Deltoid
Musculus deltoideus
// Structure & location
The deltoid sits like a cap over the shoulder joint. It has three parts arising from the collarbone (front), the acromion (middle) and the spine of the shoulder blade (rear). All three insert together about halfway down the outer side of the humerus.
// Function
According to OpenStax it is the major muscle for lifting the arm to the side (abduction). The front part also raises the arm forward and rotates it inwards; the rear part moves it back and rotates it outwards. Because the parts partly oppose each other, the muscle can move the arm in almost any direction.
// For training & sport
- Overhead pressing mainly trains the front and middle parts.
- Lateral raises target the middle part, which makes the shoulders look wider.
- Reverse flyes, face pulls and rows with the elbows flared work the rear part.
- The front part already works hard in bench press and push-ups.
// Good to know
The name comes from the Greek letter delta (Δ), because from the side the muscle looks like an upside-down triangle. Underneath lie the rotator cuff muscles, which keep the head of the humerus centred in the shallow socket.