Figures 2 and 3). The muscles that lift the humeral head are the deltoid and supraspinatus, and the muscles that pull down the humeral head are the infraspinatus and subscapularis and the teres minor (see Figure 2). Cool sports and fitness friends tend to practice pulling down these small muscles of the humeral head less, thus causing an imbalance between these two groups of muscles and excessive lifting of the humeral head thus hitting the acromion above it, causing inflammation of the bursa and supraspinatus muscles caught in between. In addition to excessive supination of the humeral head, which causes narrowing of the subacromial space, another aspect of the scapula that is abnormally positioned during supination can also cause excessive descent of the acromion, also causing impingement, which is often overlooked by clinicians. Postural assessment and movement analysis are precisely the specialty of the rehabilitation department. As shown in the figure, the patient’s left scapula shows a different position than the healthy side (right side) during both resting arm placement on the body side and during supination. The reason for this is the lack of strength of the muscles around the scapula, especially the anterior serratus (Figure 5). Figure 2: Rotator cuff small muscle groups: supraspinatus, infraspinatus, subscapularis, and teres minor Figure 3: thick deltoid Figure 4: abnormal position of the left scapula during arm raising Figure 5: anterior serratus After analyzing the above mechanism, it is easy to develop the following 3-step treatment measures: 1. Use traditional physical therapy for inflamed tissues to relieve inflammation and pain, commonly used are microwave and laser. The effect is better than oral anti-inflammatory and analgesic drugs, not only avoiding the gastrointestinal reaction of oral drugs, but also more efficient for direct irradiation of inflamed tissues. 2. Use joint release to increase the space between the acromion and the humeral head to reduce impingement, mainly by tugging on the joint capsule below the shoulder joint 3. The most crucial step is still muscle exercises to strengthen the muscles that pull down the humeral head and/or the muscle strength around the scapula, so that the humeral head is tugged by the pull down muscles during the upper limb lifting without producing too much upward movement to impingement on the acromion above. With this treatment, not only are the symptoms relieved, but the patient also learns the correct muscle exercises to effectively avoid recurrence of symptoms and safely return to sports.