Left shoulder internal tendon pain may be caused by physiological factors such as prolonged labor, or it may be caused by pathologic reasons such as frozen shoulder and rotator cuff injury. 1. Physiological factors: If prolonged labor or excessive exercise can lead to fatigue of the soft tissues around the left shoulder and accumulation of lactic acid in the body, causing the symptoms of pain in the tendon of the left shoulder, which can be gradually relieved by appropriate rest and local hot compresses. 2. Frozen shoulder: Frozen shoulder is caused by the degeneration of the soft tissues around the shoulder, chronic strain and other factors, resulting in chronic inflammation and fibrosis of the joint capsule and surrounding ligamentous tissues, which causes the elasticity of the soft tissues around the shoulder to decrease and the effective volume of the joint to reduce, and the symptoms of pain around the shoulder joint and activity limitation appear. 2. Rotator cuff injury: the rotator cuff consists of the tendinous part of supraspinatus, infraspinatus, subscapularis and teres minor muscles. Under the influence of trauma and degeneration, rotator cuff injury can occur, which may cause shoulder pain and limitation of joint activities. There may be other causes of left shoulder internal tendon pain, patients are advised to go to the hospital to further identify the cause of the disease, and then targeted treatment.