The deltoid (also deltoid muscle, deltoideus, delt, latin: musculus deltoideus) is a thick triangular-shaped muscle of the shoulder girdle.
The deltoid arises from two bones - the scapula and the clavicle. The deltoid muscle originates from the acromion of the scapula (laterally), the spine of the scapula (posteriorly) and from the acromial extremity (end) of the clavicle (anteriorly).
According to the origin of the fibers of the deltoid, the muscle can be divided into three parts: clavicular, acromial and spinal parts.
Fibers of the deltoid muscle merge together and insert onto the deltoid tuberosity of the humerus, which is located in the middle of the lateral aspect of the shaft of the humerus.
Depending on the activated part the deltoid muscle provides the following movements of the upper limb:
- The clavicular or anterior part - internal rotation of the upper arm.
- The spinal part or posterior part - external rotation of the upper arm.
- The acromial part or the lateral part - abduction of the upper arm.
- The clavicular and spinal parts - adduction of the upper arm.
- The clavicular and acromial parts - flexion of the upper arm.
- The acromial and spinal parts - extension of the upper arm.
The deltoid is innervated by the axillary nerve which arises from the brachial plexus carrying the fibers of C5-C6.