Upper limb
Deltoid, acromial part (middle fibers)
Musculus deltoideus (pars acromialis)
- Origin
- Acromion
- Insertion
- Deltoid tuberosity of the humerus
- Function
- Abduction of the shoulder from ~15° onward
- Innervation
- Axillary nerve (C5–C6)
- Movement & chain
- The only one of the three parts that produces pure abduction; the other two actually oppose each other in pure abduction. Below ~15°, supraspinatus initiates abduction, after which this part takes over.
- Palpation
- Easily grasped over the lateral shoulder, directly below the acromion, during resisted abduction.
- Clinical
- Injury to the axillary nerve mainly causes loss of abduction strength, since this is the largest and strongest of the three parts.
- Segmental innervation
- Axillary nerve, C5–C6.
- In clinical practice
- Standard site for intramuscular injection in adults: in the muscle belly, roughly three finger-widths below the acromion, to spare the axillary nerve. Maximum 1–2 mL.
- Region in the model
- Upper limb