The answer is d. The posterior fibres of deltod assist latissimus dorsi in extending the shoulder (moving the arm backwards from the resting position).
- Origin: Lateral third of clavicle, acromion process and spine of scapula.
- Insertion: Deltoid tuberosity of humerus.
- Nerve supply: Axillar nerve (C5,6)
- Action: Flexion, extension and abduction of shoulder.
Deltoid is a triangular muscle covering the rounded tip of the shoulder. It has three distinct sets of fibres (parts); the anterior, lateral and medial fibres. The three parts work together to abduct the shoulder. Individually the anterior and posterior fibres flex and extend the shoulder respectively.
Supraspinatus initiates abduction of shoulder up to about 15 degrees. The greater power of deltoid then takes over to further abduct the arm. Lateral rotation of the scapula facilitates abduction beyond 90 degrees.