Pars clavicularis musculi deltoidei

The deltoid muscle is the muscle forming the rounded contour of the human shoulder.

It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat.

Anatomically, the deltoid muscle appears to be made up of three distinct sets of muscle fibers, namely the

• anterior or clavicular part (pars clavicularis)
• posterior or scapular part (pars scapularis)
• intermediate or acromial part (pars acromialis)

However, electromyography suggests that it consists of at least seven groups that can be independently coordinated by the nervous system.

It was previously called the deltoideus (plural deltoidei) and the name is still used by some anatomists.

It is called so because it is in the shape of the Greek capital letter delta (Δ).

Deltoid is also further shortened in slang as "delt". A study of 30 shoulders revealed an average mass of 191.9 grams (6.77 oz) in humans, ranging from 84 grams (3.0 oz) to 366 grams (12.9 oz).