Shoulder Girdle and Muscle Functions

Shoulder: Clinical Anatomy & Biomechanics

• The shoulder complex comprises multiple joints
• Glenohumeral joint, acromioclavicular joint, sternoclavicular joint, scapulothoracic articulation, and subacromial space are included.

Shoulder Complex

• These joints facilitate a wide range of upper extremity movements
• The glenohumeral joint, acromioclavicular joint, and sternoclavicular joint facilitate specific movements
• The scapulothoracic articulation, which is not a true joint, supports movement.
• The subacromial space is a crucial area for shoulder function.

Anatomical Orientation of the Shoulder Complex

• The scapula's position relative to the humerus is critical for movement.
• Scapular plane inclination, retroversion, and angles are important factors to consider. Values are given for reference.

Stability Functions of Shoulder Girdle

• The shoulder girdle provides a stable base for shoulder muscles to generate force.
• Shoulder girdle muscles, acting as stabilizers, maintain appropriate force-length relationships.
• The shoulder girdle ensures maximum congruence at the shoulder joint.

Mobility Functions of Shoulder Girdle

• The shoulder girdle provides the largest range of motion (ROM) of any complex in the body.
• The scapulohumeral rhythm improves ROM with less compromise to stability (4 joints vs. 1 joint).
• Shoulder girdle movements position the glenohumeral joint (GHJ) optimally for upper extremity movements.

Shoulder Girdle Motion

• Elevation and depression: upward and downward movement of the shoulders.
• Protraction and retraction: forward and backward movement of the shoulders.
• Upward and downward rotation: movement of the scapulae.

Shoulder Joint Muscles

• Pectoralis major: flexion, horizontal adduction, internal rotation of the humerus
• Deltoid: majority of shoulder movements, e.g., internal rotation, abduction, hyperextension, flexion, and horizontal adduction
• Coracobrachialis: forward movements of the humerus
• Subscapularis: internal rotation of the humerus and stabilizes glenohumeral joint
• Biceps brachii: flexion and adduction, horizontal adduction
• Supraspinatus: abduction of the humerus
• Infraspinatus: external rotation, horizontal abduction, stabilizes the humerus during flexion
• Teres minor: external rotation, horizontal abduction, stabilizes the humerus during flexion
• Latissimus dorsi: extension and adduction of the arm.
• Teres major: shoulder extension, adduction, medial rotation
• Triceps brachii: adduction, extension, hyperextension of the humerus

Scapulothoracic Muscles

• Elevators (upper trapezius, levator scapulae, rhomboids): move the scapula upward
• Depressors (lower trapezius, pectoralis minor): move the scapula downward
• Protractors (serratus anterior, pectoralis minor): move the scapula forward
• Retractors (mid & lower trapezius, rhomboids, levator scapulae): move the scapula backward
• Upward rotators (serratus anterior, upper trapezius): rotate the scapula upward
• Downward rotators (rhomboids, pectoralis minor, levator scapulae): rotate the scapula downward

Glenohumeral Joint Muscles

• Abductors (deltoid, supraspinatus): move the humerus away from the body
• Adductors (pectoralis major, latissimus dorsi, teres major): move the humerus towards the body
• Internal rotators (pectoralis major, subscapularis, anterior deltoid, latissimus dorsi, teres major): rotate the humerus inward.
• External rotators (infraspinatus, teres minor, posterior deltoid): rotate the humerus outward
• Flexors (biceps brachii, coracobrachialis, anterior deltoid): bend the arm at the shoulder.
• Extensors (triceps brachii, posterior deltoid, latissimus dorsi, teres major): straighten the arm at the shoulder.

Specific Joints Structure and Function

Sternoclavicular Joint

• Clavicle, sternum, first rib, sternoclavicular ligaments, costoclavicular ligament, articular disc, muscular attachments

• The sternoclavicular joint allows for elevation, depression, protraction, retraction, and axial rotation.

Acromioclavicular Joint

• Gliding or planar joint, articular disc, acromioclavicular ligament, coracoclavicular ligament.

• Upward and downward rotation, horizontal and sagittal plane adjustments.

Coracoclavicular Ligaments

• Conoid ligament and trapezoid ligament are part of the coracoclavicular ligaments
• They maintain the acromioclavicular joint and support the scapula.

Scapulothoracic Articulation

• Not a true joint, providing movement base for the humerus.
• Scapulothoracic motion is the sum of the acromioclavicular and sternoclavicular motions

Primary Movements of the Scapulothoracic Articulation

• Elevation and depression
• Retraction and protraction
• Upward and downward rotation

Scapulothoracic Elevation

• Scapulothoracic elevation is a combination of sternoclavicular joint (SCJ) elevation + downward rotation at the acromioclavicular joint (ACJ).

Scapulothoracic Upward Rotation

• This is a combination of SCJ elevation + upward rotation at the ACJ.

Scapular Rotation

• Enhances glenohumeral stability
• Elevates the acromion to prevent impingement
• Maintains effective length tension relationship of scapulohumeral muscles

Glenohumeral Joint

• Humeral head, Glenoid Fossa, Glenoid Labrum, Rotator Cuff Muscles (SITS), Capsuloligamentous complex, Coracohumeral Ligament, Long Head of Biceps, Subacromial Space

Glenohumeral Ligaments

• Superior glenohumeral ligament (SGHL)
• Middle glenohumeral ligament (MGHL)
• Inferior glenohumeral ligament (IGHL)
• Anterior band
• Posterior band
• Axillary pouch

Coracohumeral Ligament

• Anterior band
• Posterior band

Glenohumeral Joint Motions

• Flexion/Extension (120/45 degrees)
• Abduction/Adduction (120 degrees)
• Internal/External Rotations (80/70 degrees)
• Cannot be isolated from other shoulder girdle elements

Glenohumeral Joint Stability

• Static restraints: bony structures, labrum, capsuloligamentous structure, negative joint pressure
• Dynamic restraints: rotator cuff, biceps, proprioceptive receptors, neuromotor coordination

Kinematics and Biomechanics

Convex Concave Rules

• Convex stationary + concave moving = gliding in the same direction as angular motion
• Concave stationary + convex moving = gliding in the opposite direction of angular motion

Glenohumeral Joint Motions

• Normal arthrokinematics: combines rotation and translation to keep the humeral head centered on the glenoid

Glenohumeral Roll / Glide

• Describes the complex combined movements.

Glenohumeral Joint Motions (Abnormal)

• Abnormal arthrokinematics
• Capsular tightness
• Rotator cuff tears

Subacromial Space

• Clinical relevance: avoiding impingement during arm elevation.
• External rotation of humerus
• Upward rotation of the scapula to elevate acromion

Dynamic Stabilization Mechanisms

• Passive muscle tension
• Compressive forces from muscle contraction
• Joint motion that tightens passive structures
• Redirection of joint force toward center of GH joint
• Force-length relationships are variable due to multiple joints.
• Tension development in agonist frequently requires tension development in antagonist to prevent humeral head dislocation.
• Force couples involve two forces equal in magnitude but opposite in direction, important for maintaining stability

Force Couples Acting on Glenohumeral Joint

• Transverse plane: anterior vs. posterior rotator cuff muscles
• Coronal plane: deltoid vs. inferior rotator cuff

Force Couples Disruption

• Diagrams illustrate disruption of force couples (abnormal movements).

Rotator Cuff Muscles

• Supraspinatus, Infraspinatus, Teres Minor, Subscapularis

Rotator Cuff – Dynamic GH Stabilizer

• Illustrates how the rotator cuff muscles work together to maintain dynamic stability.

Rotator Cuff Function

• Approximates humerus to function
• Supraspinatus helps the deltoid with abduction
• Subscapularis, infraspinatus and teres minor depress humeral head

Scapulohumeral Rhythm

• Movement relationship between the humerus and scapula during arm raising movements.

• Ratio of 2:1 - for every 3 degrees of shoulder abduction, 2 degrees occur at GH joint and 1 at ST joint (SC & AC joint)

• Preserves length-tension relationships of the glenohumeral muscles

• Sustains force production through a larger portion of the range of motion

• Prevents impingement between the humerus and the acromion.

• Movement limits relative movement between the two bones

Scapulohumeral Rhythm Clinical Implications

• Rotator cuff pain early in abduction
• Impingement pain mid-abduction
• AC pain late in abduction

Effects of Poor Scapular Stabilization

• Poor resting scapular posture (protracted)
• Loss of upward tilt of scapula
• Loss of GH joint stability.
• Inferior humeral translation and internal rotation
• Rotator cuff ischemia, supraspinatus in particular
• Capsular laxity, poor dynamic stabilization

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