According to NASM Essentials of Corrective Exercise Training subacromial impingement syndrome (SAIS) may be the result of bony deformity of the acromion, underlying rotator cuff weakness, shoulder instability, or scapular dyskinesis. Rotator cuff weakness and shoulder instability results in excessive superior and anterior translation and inadequate external rotation of the humeral head, limiting clearance of the greater tuberosity under the acromion process. Decreases in the normal scapular upward rotation and external rotation of the humerus combined with posterior tilting on the thorax cause a decrease in the physiologic space under the coracoacromial arch.”
There are other contributing factors mentioned in the textbook, however, many personal trainers are certified through the NASM and have access to this resource so I figured it is in the best to include just a few.
Thank you for your question.