The deltoid is sometimes considered one muscle with three sections and sometimes treated as three separate muscles.
Together they form the bulk of your shoulder and are used to raise your arm from your side and flex and extend your arm at the shoulder.
Problems with the deltoid muscles are typically either related to underlying dysfunction in the rotator cuff muscles, chronic overload of the muscles from poor ergnomics and sports activities, or impact trauma to your shoulder.
The relationship between the deltoids and the rotator cuff muscles, especially supraspinatus, is just being fully understood.
Deltoid trigger points are often misdiagnosed as rotator cuff tears, bicipital (biceps) tendonitis, subdeltoid bursitis, subacromial impingment syndrome or C5 radicular pain.