Anterior fibres flex and medially rotate, middle fibres abduct, posterior fibres extend and laterally rotate
Mnemonic
“Front flexes, middle abducts, back extends”:
- Anterior (clavicular) fibres - flexion and internal rotation
- Middle (acromial) fibres - abduction, from about 15 to 90 degrees
- Posterior (spinous) fibres - extension and external rotation
Supplied by the axillary nerve (C5, C6).
“Supraspinatus starts it, deltoid continues it, trapezius and serratus finish it” is the sequence of abduction: supraspinatus initiates the first 15 degrees, deltoid takes it to 90, and beyond that scapular rotation by trapezius and serratus anterior takes over, in a 2:1 glenohumeral to scapulothoracic ratio.
Deltoid wasting with loss of the shoulder contour gives the squared-off shoulder of anterior dislocation, and it is the sign that accompanies axillary nerve injury.
The deltoid is a common intramuscular injection site, and the axillary nerve is protected by injecting into the upper third, at least two finger breadths below the acromion.
Expansion
- Anterior fibres - flexion and medial rotation of the arm
- Middle fibres - abduction (the main power from 15 to 90 degrees)
- Posterior fibres - extension and lateral rotation
The whole muscle is supplied by the axillary nerve (C5 to C6).