Anatomy and Physiology 2e · The Muscular System
Muscles of the Pectoral Girdle and Upper Limbs
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In 30 seconds
The upper limb is built for mobility: reaching, lifting, grasping, and fine manipulation. Unlike the lower limb, built for weight bearing and stability, the upper limb trades stability for a large range of motion — made possible by muscles arranged in layers from the shoulder blade to the fingertips.
The scapula is the key to the whole system. It has almost no direct bony connection to the axial skeleton; the only link is the clavicle, which attaches at the sternoclavicular joint. Muscles of the pectoral girdle The clavicle and scapula connecting the upper limb to the axial skeleton Full entry → must therefore position and hold the scapula while muscles of the shoulder use it as a stable base to move the humerus. From there, muscles of the arm move the forearm, and muscles of the forearm move the wrist and hand. Study the region as four functional groups: muscles that position the pectoral girdle, muscles that move the humerus, muscles that move the forearm, and muscles that move the wrist, hand, and fingers.
Why this matters
Knowing which muscle does what is directly useful in rehabilitation, sports medicine, and nursing. After a stroke, shoulder injury, or nerve damage, clinicians assess which movements are weak and reason backward to which muscles are affected. rotator cuff The four SITS muscles and their tendons around the humeral head Full entry → injuries are among the most common shoulder problems, so the muscles that stabilize the glenohumeral joint are a favorite exam topic. For movement-based careers — physical therapy, athletic training, occupational therapy — this is the vocabulary of daily practice.
The college version
Core Concepts
Muscles that position the pectoral girdle
- Trapezius — a large, diamond-shaped muscle with three fiber groups: superior fibers elevate the scapula (the "shrug" motion), middle fibers retract it (pull the shoulder blades together), and inferior fibers depress it.
- Rhomboids (major and minor) — retract and slightly elevate the scapula, running from the spine to the medial border of the scapula.
- Levator scapulae — elevates the scapula.
- Serratus anterior — protracts the scapula (pulls it forward around the rib cage, like a boxer's punch) and holds the medial border against the thoracic wall. When it is weak, the medial border of the scapula can "wing" away from the rib cage — a commonly taught clinical sign.
- Pectoralis minor — protracts and depresses the scapula.
- Subclavius — a small muscle under the clavicle that steadies the clavicle during shoulder movements.
Muscles that move the humerus
- Pectoralis major — flexes, adducts, and medially rotates the arm (the muscle used to hug or throw across the body). The clavicular head assists flexion; the sternocostal head helps pull an elevated arm back down.
- Latissimus dorsi — extends, adducts, and medially rotates the arm ("swimmer's muscle"). Although it lies on the back, its action is on the humerus.
- Deltoid — three parts of one muscle: anterior fibers flex and medially rotate; middle fibers abduct (the prime mover of arm abduction Moving a body part away from the midline Full entry →); posterior fibers extend and laterally rotate.
- Teres major — adducts, medially rotates, and extends the arm; works with the latissimus dorsi.
- Coracobrachialis — flexes and adducts the arm.
- Rotator cuff (SITS muscles) — four muscles whose tendons form a cuff around the humeral head, acting as dynamic stabilizers of the joint: supraspinatus (initiates abduction), infraspinatus (lateral rotation), teres minor (lateral rotation), and subscapularis (medial rotation). Remember "SITS."
Muscles that move the forearm
- Biceps brachii — flexes the elbow and supinates the forearm (turns the palm up); both heads insert on the radial tuberosity.
- Brachialis — the primary, most powerful elbow flexor; lies deep to the biceps.
- Brachioradialis — flexes the elbow, most effectively with the forearm in the neutral "thumb-up" position.
- Triceps brachii — the primary elbow extensor; its long head also extends and adducts the arm.
- Anconeus — a small muscle that assists elbow extension.
Muscles that move the wrist, hand, and fingers
The forearm is organized into an anterior (flexor) compartment and a posterior (extensor) compartment, plus intrinsic muscles within the hand.
- Anterior compartment: flexor carpi radialis (flexes and abducts the wrist), flexor carpi ulnaris (flexes and adducts the wrist), palmaris longus (weak wrist flexion), flexor digitorum superficialis (flexes the middle phalanges), flexor digitorum profundus (flexes the distal phalanges), and flexor pollicis longus (flexes the thumb).
- Posterior compartment: extensor carpi radialis longus and brevis (extend and abduct the wrist), extensor carpi ulnaris (extends and adducts the wrist), extensor digitorum (extends the fingers), and thumb muscles — extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.
- Intrinsic hand muscles: the thenar group moves the thumb (including opposition), the hypothenar group moves the little finger, and the interossei and lumbricals produce fine finger movements (flexion at the knuckles and spreading the fingers).
How It Works / Step-by-Step Process
Trace what happens when you reach up to a high shelf:
- Stabilize the scapula: the trapezius and rhomboids retract it; the serratus anterior holds its medial border against the rib cage.
- Lift the arm: the supraspinatus initiates abduction; the middle deltoid then lifts the humerus as the arm rises.
- Bend the elbow: the brachialis and biceps brachii flex the elbow to bring the hand to shelf height.
- Grasp: the finger flexors curl the fingers and the thenar muscles oppose the thumb for a secure grip.
- Lower: the triceps extends the elbow, the latissimus dorsi and teres major pull the arm down, and the inferior trapezius depresses the scapula.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Deltoid as the only shoulder abductor | Supraspinatus | Supraspinatus initiates abduction; the middle deltoid is the prime mover for the rest |
| Biceps as the main elbow flexor | Brachialis | Brachialis is the strongest flexor; biceps adds supination |
| "Rotator cuff" as muscles that only rotate | Stabilizers | The cuff's main job is holding the humeral head in the joint socket |
| Muscle location vs. muscle action | E.g., latissimus dorsi | It lies on the back but extends, adducts, and medially rotates the humerus |
| Wrist/finger flexors vs. extensors | Compartment locations | Flexors sit in the anterior forearm; extensors in the posterior forearm |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your shoulder blade is like a skateboard that can slide around your rib cage. Muscles pull it up, down, forward, and back — then other muscles use it as a base to swing your arm. To reach a cookie jar on a high shelf, your body steadies the shoulder blade, lifts the arm, bends the elbow, and curls your fingers around the jar.
Worked example
A physical therapist watches a client lift a heavy box from the floor to a table and notices the shoulder blade lifts away from the rib cage ("winging") during the push — evidence of a weak serratus anterior, which normally holds the scapula flat against the thorax. The therapist also sees the client keep the forearm pronated and struggle to turn the palm up. Because the biceps brachii is the major supinator as well as an elbow flexor, difficulty supinating a bent elbow suggests a biceps problem, not a pure elbow-flexion problem. Reasoning from muscle actions to observed movements is how anatomy becomes clinical thinking.
Key takeaways
- Scapula first, arm second: pectoral girdle muscles position the scapula; shoulder muscles then use it as a stable base.
- Rotator cuff = SITS: supraspinatus (initiates abduction), infraspinatus and teres minor (lateral rotation), subscapularis (medial rotation) — the dynamic stabilizers of the glenohumeral joint.
- Deltoid abducts, but supraspinatus starts the motion.
- Brachialis, not biceps, is the strongest elbow flexor: the biceps also supinates.
- Compartment rule of thumb: anterior forearm = flexors, posterior forearm = extensors.
- Classify by action, not location: latissimus dorsi lies on the back but moves the humerus.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Name the four rotator cuff muscles and one action of each.
Show answer
Supraspinatus (initiates abduction), infraspinatus (lateral rotation), teres minor (lateral rotation), subscapularis (medial rotation) — "SITS."
Which muscle initiates abduction of the arm, and which is the prime mover once the arm is moving?
Show answer
The supraspinatus initiates abduction; the middle deltoid is the prime mover for the rest of the motion.
Why is the biceps brachii both an elbow flexor and a supinator, and which muscle is the strongest elbow flexor?
Show answer
The biceps inserts on the radial tuberosity, so it supinates while flexing. The brachialis is the strongest pure elbow flexor.
If the medial border of the scapula "wings" away from the rib cage during a push, which muscle is likely weak?
Show answer
The serratus anterior, which holds the medial scapular border against the rib cage.
Where are the wrist/finger flexors and extensors located?
Show answer
Flexors: anterior forearm compartment; extensors: posterior compartment.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- pectoral girdle
- The clavicle and scapula connecting the upper limb to the axial skeleton
- retraction
- Pulling the scapula toward the vertebral column
- abduction
- Moving a body part away from the midline
- adduction
- Moving a body part toward the midline
- rotator cuff
- The four SITS muscles and their tendons around the humeral head
- supination
- Rotating the forearm so the palm faces up
Sources & references
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