There are many causes of shoulder pain, which are different for young people and middle-aged and older people, as follows: 1. Young people: when shoulder pain occurs, it is generally considered to be caused by muscle strain or strain, such as during excessive sports training or fitness, which can cause damage to the deltoid muscle, manifesting as pain on the lateral side of the shoulder, or synovitis of the inter-articular joint, mainly manifesting as pain on the internal extensor side of the shoulder; 2. In patients with rotator cuff injury, in addition to pain, there is also weakness in supination. In people around 50 years of age, if there is shoulder pain along with limited active and passive movement of the shoulder joint, it is usually considered to be frozen shoulder.