The deltoid is a thick, triangular muscle that caps the shoulder joint and gives the shoulder its rounded contour. It consists of three distinct portions (anterior, middle, and posterior) that work together or independently. This muscle primarily abducts the arm, while its different portions assist in forward flexion and extension of the shoulder.
The name comes from the Greek letter "delta" (Δ) due to its triangular shape, with the "-oid" suffix meaning "resembling."
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