Musculoskeletal System
Supraspinatus versus deltoid in abduction
In standard anatomy teaching, supraspinatus starts lifting the arm out to the side over about the first 15 degrees, and after that the deltoid is the main mover.
Remember
Difficulty starting abduction points to supraspinatus, which does the first 15 degrees; difficulty beyond about 15 degrees points to the deltoid.
Abduction means lifting the arm out to the side, away from the body. In standard anatomy teaching, supraspinatus and the deltoid share the job, and the handover comes at about 15 degrees.
Supraspinatus starts abduction. It is one of the four rotator cuff muscles, the small muscles that hold the head of the humerus in its socket. It arises from the supraspinous fossa (the hollow above the spine of the scapula) and attaches to the greater tubercle (a bony bump at the top of the humerus). Its nerve is the suprascapular nerve.
The deltoid, the large rounded muscle over the shoulder, then takes over as the main abductor, while supraspinatus goes on assisting it. The deltoid is supplied by the axillary nerve, which winds around the surgical neck of the humerus (the narrow part of the bone just below its head). A fracture there, or a shoulder dislocation, can injure the nerve.
A patient who struggles to start lifting the arm from rest, but can abduct further once it is lifted slightly, points to supraspinatus. A patient who can start the lift but struggles beyond about 15 degrees points to a deltoid that is not working, and the skin over the lower deltoid may be numb as well.
You can think of supraspinatus as the starter motor and the deltoid as the engine. The four cuff muscles are supraspinatus, infraspinatus, teres minor and subscapularis, in that order, and supraspinatus is the first of them, the one that starts the lift.
Mnemonic
SITS
- S Supraspinatus
- I Infraspinatus
- T Teres minor
- S Subscapularis
The letters give the four rotator cuff muscles in order, and the first letter is supraspinatus, the muscle that starts abduction.
Key facts
- Starts abduction
- supraspinatus, over roughly the first 15 degrees.
- Main abductor
- the deltoid, which takes over after the first 15 degrees.
- Supraspinatus nerve
- the suprascapular nerve.
- Deltoid nerve
- the axillary nerve, which can be injured in a fracture of the surgical neck of the humerus.
- Cannot start the lift
- when a patient cannot start to abduct the arm but can continue once it is lifted, think of supraspinatus.
- Other cuff muscles
- infraspinatus and teres minor turn the arm outwards, and subscapularis turns it inwards.
- Skin sign
- numbness over the lower deltoid (the regimental badge area) may go with an axillary nerve injury.
Exam traps
- A patient with a deltoid that is not working should still be able to start the movement, because supraspinatus, supplied by the suprascapular nerve, performs the first 15 degrees or so of abduction.
- Supraspinatus is not the main abductor once the first 15 degrees or so are complete, because after that point it assists the deltoid. It is also supplied by the suprascapular nerve rather than the axillary nerve.
- The deltoid is the major abductor of the arm, but it takes over from supraspinatus after the first 15 degrees, so it is not the muscle that starts the movement.
- Teres major is not part of the rotator cuff, and it pulls the arm towards the body rather than lifting it away.
- Teres minor is supplied by the axillary nerve and can be weak in the same injury as the deltoid, but it turns the arm outwards rather than abducting it.
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