Shoulder Muscle and Joint Interaction Flashcards
2 functional muscle categories of the shoulder complex
Proximal stabilizers
Distal mobilizers
Where do proximal stabilizers originate and insert?
On the spine, ribs, and cranium and insert on the scapula and clavicle
Examples of proximal shoulder stabilizers
Serratus anterior and trapezius for example
Where do distal stabilizers originate and insert?
Originate on the scapula and clavicle and insert on the humerus or forearm
Examples of distal shoulder stabilizers
Deltoid and biceps brachii
In order for the shoulder complex to function optimally, what must occur?
There must be an interaction between the proximal stabilizers and distal mobilizers
Give an example of how the proximal and distal stabilizers interact to increase shoulder function
In order for the deltoid to generate effective abduction torque at the GH joint, the scapula must be firmly stabilized against the thorax by the serratus anterior and trapezius muscles. If the serratus anterior is paralyzed the deltoid muscle is unable to express its full abduction function
How are muscles of the ST joint categorized? (examples)
According to their actions
- elevators or depressors
- protractors or retractors
- upward and downward rotators
3 primary elevators at the ST joint
- Upper trapezius
- Levator scapulae
- Rhomboids
4 primary depressors at the ST joint
- Lower trapezius
- Latissimus dorsi
- Pectoralis minor
- Subclavius
Primary protractor at the ST joint
- serratus anterior
3 primary retractors at the ST joint
- Middle trapezius
- Rhomboids
- Lower trapezius
2 primary upward rotators of the ST joint
- Serratus anterior
- Upper and lower trapezius
2 primary downward rotators of the ST joint
- Rhomboids
- Pectoralis minor
Functionally, what do the shoulder elevators do?
Support posture of shoulder girdle (scapula and clavicle) and upper extremity
ideal posture of shoulder girdle incorporates slightly _____ and relatively _____ scapula, with glenoid fossa facing slightly ____.
elevated
retracted
upward
Long-term paralysis of the upper trapezius may lead what pathology at the GH joint and why?
inferior dislocation (or subluxation) of GH joint
The glenoid fossa loses its upwardly rotated position which allows the humerus to slide inferiorly. This coupled with the downward pull of gravity may strain the GH capsular ligaments leading to irreversible dislocation. Someone with flaccid hemiplegia may need a sling
People with functional impairments at the ST joint may lead to what?
Scapulae slightly depressed, downwardly rotated, and protracted
While the lower trapezius and pec minor act directly on the scapula, the latissimus dorsi depresses the shoulder girdle indirectly how?
It pulls the humerus inferiorly
Why is the serratus anterior the prime protractor at the STJ?
It has excellent leverage for protraction around the sternoclavicular joint’s vertical axis of rotation
Persons with serratus anterior weakness have difficulty doing what?
Performing forward pushing motions
Describe which muscles are at work during a standard prone push up
The early phase of the push-up is performed primarily by triceps and pectoral musculature. But, after the elbows are completely extended, the chest can be raised farther from the floor by deliberate protraction of both scapulas (serratus anterior contraction)
The serratus anterior is primarily responsible for ST protraction, however it also assists with ST depression, why?
The SA fibers attaching near the inferior angle may assist with scapulothoracic depression
The strength of the protraction force is primarily the result of what?
Muscle force multiplied by the internal moment arm originating at vertical axis of rotation at the SC joint
Of the 3 primary retractors of the ST joint, which one produces the most retracting force and why?
The middle trapezius has an optimal line of force to retract the scapula.
In what direction is the line of force for the rhomboids and lower trapezius (secondary retractors)
The rhomboids elevate, whereas the lower trapezius depresses, but when they work together they yield a single retraction force
The retractor muscles are active during what type of activities?
pulling activities such as climbing and rowing
What occurs if there is complete paralysis of retractors (primarily the trapezius)?
There is a significant reduction in the retraction potential of the scapula which results in the scapula tending to “drift” slightly into protraction as result of partially unopposed protraction action of serratus anterior muscle
Elevation of the arm is performed by what 3 groups of muscles?
1) muscles that elevate (i.e., abduct or flex) humerus at GH joint
2) scapular muscles that control upward rotation of scapulothoracic joint
3) rotator cuff muscles that control dynamic stability and arthrokinematics at GH joint
4 GH joint muscles that are primarily responsible for arm elevation
- Anterior and middle deltoid
- Supraspinatus
- Coracobrachialis
- Biceps (long head)
- First 2 abduct
- anterior deltoid and last 2 flex
During flexion, where does the internal moment arm intersect?
The line of force of the anterior deltoid
When do the middle deltoid and supraspinatus muscles reach their maximum level during elevation?
near 90° of abduction
Is abduction possible with deltoid paralysis?
Yes, the supraspinatus is capable of fully abducting the GH joint, although abduction torque reduced
Is abduction possible with supraspinatus paralysis or tendon ruputre?
No, because there is altered arthrokinematics at GH joint
Is abduction possible with paralysis of both deltoid and supraspinatus?
No
2 ST joint muscles that are primarily responsible for arm elevation
- Serratus anterior
- Trapezius
4 RC muscles that are primarily responsible for arm elevation
All 4
- Supraspinatus
- Infraspinatus
- Teres minor
- Subscapularis
Describe how the rotator cuff muscles abduct the GH joint
The upper fibers of these muscles pass slightly superior to the joint’s AP axis of rotation
Although the RC muscles are considered only secondary abductors, they play primary role in establishing what?
dynamic stabilization and directing the joint’s arthrokinematics
Of the 2 upward rotators of the GH joint, which one provides most of the force?
Serratus anterior
What are the 2 secondary actions of the serratus anterior?
- The lower fibers of the serratus anterior upwardly rotates the scapula that may help to tilt the glenoid fossa posteriorly
- Externally rotates the scapula which helps to secure the medial border firmly against the thorax
During shoulder abduction, when are the serratus anterior, upper trapezius, and lower trapezius active?
- Serratus anterior is pretty constant throughout
- Upper trap is active early
- Lower Trap is active late
How do the upper and lower trapezius upwardly rotate the scapula?
The upper trapezius upwardly rotates the scapula indirectly by its superior-and-medial pull on clavicle
The lower trapezius upwardly rotates the scapula by its inferior-and-medial pull on the root of the spine of scapula
Which of the 3 portions of the trapezius muscle is most active during shoulder abduction?
Middle trapezius
What determines the final retraction-protraction position of upward rotated scapula?
Net force dominance between the middle trapezius and serratus anterior during elevation of the arm
Between the serratus anterior and the trapezius, which has greater leverage for arm elevation?
Serratus anterior has greater leverage for this motion
What will occur if there is complete paralysis of the trapezius?
They will have moderate to marked difficulty in elevating arm overhead, but they should still be able to accomplish full ROM as long as the serratus anterior Is fully innervated
Elevation of arm in pure ____ plane is particularly difficult with trapezius paralysis, why?
Frontal
Because this action requires the middle trapezius to generate strong retraction force on scapula
Paralysis of the serratus anterior is due to what nerve injury?
long thoracic nerve
Why do people with complete paralysis of the serratus anterior have such difficulty actively elevating the arm even though the middle deltoid and supraspinatus are fully innervated?
Normal contraction of the serratus anterior upwardly rotates the scapula, which allows the middle deltoid and supraspinatus to rotate the humerus upward. When the SA is paralyzed, the middle deltoid and supraspinatus downwardly rotate the scapula
The scapula is ___ tilted and _____ rotated with SA paralysis
anteriorly
internally
aka scapular “winging”
Scapular winging may also lead to what?
Shortening of the pectoralis minor muscle which will further promote anterior tilt and internal rotation of the scapula
What is the primary function of the rotator cuff muscles?
Regulators of dynamic joint stability and controllers of arthrokinematics
Anatomic design of GH joint favors _____ at expense of _____.
mobility
stability
The supraspinatus, infraspinatus, and teres minor blend into and support what aspects of the GH joint before attaching to the humerus?
superior and posterior aspects
The subscapularis muscle blends into and supports what aspect of the GH joint before attaching to the lesser tubercle of the humerus?
anterior
Which RC muscle forces the humeral head into the glenoid fossa? Why is this important?
The supraspinatus muscle force stabilizes the humeral head firmly against fossa during its superior roll into abduction
The supraspinatus is ideal for directing arthrokinematics of what motion?
abduction
The subscapularis, infraspinatus, and teres minor muscles have line of force that exert an ____ directed force on humeral head during abduction. What other muscle helps to do this?
inferior
Long head of biceps
Supraspinatus rolls humeral head _____ toward abduction while also compressing joint.
superiorly
Which two muscles also exert an inferior force on the humeral head during abduction?
latissimus dorsi and teres major exert passive forces from being stretched
What is the importance of the inferior slide of the humeral head in the glenoid fossa?
Without active and passive inferior-directed forces the humeral head would jam or impinge against the coracoacromial arch, thereby blocking abduction
Which muscles externally rotate the humerus to increase the clearance between the greater tubercle and acromion.
infraspinatus and teres minor muscles
3 functions of the supraspinatus
- Drives superior roll of humeral head
- Compresses humeral head firmly against glenoid fossa
- Creates semi-rigid spacer above humeral head, restricting excessive superior translation of humerus
Function of the Subscapularis
Exerts a depression force on the humeral head
2 Functions of the Infraspinatus and Teres Minor
- Exerts a depression force on the humeral head
- Externally rotate humerus
Muscles That Adduct & Extend Shoulder (which have the longest moment arms?)
- posterior deltoid
- latissimus dors
- teres major
- long head of triceps brachii
- sternocostal head of pectoralis major
Latissimus dorsi, teres major, and pectoralis major have largest moment arms
Which RC muscles also assist with adduction & extension?
Infraspinatus (lower fibers) and teres minor muscles
Which group of muscles are capable of generating the largest torques of any shoulder muscle group?
Extensor and adductor muscles
Which muscles assist the rhomboids with downward rotation of the scapula?
pectoralis minor and latissimus dorsi
How many adductor-extensor muscles have their primary proximal attachments on the scapula?
5 of 7
What is the primary responsibility of the rhomboid muscles during active adduction and extension of GH joint?
to stabilize the scapula (downwardly rotate and retract it)
Shoulder extension is increased by ____ tilting the scapula. This involves the activation of what muscle?
anterior
pec minor
5 muscles that internally rotate the GH joint
- subscapularis
- anterior deltoid
- pectoralis major
- latissimus dorsi
- teres major
Many of the internal rotators are also powerful at what other 2 motions?
extensors and adductors
Is there more internal or external muscle mass?
internal rotators greatly exceeds that of external rotators
Muscles that internally rotate the GH joint are often described as rotators of humerus relative to what?
relative to the scapula
3 muscles that externally rotate GH joint
- infraspinatus
- teres minor
- posterior deltoid
Do the external rotator muscles constitute a large or small percentage of total shoulder mass?
Small
____ rotators produce lowest maximal-effort torque of any muscle group at shoulder
External
Why are the shoulder external rotators at risk for tear?
They must decelerate shoulder internal rotation at release phase of pitching, which can reach velocity of near 7000 °/sec
Which shoulder pathology is the most common?
Subacromial impingement syndrome
What structures are compressed between to humeral head and coracoacromial arch in impingement syndrome?
- supraspinatus tendon
- tendon of long head of biceps
- superior capsule
- subacromial bursa
Which area of the shoulder is painful with Subacromial ImpingementSyndrome?
anterior shoulder region
Subacromial ImpingementSyndrome is usually aggravated by active abduction of __° to ___°.
60 - 120
Subacromial ImpingementSyndrome in what type of person?
Most common in athletes and laborers who repeatedly abduct their shoulders over 90°, but can also occur in relatively sedentary persons
5 Causes of Impingement Syndrome
1) Abnormal kinematics at GH and scapulothoracic joints (STJs)
2) “Slouched” posture affecting alignment of STJ
3) Fatigue, weakness, poor control, or tightness of muscles that control GH or STJ motions
4) Inflammation and swelling of tissues within and around subacromial space
5) Excessive wear and subsequent degeneration of tendons of rotator cuff muscles
