Nerve Supply and Actions of Deltoid
Nerve Supply
The deltoid is supplied by the axillary nerve (C5, C6).
Actions — “Three Muscles in One”
The deltoid functions as three muscles in one, with each of its three parts producing a different action at the shoulder joint:
| Part | Action |
|---|---|
| Anterior (clavicular) fibres | Flexion and medial rotation of the arm |
| Middle (acromial) fibres | Abduction of the arm from 15° to 90° |
| Posterior (spinal) fibres | Extension and lateral rotation of the arm |
Abduction — The 0°–15° Problem
- The multipennate acromial fibres are powerful abductors of the arm from 15° to 90°.
- These fibres cannot initiate abduction from 0° to 15° when the arm is by the side of the body, because at this position the vertical pull of the acromial fibres corresponds to the long axis of the arm — there is no abductor moment.
- The first 15° of abduction is initiated by the supraspinatus, after which the deltoid takes over and continues abduction up to 90°.
- Beyond 90°, further elevation occurs through scapular rotation (scapulohumeral rhythm), involving trapezius and serratus anterior.
Clinical Testing
- The deltoid can be tested by asking the patient to abduct the arm against resistance, while the examiner palpates the contracting muscle with the other hand.
- The muscle can be easily seen and felt to contract during resisted abduction.