Shoulder Girdle Kinesiology

Shoulder Girdle Overview

  • The shoulder girdle is a critical structure that supports the upper extremity.

  • It serves as the sole point of attachment for the upper extremity to the axial skeleton, through the scapula and its connection to the clavicle at the sternoclavicular joint.

Bones of the Shoulder Girdle

Key Components

  • Scapula and Clavicle:

    • They function as a unit, allowing coordinated movement.

    • The clavicle articulates with the sternum, forming the only bony link to the axial skeleton.

Key Bony Landmarks

  • Manubrium: The upper part of the sternum, providing attachment to the clavicle.

  • Clavicle: An S-shaped bone that provides structural support.

  • Coracoid Process: A hook-like structure on the scapula providing attachment for ligaments and muscles.

  • Acromion Process: The highest point of the scapula, forming the bony prominence at the shoulder.

  • Glenoid Fossa: The shallow socket in the scapula that receives the head of the humerus.

  • Lateral Border: The edge of the scapula closest to the arm.

  • Medial Border: The edge of the scapula closest to the spinal column.

  • Inferior Angle: The lowest point of the scapula.

  • Superior Border: The edge of the scapula opposite the inferior angle.

  • Superior Angle: The top point of the scapula.

  • Spine of the Scapula: A prominent ridge on the posterior side of the scapula that separates the supraspinous fossa from the infraspinous fossa.

  • Supraspinous Fossa: The depression above the spine of the scapula.

  • Infraspinous Fossa: The depression below the spine of the scapula.

Joints of the Shoulder Girdle

Scapulothoracic Joint

  • The scapula moves along the rib cage and is influenced by movements at the sternoclavicular and acromioclavicular joints.

Sternoclavicular (SC) Joint

  • Classification: Multiaxial, arthrodial joint; also classified as a seller joint.

  • Movement Capabilities:

    • Anterior (15 degrees) during protraction.

    • Posterior (15 degrees) during retraction.

    • Includes slight rotary gliding movements.

Acromioclavicular (AC) Joint

  • Classification: Arthrodial joint.

  • Total Movement: 20 to 30 degrees of gliding and rotational motion during shoulder girdle movements.

  • Support Structures include:

    • Coracoclavicular ligaments (conoid and trapezoid ligaments).

    • Superior acromioclavicular ligament.

    • Inferior acromioclavicular ligament.

  • This joint is commonly injured during falls or intense shoulder activities.

Scapulothoracic Interface

  • Not a true synovial joint and lacks standard synovial features.

  • Movements depend heavily on SC and AC joints which allow:

    • 25 degrees of abduction-adduction.

    • 60 degrees of upward-downward rotation.

    • 55 degrees of elevation-depression.

  • The movement is dynamically supported by muscles but lacks ligamentous support.

Movements of the Shoulder Girdle

Fundamental Principles

  • Shoulder girdle movements correspond directly to scapular movements; when the scapula moves, the clavicle follows.

Types of Movements

Abduction and Protraction

  • Mechanism: The scapula moves laterally away from the spinal column.

Adduction and Retraction

  • Mechanism: The scapula moves medially towards the spinal column.

Downward and Upward Rotation

  • Downward Rotation: The inferior angle of the scapula returns medially and inferiorly towards the spinal column, positioning the glenoid fossa to its standard location.

  • Upward Rotation: The glenoid fossa turns upward and the inferior angle shifts superiorly and laterally away from the spinal column, aiding in arm elevation.

Elevation and Depression

  • Elevation: Upward movement of the scapula, e.g., during shoulder shrugs.

  • Depression: Downward movement of the scapula, returning from elevated positions.

Tilt Movements

  • Lateral Tilt (outward tilt): Occurs during abduction, rotating about the scapula's vertical axis.

  • Medial Tilt (inward tilt): Occurs during extreme adduction, where the medial border moves anteriorly.

  • Anterior Tilt: Rotational movement during glenohumeral hyperextension (superior border moves anteroinferiorly).

  • Posterior Tilt: Rotational movement during glenohumeral hyperflexion (superior border moving posteroinferiorly).

Synergy with Glenohumeral Joint Muscles

  • The shoulder joint and shoulder girdle collaborate to facilitate upper-extremity movements.

  • Necessity of Scapular Movement: Without scapular movement, the humerus's elevation is limited to approximately 90 degrees of shoulder abduction and flexion.

  • Synergistic Action: As the shoulder joint raises the arm laterally, muscles like serratus anterior and trapezius are engaged to upwardly rotate the scapula, enhancing overall arm movement.

Muscles Involved in Shoulder Girdle Movements

  • There are five primary muscles that mediate shoulder girdle movements:

    • None attach directly to the humerus; instead, they stabilise the scapula.

    • These muscles play a significant role in maintaining scapular position and supporting shoulder joint activities (e.g., throwing and batting).

Postural Considerations

  • Scapular muscle imbalances can affect posture, leading to conditions like increased kyphosis and forward shoulder posture.

  • Maintaining lumbar lordosis and adequate head alignment is crucial for balance and functionality in movement and breathing.

List of Key Shoulder Girdle Muscles

  • Trapezius: Upper, middle, and lower fibers with various actions including elevation, adduction, and upward rotation.

  • Rhomboid: Assists in adduction, downward rotation, and slight elevation of the scapula.

  • Levator Scapulae: Elevates the medial margin of the scapula.

  • Serratus Anterior: Key in abduction and upward rotation of the scapula.

  • Pectoralis Minor: Engages in abduction, downward rotation, and depression of the scapula.

Detailed Actions by Muscle

Trapezius Muscle

  • Functions:

    • Upper fibers: Elevation of the scapula and extension of the head.

    • Middle fibers: Elevation, adduction, and upward rotation of the scapula.

    • Lower fibers: Adduction, depression, and upward rotation.

Levator Scapulae Muscle

  • Actions: Primarily elevates the medial margin of the scapula, along with weak adduction and downward rotation.

Rhomboid Muscles

  • Functions:

    • Adduction (retraction) of the scapula toward the spinal column.

    • Downward rotation and slight elevation accompanying adduction.

Serratus Anterior Muscle

  • Functions:

    • Abduction and protraction of the scapula.

    • Upward rotation of the scapula enhancing arm mobility.

Pectoralis Minor Muscle

  • Functions:

    • Abduction and protraction, tilting the lower border of the scapula away from the ribs.

    • Engages in downward rotation and assists in scapular depression.

Specific Movement Actions

Scapular Movements

  1. Abduction: Scapula moves laterally away from the spinous processes; agonists include pectoralis minor and serratus anterior.

  2. Adduction: Occurs through retraction; agonists are middle and lower trapezius, rhomboid muscles.

  3. Upward Rotation: Involves upward movement, supported by middle and lower trapezius and serratus anterior.

  4. Downward Rotation: Mediated by pectoralis minor and rhomboid muscles.

  5. Elevation: Lifting scapula upwards without rotation, agonists include levator scapula, upper trapezius, middle trapezius, and rhomboid.

  6. Depression: Downward movement; agonists include lower trapezius and pectoralis minor.