Primary Table
| DEGREE | MUSCLE | NERVE |
|---|
| 0°–15° | Supraspinatus | Suprascapular |
| 15°–90° | Deltoid | Axillary |
| > 90° | Trapezius | Accessory |
| > 90° | Serratus Anterior | Long Thoracic (SALT) |
Breakdown of Arm Abduction
- 0°–15°: Supraspinatus (Suprascapular nerve)
The "Starter": The supraspinatus is responsible for the initial "tug" to get the arm moving away from the body.
Clinical Tip: If a patient has to lean to one side or "hitch" their shoulder to start abduction, suspect a supraspinatus tear or suprascapular nerve impingement.
- 15°–90°: Deltoid (Axillary nerve)
The "Workhorse": This is the most powerful phase of abduction.
Clinical Tip: Axillary nerve damage (often from surgical neck of the humerus fractures or anterior shoulder dislocations) will make lifting the arm to 90 degrees nearly impossible.
- > 90°: Trapezius (Accessory nerve) & Serratus Anterior (Long Thoracic nerve)
The "Finishers": To get the arm above the head, the scapula must rotate upward. The trapezius and serratus anterior work together to "swing" the bottom of the scapula outward.
Clinical Tip: If the serratus anterior is weak, you'll see "winged scapula" when the patient pushes against a wall.
Mnemonics & Learning Tips
S-D-T-S (The Order of Muscles):
Some Days Trash Smells.
Supraspinatus → Deltoid → Trapezius → Serratus anterior.
SALT (Serratus Anterior, Long Thoracic):
Serratus Anterior = Long Thoracic.
Think: "Salt stays on the long table."
The "90-Degree Rule":
Everything under 90° involves the Glenohumeral joint (the ball and socket).
Everything over 90° requires Scapulothoracic movement (the shoulder blade moving on the ribcage).
Anatomy Tip:
The Suprascapular nerve (0-15°) and the Axillary nerve (15-90°) both arise from the Brachial Plexus. The Accessory nerve (CN XI), which controls the trapezius, is a cranial nerve—this is a common distractor in questions!