TempoLife › Features › Muscle and body-part reference › deltoid
Deltoid
The triangular muscle that caps the shoulder and lifts the arm away from the body in every direction.
Latin name: Musculus deltoideus. In plain English, the cap of the shoulder. It is filed here under shoulder and shoulder blade.
Reference information, not a diagnosis. Where a symptom is described below, it is there so you can describe your own more precisely to a clinician — not so you can name what you have.
Where it is
The deltoid is the muscle you can see and grab: it wraps the point of the shoulder from the collarbone at the front, round the bony ridge of the acromion at the side, to the spine of the shoulder blade at the back. All three parts converge into one short tendon that lands on a rough patch halfway down the outside of the upper arm bone, the deltoid tuberosity. That convergence is why the muscle is triangular, and why the Greek letter delta gave it its name.
Because the three parts approach the arm from three different directions, anatomists treat them almost as three muscles sharing one insertion. The front (clavicular) fibres pull the arm forward, the side (acromial) fibres pull it straight out, and the back (spinal) fibres pull it behind you. The nerve that supplies all three, the axillary nerve, wraps around the surgical neck of the humerus — which is why a broken upper arm or a dislocated shoulder can leave a numb patch over the deltoid and a weak lift.
| Feature | Detail |
|---|---|
| Origin | Lateral third of the clavicle, the acromion, and the spine of the scapula |
| Insertion | Deltoid tuberosity on the lateral shaft of the humerus |
| Nerve | Axillary nerve (C5-C6) |
| Main actions | Abduction of the arm; the front fibres flex and internally rotate, the back fibres extend and externally rotate |
What it does
The side fibres are the main abductors of the arm, but they do not start the movement. The first fifteen degrees or so of lifting the arm away from the body belongs to supraspinatus in the rotator cuff; once the arm is clear, the deltoid takes over and carries it up to shoulder height. Beyond that, the shoulder blade has to rotate underneath for the arm to keep rising, so the deltoid is only ever half the story of raising your arm overhead.
The front and back parts do quieter but constant work. Anything you press, throw or reach for uses the anterior deltoid; anything you pull toward you or reach behind you uses the posterior deltoid. In everyday life the muscle mostly acts as a stabiliser, holding the arm bone into the socket while the hand does something at the end of a long lever.
Exercises that load it
| Exercise | Category | Muscles the exercise library lists |
|---|---|---|
| Overhead press | Strength | Shoulders, triceps, core |
| Push-ups | Strength | Chest, triceps, front shoulder, core |
| Dips | Strength | Triceps, chest, front shoulder |
| Bench press | Strength | Chest, triceps, front shoulder |
| Swimming, freestyle | Cardio | Whole body, heart, lungs |
Overhead pressing is the most direct load: the arm travels through the deltoid's strongest range against gravity. Push-ups, dips and bench pressing load the front fibres hard as part of a bigger movement, which is why people who only press end up front-heavy. Rowing and any pulling done with the elbows wide loads the back fibres. If you train the shoulder deliberately, the useful test is whether all three directions get work in a week, not whether any single session felt hard.
How it is lengthened
The deltoid rarely needs stretching in its own right; it is short and its fibres run a short distance. What usually feels tight at the top of the shoulder is the upper trapezius or the joint capsule underneath. A cross-body arm hug lengthens the posterior fibres, and reaching the arm behind you along a doorframe lengthens the anterior fibres along with the front of the capsule. Take both to a gentle stretch, not to pain: the shoulder capsule does not respond well to being forced.
When it goes wrong
| Problem | What it typically feels like | Usual setting |
|---|---|---|
| Deltoid strain | A sharp local ache over the cap of the shoulder that hurts to lift the arm sideways but is fine at rest | A sudden overload — a heavy press, a fall onto an outstretched arm, or a big jump in overhead volume |
| Axillary nerve injury | Weak lifting of the arm sideways with a numb patch of skin over the outer shoulder | After a shoulder dislocation or a fracture of the upper humerus |
| Deltoid tightness after immobilisation | A stiff, weak shoulder that does not fully lift after a sling | Weeks in a sling or avoiding the arm after an injury |
Weakness lifting the arm sideways after a dislocation or a fall, or a numb patch over the outer shoulder, needs an in-person assessment — that pattern points at the axillary nerve rather than at a sore muscle.
Nothing on this page is a treatment plan. If a problem is limiting what you do, is not improving, or is getting worse, the right next step is an assessment by a doctor or a physiotherapist who can examine you.
Sources: descriptive anatomy from Gray's Anatomy of the Human Body, 20th US edition (1918), which is in the public domain; Latin names follow Terminologia Anatomica, published by the Federative International Programme for Anatomical Terminology. Nerve root levels are the conventional ranges taught from those sources and vary between individuals and between textbooks. Exercise cross-links use the TempoLife exercise library. Checked 2026-09-06.
Frequently asked questions
Which part of the deltoid do lateral raises work?
Mostly the middle, acromial fibres, which are the ones that carry the arm straight out to the side. Raising the arm slightly in front of the body brings the front fibres in; raising it slightly behind brings the back fibres in.
Why does my shoulder hurt at the side when I lift my arm?
Pain felt over the outer arm on lifting is often referred from structures under the acromion rather than from the deltoid itself — the rotator cuff tendons and the bursa sit directly beneath. A muscle strain usually hurts when you press on the sore spot; tendon and bursa problems often do not.
Can you build the deltoid without weights?
Yes, up to a point. Push-ups, dips, pike push-ups and any pressing pattern where your own body weight is the resistance load the front and middle fibres well. Progressing then means changing the leverage — elevating the feet, slowing the lowering — because you cannot add kilograms.
Nearby in this reference
Rotator cuff
The four small muscles that hold the shoulder in its socket
Trapezius
The big diamond across the upper back and neck
Rhomboids
The straps that pull the shoulder blades together
Serratus anterior
The finger-like muscle on the ribs that pins the shoulder blade down
Biceps brachii
The two-headed muscle at the front of the upper arm
Triceps brachii
The three-headed muscle at the back of the upper arm
All 34 body parts · Exercise library · Body map
Training is easier to judge when it is written down
TempoLife logs workouts, steps and sleep together, so a week of training and a week of recovery can be read side by side.