The inability to lift the left arm may be caused by the following reasons: First, it may be caused by frozen shoulder, which is common in women around 50 years old. Inflammation around the shoulder joint can cause widespread pain in the shoulder joint, accompanied by restricted function and joint adhesions, so the inability to lift the shoulder joint may also cause the inability to lift the arm. Second, it may be caused by cerebrovascular accident, commonly cerebral infarction or cerebral hemorrhage. In this case, besides the symptoms of the hand, there will also be symptoms of the ipsilateral lower limb, such as weakened muscle strength, inability to walk, and there may be symptoms of dizziness, headache, facial palsy, aphasia, etc. in the head, and in serious cases, coma may occur. Thirdly, it is caused by trauma, which leads to shoulder or arm fracture after trauma, and can also cause the inability to lift the arm.