The most common causes of left arm bending pain are soft tissue injury, osteoarthritis, humeral epicondylitis and so on. 1. Soft tissue injury: if the patient’s left arm is hit by external impact, the local soft tissue is damaged and then inflammatory reaction occurs, in the process of bending to pull the damaged soft tissue, there may be pain in the arm. 2. Osteoarthritis: If the patient has osteoarthritis, it may cause degenerative changes in the articular cartilage of the left arm, and when subjected to weather changes or humidity and other factors, bending the left arm may cause pain. 3. Epicondylitis of the humerus: If the patient repeatedly twists the arm to cause overuse of the forearm muscles, it is easy to induce epicondylitis of the humerus, which may cause pain when bending the arm. Causes of left arm bending pain is more, may also be related to local infection, fasciitis and other reasons, if the symptoms are not relieved after rest, it is recommended to go to the hospital in time to identify the cause of the disease, by the doctor targeted treatment.