Deltoid muscle

The deltoid muscle is the thick, triangular muscle that forms the rounded outline of the shoulder. It is named after the Greek letter delta (Δ), which its shape resembles (Kenhub). It is a key muscle for both moving and steadying the shoulder, and it is a common site for injections, including vaccinations (Elzanie & Varacallo, 2024).

Where is the deltoid muscle and what does it do?

The deltoid starts from three places: the outer third of the collarbone, the bony tip of the shoulder (the acromion) and the ridge along the top of the shoulder blade. Its fibres wrap over the shoulder joint from the front, side and back, and meet on a single tendon about halfway down the outside of the upper arm (Kenhub).

Each of its three parts has its own job (Kenhub):

• Front (clavicular) part: raises the arm forward and turns it inwards.
• Middle (acromial) part: the largest and strongest. It is the main muscle for lifting the arm out to the side, but only once the arm is about 15 degrees away from the body. The first part of that movement comes from the supraspinatus, one of the rotator cuff muscles.
• Back (spinal) part: draws the arm back and turns it outwards.

Together with the rotator cuff, the deltoid holds the ball of the upper arm steady in its socket, and it controls the arm as you lower it. Its nerve supply is the axillary nerve, from the C5 and C6 levels of the brachial plexus, which also supplies a patch of skin over the lower deltoid (TeachMeAnatomy).

Why does the deltoid matter at a desk?

At a desk, the deltoid works mostly to hold the arm in position for the keyboard and mouse. How much depends on the setup:

• Arm support. In 13 people typing for 20 minutes, resting the wrists on a support reduced activity in the trapezius and front deltoid compared with typing with the arms “floating”. Supporting the forearms reduced sideways bending at the wrist and discomfort (Cook et al., 2004).
• Mouse position. In 30 adults, a mouse placed high on the desk while the keyboard sat in a drawer below gave the least neutral posture and the highest muscle activity. A mouse placed centrally, between the keyboard and the body, gave the most neutral posture (Dennerlein & Johnson, 2006).
• Prevention trials. A Cochrane review found moderate-quality evidence from two studies that an arm support combined with an alternative mouse reduced new neck or shoulder disorders, but inconsistent evidence for either one alone (Hoe et al., 2018).

These are short laboratory studies, so they say more about load than about pain over months. For setup details, see setting up an office chair and Is a vertical mouse better?

Is pain over the deltoid coming from the deltoid?

Often not. A small fluid-filled sac, the subacromial bursa, lies under the deltoid between the bony tip of the shoulder and a rotator cuff tendon. It can become irritated and swollen if it is pinched repeatedly during overhead movements, and that pain is felt under the deltoid (Kenhub). When the rotator cuff is injured, the deltoid may also have to work harder to compensate (Elzanie & Varacallo, 2024). Pain that is worse when you use the arm is often linked with tendonitis, bursitis or impingement, while pain and stiffness lasting months can point to a frozen shoulder or arthritis (NHS). The wider picture is in shoulder pain from computer work.

When to see a doctor

See a GP if shoulder pain is getting worse or hasn’t improved after two weeks, or it is very hard to move your arm. Ask for an urgent appointment or call 111 if the pain is sudden or very bad, you can’t move your arm, the shoulder has changed shape or is badly swollen, pins and needles won’t go away, the arm is hot or cold to the touch, or the pain followed a fall or injury (NHS). Call 999 if chest pain spreads to your arm, neck or jaw (NHS).