Shoulder Joint: Structure, Movements & Stability

• The shoulder joint is highly mobile, which allows movement in all planes.
• The shoulder joint works in conjunction with the shoulder girdle that includes the scapula and clavicle.
• A trade-off exists between mobility and stability, more mobility equals less stability, which presents a higher risk of injury.

Structure & Stability

• A shallow glenoid fossa allows a wide range of motion (ROM).
• The humeral head fitting loosely increases motion, but reduces stability.
• Laxity in loose connective tissue allows extreme mobility, which raises risk of dislocations, subluxations, and rotator cuff impingements.

Shoulder Joint Movements & Pairings

• Shoulder movements are paired with scapular (shoulder girdle) movements.
• Abduction of the shoulder pairs with upward rotation of the scapula.
• Internal/External rotation involves abduction/adduction of the scapula.

Planes of Motion

• Sagittal Plane movements include Flexion/Extension.
• Frontal Plane movements include Abduction/Adduction.
• Horizontal Plane movements include Horizontal abduction/adduction.
• Transverse Plane movements include Internal/External rotation.

Key Bony Landmarks - Scapula

• Glenoid fossa is a shallow socket that accommodates humeral head.
• Acromion process connects to clavicle at the AC joint.
• Coracoid process is an anterior projection (Pec minor attachment).
• Scapular spine is a posterior ridge separating supraspinous/ infraspinous fossae.
• Supraspinous fossa is located above the spine and is the (Supraspinatus origin).
• Infraspinous fossa is below the spine and is the (Infraspinatus origin).
• Subscapular fossa is an anterior depression, and point of (Subscapularis origin).
• Inferior angle is a key landmark for scapular movement.
• Lateral/Medial borders are muscle attachment sites

Key Bony Landmarks - Humerus

• Humeral head articulates with glenoid fossa.
• Greater/Lesser tubercles are muscle attachment sites.
• Intertubercular (Bicipital) groove is the pathway for biceps tendon.
• Deltoid tuberosity is the deltoid muscle insertion.

Key Takeaways

• Shoulder joint movement depends on scapular movement.
• Understanding muscle origins/insertions is crucial for movement analysis.
• High mobility leads to common injuries like dislocations and impingements.

Glenohumeral Joint & Movements - Summary Notes

• Glenohumeral Joint Structure & Stability
• In the glenohumeral joint, the humeral head meets the glenoid fossa.
• Ligaments provide primary stability (anterior & inferior) while allowing flexibility, including;
  • Superior glenohumeral ligament
  • Inferior glenohumeral ligament
  • Coracohumeral ligament
• Glenoid labrum is the cartilage ring enhancing stability.

Tendons

• Surround ligaments and are key for movement, examples include; supraspinatus, infraspinatus, teres minor.
• Scapular tracking is necessary to prevent impingement of tendons under the acromion.

Movements of the Glenohumeral Joint

• Frontal Plane
  • Abduction involves lateral upward movement of the humerus.
  • Abduction requires scapular upward rotation & elevation to avoid impingement.
  • Adduction is characterized by a medial downward movement partnered with scapular downward rotation, and can occur without crossing the midline
• Sagittal Plane
  • Flexion means the humerus moves anteriorly and is paired with scapular upward rotation & elevation.
  • Extension means the humerus moves posteriorly and is paired with scapular downward rotation & depression.
• Horizontal Plane
• Horizontal Adduction means the humerus moves toward and across the chest and involves scapular protraction (abduction).
• Horizontal Abduction means the humerus moves away from the chest.
• Rotational Movements
  • External Rotation means the humerus rotates away from midline and involves scapular retraction.
  • Internal Rotation means the humerus rotates toward midline and involves scapular protraction
• Critical for throwing mechanics.

Combined & Diagonal Movements

• Diagonal abduction/adduction combines of flexion, abduction, or extension, and adduction.
• Break movements into cardinal planes instead of naming all combinations. Circumduction is a mix of flexion, extension, abduction, adduction, & rotation.
• The glenohumeral joint relies on ligaments, tendons, and scapular movement to maintain stability and prevent impingement while allowing a wide range of motion.

Glenohumeral Adduction

• Movement of the humerus toward the midline, typically from an abducted position.
• An example is a pull-up in which the humerus moves from an abducted to an adducted position.
  • Primary Muscles Involved:
    • Latissimus dorsi is a major mover supporting internal rotation and extension.
    • Teres major assists adduction, internal rotation, and extension.
    • Lower pectoralis major helps adduction, even when arm is not fully abducted.

Glenohumeral Extension

• Moving the arm posteriorly in the sagittal plane.
• Examples:
  • Bent-over barbell row means shoulder extension with elbow flexion.
  • Paddling motion describes the shoulder moving from a flexed to an extended position.
• Primary Muscles Involved:
  • Latissimus dorsi
  • Teres major
  • Lower pectoralis major from a flexed position
  • Triceps brachii (long head) assists in shoulder extension.

Glenohumeral Horizontal Abduction

• Moving the arm backward in a horizontal plane.
  • Examples:
    • Rear delt fly is a posterior deltoid-driven movement.
    • Tennis backhand involves horizontal abduction, external rotation, and some flexion. -Primary Muscles Involved:
      • Posterior deltoid is the main agonist.
      • Infraspinatus helps rotate and stabilize the humeral head.
      • Teres minor works similarly to infraspinatus.

Glenohumeral External Rotation

• Rotating the humerus outward from the body, an example is a table tennis backhand stroke.
  • Primary Muscles Involved:
    • Infraspinatus (rotator cuff)
    • Teres minor (rotator cuff)
    • Posterior deltoid (minor role)

Glenohumeral Internal Rotation

• Rotating the humerus inward toward the body includes examples such as
  • Volleyball spike that starts in external rotation, then internally rotates to accelerate the arm.
• Baseball pitch moves from external to internal rotation for ball release.
• Primary Muscles Involved:
  • Latissimus dorsi.
  • Teres major.
  • Pectoralis major
  • Subscapularis (rotator cuff) is a primary internal rotator, located on the anterior scapula.

Rotator Cuff Muscles & Their Roles

• Infraspinatus enables external rotation, horizontal abduction, extension, and stabilization.
• Teres minor enables external rotation, horizontal abduction, extension, and stabilization.
• Subscapularis enables internal rotation, adduction, extension, and stabilization.

Key Takeaways

• Shoulder movements often involve coordinated elbow motion.
• Many muscles contribute to multiple shoulder actions.
• The rotator cuff muscles play a major role in stabilizing the humeral head.
• Overhead athletes often experience rotator cuff injuries due to repetitive stresses.