Exam 2: Muscles of the Shoulder Complex

Pt. 2 of Shoulder Complex lecture

Axioscapular Muscles: Originate on spine, ribs, cranium = Insert on scapula & clavicle

Posterior muscles

• Trapezius (upper, middle, lower)

• Levator scapulae

• Rhomboids

Anterior muscles

• Serratus anterior

• Pectoralis minor

• Subclavius

Axiohumeral Muscles: directly link axial skeleton & humerus

• Anterior: Pectoralis major

• Posterior: Latissimus dorsi *function on the scapula and GH joint vital

Trapezius 🛶

• Purposeful activity: reaching for overhead object (upper/lower trapezius), rowing kayak or canoe (middle trapezius)

• Upper trap

• Middle trap

• Lower trap

• Innervation from: spinal part of CN XI (accessory) + ventral ramus C2-4

Levator Scapulae 🤷

• Elevates & downwardly rotates scapula, laterally flexes head & neck, rotate head & neck to same side

• Muscle assisting scapula mvmt

• Shrug shoulders

• Innervation from: C3-4, dorsal scapular C4-5

Rhomboids 👖

1. Adduct, elevate, downwardly rotate scapula (ST joint)

2. Reaching into back pocket 👖

3. Innervation from: dorsal scapular n. C4-5

Latissimus Dorsi ♿🧍

1. Purposeful activity: W/C mobility (propel forward), standing from sitting (push down on chair arms) ♿🧍

2. Extend, adduct, internally/medially rotate shoulder (GH)

3. Innervation from: Thoracodorsal n. C6-C8

Serratus Anterior 🚪

1. Pushing open heavy door🚪Protractor!

2. W/ origin fixed: abducts, upwardly rotates, and depresses scapula (ST joint). Hold medial border of scapula against rib cage.

3. W/ scapula fixed: elevate thorax during forced inhalation

4. Innervation from: long thoracic n. C5-8

Pectoralis Minor 😤🩼

1. Taking deep breath, walk w/ crutches 😤🩼

2. Depress, abduct, downwardly rotate scapula (ST joint)

3. W/ scapula fixed: assist to elevate thorax in forced inhalation

4. Innervation from: medial pectoral, w/ fibers communicating to branch of lateral pectoral C6-C8, T1

Subclavius: Stabilizer

1. Action: anchors & depresses clavicle

2. Origin: junction of 1st rib & its costal cartilage

3. Insertion: inferior surface of middle third of clavicle

4. Innervation: subclavian nerve (C5,6) = closer to neck

Scapular muscles by fxn: Elevators 🤷

• Primary movers: Upper trapezius & levator scapulae

• Assist: Rhomboids

Scapular muscles by fxn: Protractors ✋🚪(anterior muscles)

Serratus anterior: punching out muscle🥊(pulls scapula in protracted way against ribs)

Scapular muscles by fxn: Upward rotators

Force-couple: 2 equal forces acting in opp. directions to rotate a part about its own axis of mvmt

1. Serratus anterior

2. Upper & lower trapezius

EX: turning steering wheel (1 hand pulls this way, other assists to create upward rotation)

Scapular muscles by fxn: Depressors 🔽 Eccentric mvmt

1. Lower trapezius

2. Latissimus dorsi

3. Pectoralis minor

4. Subclavius

   

Scapular muscles by fxn: Retractors (posterior muscles)

1. Middle & lower trapezius

2. Rhomboids

   

Scapular muscles by fxn: Downward rotators (bring arm back)/pull forcefully down)

Primary Movers:

1. Rhomboids

2. Levator scapulae

3. Pectoralis minor

4. Assist muscle: Latissimus dorsi

Long Thoracic Nerve Palsy, C5-7

Lack of serratus anterior ➡

• Unopposed deltoid action (tries to bring arm up by bringing humerus closer to scapula) ➡

• Scapula paradoxical downward rotation (more deltoid contracts, more scapula goes other way)

Clinical Presentation: Scapular dyskinesis

Loss of control of normal scapular physiology, mechanics, motion

• Not an injury in and of itself; not always directly related to a specific injury!

• Reduces efficiency of shoulder function!

Presentation:

• Prominence of medial or inferomedial border

• Early scapular elevation/shrugging on arm elevation 🤷

• Rapid downward rotation on lowering of arm

Scapula winging (type of scapular dyskinesis, medial border prominence) 🪽

Most common cause: paralysis of serratus anterior due to injury of long thoracic n. (C5-C7)

Scapulohumeral muscles: connect scapula & proximal humerus

1. Deltoid, teres major, coracobrachialis (in front)

2. Rotator cuff muscles (SITS)

   1. Supraspinatus

   2. Infraspinatus

   3. Teres minor

   4. Subscapularis

Coracobrachialis 👕🧺 (down low in front)

• Placing groceries in fridge, carry laundry basket

• Flex & adduct shoulder (GH)

• Innervates from: musculocutaneous n. C6-7

• O: Coracoid process of scapula

• I: Medial surface of midhumeral shaft

Teres major: bottom inferior tip of scapula, wraps around to anterior side to medial side of lesser tubercule of humerus

• Toileting, perineal care 🚽

• Extend, adduct, internally rotate shoulder (GH)

• Innervates from: lower subscapular n. C5-7

Deltoid

• Yoga poses w/ arms overhead (all fibers), reaching for (anterior fibers) & putting on (posterior) seat belt

• Innervates from: Axillary n. C5-6

Rotator Cuff Muscles

• Group of 4 muscles forming protective, dynamic, muscular “cuff” around GH joint

• Key dynamic stabilizers of GH joint

Rotator Cuff: Supraspinatus

• Painting overhead, washing hair 💇‍♀🧼🫧

• Abduct shoulder (GH), stabilize head of humerus in glenoid cavity

• Innervates from: suprascapular n. C4-6

Rotator Cuff: Infraspinatus

• Playing racket sports, table tennis 🎾

• Externally rotate, adduct shoulder (GH) + stabilize head of humerus in glenoid cavity

• Innervates from: suprascapular C4-6

Rotator Cuff: Teres Minor

• Wash back of head & neck

• Externally rotate, adduct shoulder (GH) + stabilize head of humerus in glenoid cavity

• Innervates from: Axillary C5-6

Rotator Cuff: Subscapularis

• Toileting, donning bra, throwing baseball 🚽👙⚾

• Internally rotate shoulder (GH), stabilize head of humerus in glenoid cavity

• Innervates from: upper & lower subscapular C5-7

Rotator Cuff in Glenohumeral (GH) Abduction

1. Supraspinatus: Compressive force on humeral head against the glenoid fossa

2. Infraspinatus, Teres minor ⬅➡Subscapularis: Depressive force on humeral head

Rotator cuff: Force couple

2 or more muscles on opposing side of joints cooperate in harmony, stabilizing joint or facilitating movement

1. Deltoid rotator cuff force couple: largest amt force in shoulder (raise arm, up/out force on humerus)

2. Anterior posterior RC FC

3. Upper + Lower Trapezius & Serratus Anterior RC FC

Clinical corner: Glenohumeral subluxation

Presentation: Paralysis of deltoid & rotator cuff muscles = inability of humeral head to align w/ glenoid fossa against gravity

Key muscles involved:

1. Deltoid

2. Rotator cuff muscles: esp. the supraspinatus muscle

Causes:

1. Stroke or brain injury

2. Brachial plexus injury

Managing shoulder subluxation: Why is it NOT a good idea to position the shoulder in internal rotation for someone w/ shoulder subluxation for long?

External rotators would become weaker with internal rotation contracture if patient is left in an internal rotation position.

• We have 4 ½ internal rotators = develop more tightness

• 2 external rotators = become weaker/lax over time

Clinical corner: Rotator cuff tear

• Prevalence: 25% in people 50+

• Symptoms

  • Pain: majority of tears don’t cause pain, only a 1/3

  • Weakness: depends on size of tear

• Muscles involved:

  • Commonly, supraspinatus

  • Massive tear = 2 or more muscles involved

Rotator cuff tear: A patient is asked to bring the hands to the mouth. He can do it, but only with the elbow in a high position (right arm). The unaffected side can reach the mouth without bringing the elbow high. Why does this occur?

• Pt exhibits weakness of supraspinatus muscle = difficulty initiating shoulder abduction

• Weakness in external rotation is the cause, as this position eliminates the need to externally rotate the shoulder to bring cup to mouth

Rotator cuff tear: A patient isn’t able to lift the hand away from his back. Which rotator cuff muscle is torn?

Subscapularis tear, can’t do internal rotation in this posture

Putting it together: GH + Scapula muscles

Fxnal shoulder motion = primary motion from muscles @ GH joint + assist/stabilization from scapula muscles acting @ ST joint

• Scapulothoracic muscles DON’T move the humerus directly, but are vital for efficient GH motion by positioning & stabilizing scapula

Shoulder abductors: Primary movers 🩰

1. Deltoid

2. Supraspinatus (initiates first ~15*)

Shoulder abductors: Assist

ST joint upward rotators

1. Serratus anterior

2. Upper & lower trapezius

Shoulder abductors: Stabilizers

Rotator cuff muscles

Shoulder flexors: Primary movers

1. Anterior deltoid

2. Clavicular fibers of pectoralis major

Shoulder flexors: Assist

1. Coracobrachialis

2. Long of head of biceps brachii (initial ~30*)

3. Scapulothoracic (ST) joint upward rotators:

   1. Serratus anterior

   2. Upper & lower trapezius

Shoulder flexors: Stabilizers

Rotator cuff muscles

Shoulder extensors: Primary movers

1. Posterior deltoid

2. Latissimus dorsi

3. Teres major

Shoulder extensors: Assist

1. Long head of triceps brachii

2. Sternocostal head of pectoralis major

3. Infraspinatus

4. Teres minor

Shoulder extensors: Stabilizers

Rotator cuff muscles

Shoulder extensor & adductor muscle group has capacity to generate greatest torque compared to other shoulder muscle groups

ST downward rotators (rhomboids, pectoralis minor, levator scapulae) are both assist & stabilizers for shoulder extension & adduction

Shoulder adductors: Primary movers

1. Pectoralis major

2. Latissimus dorsi

3. Teres major

Shoulder adductors: Assist

1. Coracobrachialis

2. Long head of triceps brachii

3. Infraspinatus

4. Teres minor

Shoulder adductors: Stabilizers

Rotator cuff muscles

Shoulder External Rotators: Lowest torque muscle group, small part of the total muscle mass @ shoulder

Important muscle group in shoulder stabilization to balance strong internal rotators

1. Infraspinatus: peaks at 0* abduction

2. Teres minor: peaks at 90* abduction

3. Posterior deltoid

EX: cocking phase of pitching baseball ⚾

Shoulder Internal rotators: able to gen. much greater torque compared to external rotators

1. Subscapularis

2. Anterior deltoid

3. Pectoralis major

4. Latissimus dorsi (in btwn 2 majors)

5. Teres major

Clinical corner: Upper crossed syndrome (Not on test)

• Consistently slouched posture

• Large internal rotators adapt to become tight & short

• Leads to smaller external rotators becoming weak

Common shoulder nerve palsies

1. Suprascapular nerve palsy

2. Axillary n. palsy

3. Long thoracic n. palsy

4. Spinal accessory n. palsy

Suprascapular nerve palsy: what muscles are affected?

Supraspinatus & infraspinatus muscles of the rotator cuff

What muscles are affected in axillary nerve palsy?

Deltoid & teres minor muscles

Spinal accessory nerve palsy

• Trapezius muscle affected = affects upward rotation

• Muscle is diminished compared to other side of shoulder

• Can’t shrug shoulders 🤷❌

Which nerve root(s) is/are likely to be involved in GH subluxation? (p. 78)

C5, 6, 7

What kind of brachial plexus injury would likely result in GH subluxation: upper OR lower trunk? (p. 78)

Upper trunk

Pectoralis Major

• Reaching across body into overhead cabinet, put on seatbelt: shoulder horizontal adduction, internal adduction, flexion of extended arm 🚗

• Clavicular head, sternocostal head, abdominal part

• Innervation from: Lateral pectoral n. C5-7, lateral & medial pectoral n. C6-8, T1

• Accessory muscle to expand ribcage 🫁

• Strengthen: push-ups, chest press, dips

Suprascapular nerve palsy: Which shoulder motion would be affected?

1. Shoulder abduction (supraspinatus): Watch for “shrugging” as a compensatory action when a patient tries initiating

   1. Weakened but NOT completely absent due to deltoid, the main power abductor of shoulder

2. External rotation (Infraspinatus)

In axillary nerve palsy, which shoulder motions are affected?

1. Shoulder abduction due to loss of power abduction (deltoid)

2. External rotation weakness, but action is partly preserved by infraspinatus

Spinal accessory nerve palsy symptoms:

• Much weaker scapula elevation (shrugging)

• Disruption of scapula upward rotation = scapular dyskinesis (scapular winging)

• All shoulder elevation motions (flexion, scaption, abduction) are limited due to lack of ST upward rotation