Deltoid Muscle: The deltoid is a triangular shoulder muscle that is thick and strong. Its name comes from its resemblance to the Greek letter ‘delta’. The origin of the muscle is broad, spanning the clavicle, acromion, and scapula spine. It enters onto the humerus and passes inferiorly around the glenohumeral joint on all sides.
The clavicle (collarbone), humerus (upper arm bone), and scapula (shoulder blade) are all connected to the shoulder blade. Because the deltoid is a superficial shoulder muscle, it is only as deep as its surrounding fascia, the platysma muscle, and the skin. The deltoid is easily visible and palpable due to its superficial nature.
The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis), the pectoralis major and its tendon, the coracobrachialis tendons, both heads of the biceps brachii muscle, and the long and lateral heads of the triceps brachii muscle are all covered by the deltoid. The coracoacromial ligament, subacromial bursa, bony structures (coracoid process and proximal humerus) and neurovascular structures (axillary nerve and anterior and posterior circumflex humeral vessels) of the shoulder are likewise covered by the deltoid.
The acromial, clavicular, and scapular spinal components make up the deltoid. The acromial component (middle fibres) abducts the arm, while the clavicular and scapular spinal parts stabilise it and keep it in a stable plane of abduction. The front fibres of the clavicle can also operate as a flexor and internal rotator of the arm, whereas the posterior fibres of the scapular spine can extend and externally rotate the arm.
Sections of Deltoid Muscle
The deltoid muscle has a triangle shape thanks to its large origin and narrow base. The origins of the three sections of the deltoid are as follows:
- The superior surface and anterior border of the lateral third of the clavicle form the clavicular (anterior) component.
- The lateral edge and superior surface of the scapula’s acromion form the acromial (middle) part.
- The scapular spinal (posterior) component arises on the crest of the lateral third of the scapula’s spine.
Functions of Deltoid Muscle
At the glenohumeral joint, the deltoid muscle (acromial portion) is the primary abductor of the arm. However, it can only do so if the arm is already more than fifteen degrees abducted. The supraspinatus muscle is responsible for the first part of abduction. The abduction action is guided by the muscle’s clavicular and scapular spinal fibres.
The deltoid muscle, together with the rotator cuff muscles, helps to stabilise the glenohumeral joint. When carrying heavy goods with the arm completely abducted, the muscle creates a line of force (static contraction) that prevents the glenohumeral joint from being displaced inferiorly. When the arm is lowered, or adducted, the deltoid also contracts eccentrically. This enables for regulated adduction of the arm.
During walking or running motions, the clavicular (anterior) fibres of the deltoid work together with the pectoralis major to produce flexion of the arm. These fibres are also active when the humerus is rotated internally (medially).
Structure of Deltoid Muscle
The deltoid muscle is the shoulder’s major muscle. The posterior circumflex humeral artery supplies it with blood. The posterior cord of the brachial plexus and other nerves, such as the axillary nerves, C5 and C6 nerves, make up the majority of it.
The deltoid muscle is made up of three unique groups of fibres, each of which causes the glenohumeral joint to move in a different way. These are divided into several sections that are generally referred to as:
- Clavicular part: Musculus deltoideus pars clavicularis is the Latin name for the Clavicular Part of the Deltoid Muscle. It begins in the clavicle’s lateral third.
- Acromial Part of Deltoid Muscle: It comes from the acromial of the scapula and is also known as Musculus deltoideus pars acromialis in Latin (posterior).
- Deltoid Muscle Spinal Part: Also known as the musculus deltoideus pars spinalis in Latin, it comes from the scapula’s spine.
All three points converge on the deltoid tuberosity, which is located on the lateral surface of the humerus’ shaft.
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By Team Learning Mantras