Kinesiology The Shoulder and Shoulder Girdle

Introduction

The shoulder and shoulder girdle form one of the most complex and mobile regions of the human body. They allow a vast range of movements essential for various activities, including lifting, pushing, pulling, and rotation. The shoulder joint is primarily responsible for mobility, while the shoulder girdle provides support and stability.

Bones of the Shoulder Girdle

The shoulder girdle consists of three primary bones that serve as attachment points for muscles and contribute to the movement of the upper limb.

The Clavicle (Collarbone)

  • A long, S-shaped bone that connects the sternum to the scapula.
  • Functions as a structural support, keeping the shoulder joint away from the thorax to allow full arm mobility.
  • Helps absorb forces transmitted from the upper limb to the axial skeleton.

The Scapula (Shoulder Blade)

  • A flat, triangular bone that moves freely along the posterior thoracic wall.
  • Provides attachment points for various muscles responsible for shoulder movement.
  • Contains important anatomical structures, including:
    • Acromion: A bony projection that connects with the clavicle.
    • Glenoid Cavity: The socket for the humeral head, forming the shoulder joint.
    • Spine of the Scapula: A ridge that serves as a landmark for muscle attachments.

The Humerus

  • The long bone of the upper arm, which articulates with the scapula at the glenohumeral joint.
  • Provides structural support and allows a wide range of arm movements.

Joints of the Shoulder Girdle

The shoulder girdle consists of multiple joints that work together to enable mobility and stability.

Glenohumeral Joint

  • A ball-and-socket synovial joint between the humeral head and the glenoid cavity of the scapula.
  • Allows the greatest range of motion in the human body but is also the least stable.

Subacromial-Deltoid Joint

  • A functional space rather than a true joint.
  • Lies between the head of the humerus and the acromion, facilitating smooth movement of the shoulder.

Acromioclavicular Joint

  • A plane synovial joint between the acromion of the scapula and the clavicle.
  • Stabilized by ligaments, allowing slight gliding movements.

Sternoclavicular Joint

  • A saddle-type synovial joint connecting the clavicle to the sternum.
  • The only bony connection between the upper limb and the axial skeleton.

Scapulothoracic Joint

  • A functional joint between the scapula and the thoracic rib cage.
  • Facilitates scapular movements necessary for full arm motion.

Coracoclavicular Joint

  • A syndesmotic joint (fibrous connection) between the coracoid process of the scapula and the clavicle.
  • Provides additional stability to the shoulder girdle.

Movements of the Glenohumeral Joint

The glenohumeral joint allows a wide range of movements due to its ball-and-socket structure.

Flexion

  • Forward movement of the arm in the sagittal plane.
  • Example: Raising the arm in front of the body.

Extension and Hyperextension

  • Backward movement of the arm beyond the anatomical position.
  • Example: Swinging the arm behind the torso.

Abduction

  • Movement of the arm away from the body in the frontal plane.
  • Example: Raising the arm sideways.

Adduction

  • Bringing the arm back toward the body’s midline.
  • Example: Lowering the raised arm.

Internal (Medial) Rotation

  • Rotation of the humerus inward toward the body.
  • Example: Turning the palm inward.

External (Lateral) Rotation

  • Rotation of the humerus outward, away from the body.
  • Example: Turning the palm outward.

Horizontal Abduction

  • Moving the arm away from the body in the horizontal plane.
  • Example: Extending the arm sideways while at shoulder height.

Horizontal Adduction

  • Bringing the arm across the chest in the horizontal plane.
  • Example: Hugging motion.

Circumduction

  • A combination of flexion, extension, abduction, and adduction.
  • Creates a circular movement.

Muscles of the Shoulder

The muscles of the shoulder contribute to movement and stabilization.

Muscles of the Anterior Shoulder

  • Pectoralis Major: Responsible for flexion, adduction, and medial rotation of the arm.
  • Pectoralis Minor: Assists in scapular depression and downward rotation.
  • Serratus Anterior: A key muscle for protracting and stabilizing the scapula.

Muscles of the Posterior Shoulder

  • Latissimus Dorsi: Facilitates extension, adduction, and medial rotation.
  • Teres Major: Assists in adduction and internal rotation.
  • Deltoid (Middle and Posterior Fibers): Controls abduction and extension.

Muscles of the Upper Shoulder

  • Deltoid (Anterior Fibers): Involved in flexion and medial rotation.
  • Supraspinatus: Initiates arm abduction.

Muscles of the Lower Shoulder

  • Infraspinatus: Assists in lateral rotation.
  • Teres Minor: Supports external rotation and stabilization.

Movements of the Scapula

The scapula plays a crucial role in shoulder mobility by adjusting its position.

Elevation

  • Upward movement of the scapula.
  • Example: Shoulder shrug.

Depression

  • Downward movement of the scapula.
  • Example: Lowering the shoulders after a shrug.

Upward Rotation

  • Scapula rotates upward and outward.
  • Example: Raising the arm overhead.

Downward Rotation

  • Scapula moves downward and inward.
  • Example: Returning from an overhead position.

Protraction (Abduction)

  • Moving the scapula forward along the rib cage.
  • Example: Pushing forward.

Retraction (Adduction)

  • Pulling the scapulae closer together.
  • Example: Squeezing the shoulder blades.

Muscles of the Scapula

These muscles control the movements of the scapula.

Trapezius

  • Upper fibers elevate the scapula.
  • Middle fibers retract the scapula.
  • Lower fibers depress the scapula.

Levator Scapulae

  • Elevates the scapula and assists in downward rotation.

Rhomboids (Major and Minor)

  • Retract and stabilize the scapula.

Serratus Anterior

  • Protracts the scapula and prevents winging.

The Scapulohumeral Rhythm

  • Refers to the coordinated movement of the scapula and humerus during arm motion.
  • Essential for achieving a full range of motion in overhead movements.
  • Typically follows a 2:1 ratio (for every 2 degrees of humeral movement, the scapula moves 1 degree).

Conclusion

The shoulder and shoulder girdle form a highly complex system that enables versatile movements. Understanding the bones, joints, muscles, and movement mechanics of the shoulder is crucial for improving performance, preventing injuries, and enhancing rehabilitation strategies.

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