Anatomy Lesson time! 😁
The Deltoid muscle is a large triangular shaped muscle which lies over the glenohumeral (shoulder) joint, providing support and stabilisation for the head of the Humerous (upper arm bone). There are three distinct portions to this muscle - the anterior, mid/lateral and posterior - responsible for moving your arms forward, sideways and backward respectively.
Fun fact: The Deltoid is named after the Greek letter Delta which is shaped like an equilateral triangle.