The main clinical manifestation of deltoid muscle weakness is limited shoulder abduction, which means that the axillary nerve innervating the deltoid muscle is damaged, and the more common causes are damage or compression of the brachial plexus nerve, and cervical spondylosis compressing the nerve fibers innervating the deltoid muscle. However, no matter whether it is cervical spondylosis or brachial plexus nerve injury or compression, it is usually not only manifested as a separate weakness of the deltoid muscle, but also accompanied by weakness of other muscles or sensory abnormalities, so it needs to go to the hospital for further examination to be sure. Of course, rotator cuff injury, frozen shoulder, etc., these diseases can also cause poor shoulder abduction function, but not necessarily muscle weakness, but only because of the pain in the shoulder abduction, so the shoulder can not be lifted up, so it is necessary to identify the cause of muscle weakness.