Chapter 4: Muscles of the Shoulder Girdle

Overview

• Five primary muscles involved in shoulder girdle movements.
• Origin: Axial skeleton.
• Insertion: Scapula and/or clavicle (not the humerus).
• Function: Provide dynamic stability for shoulder joint activity.
• Importance in activities: Throwing, batting, blocking.

Importance of Scapular Muscles

• Posture:
  • Forward shoulders due to stronger/tighter protractors and depressors.
  • Weaker retractors contribute to kyphosis.
  • Maintaining lumbar lordosis and balanced head position enhances respiration.
• Scapular Winging:
  • Rare condition affecting upper extremity function.
  • Medial winging often due to serratus anterior weakness, typically from long thoracic nerve injury.
  • Lateral winging less common; may involve trapezius or rhomboid.

Muscle Actions

• Elevation:
  • Muscles: Upper and middle trapezius, levator scapulae.
• Retraction (Adduction):
  • Muscles: Rhomboids, middle and lower trapezius.
• Protraction (Abduction):
  • Muscles: Serratus anterior, pectoralis minor, lower trapezius.
• Upward Rotation:
  • Muscles: Trapezius, serratus anterior.
• Downward Rotation:
  • Muscles: Rhomboids, pectoralis minor.
• Depression:
  • Muscles: Lower trapezius, pectoralis minor.

Key Muscles

• Serratus Anterior:
  • Actions: Protraction, upward rotation.
  • Importance: Key in scapular dynamics; known as "Bruce Lee muscle".
• Pectoralis Minor:
  • Actions: Abduction, downward rotation, depression.
• Trapezius:
  • Upper fibers: Elevation, head extension.
  • Middle fibers: Elevation, abduction, upward rotation.
  • Lower fibers: Adduction, depression, upward rotation.
• Rhomboids:
  • Actions: Adduction, downward rotation, elevation.
• Levator Scapulae:
  • Actions: Elevation, weak downward rotation.

Nerve Innervation

• Muscles innervated by cervical and brachial plexus.
• Key nerves:
  • Dorsal Scapular Nerve: Innervates levator scapulae, rhomboids.
  • Long Thoracic Nerve: Innervates serratus anterior.
  • Medial Pectoral Nerve: Innervates pectoralis minor.

Clinical Application

• Understanding muscle actions helps in diagnosing and treating shoulder issues.
• Focus on strengthening weak muscles and stretching tight muscles for balance.

Exercises for Muscle Activation

• Recommended exercises for specific muscles:
  • Upper Trapezius: Rack pulls, barbell shrugs.
  • Middle Trapezius: Prone reverse flies.
  • Lower Trapezius: Prone Y's.
  • Rhomboids: Rows, pull-ups.

Resources for Further Study

• Websites and textbooks for diagnosing and treating shoulder injuries: Wheelist textbook of orthopedics, Baseball Almanac.

This summary provides an outline of the shoulder girdle muscles, their functions, nerve innervations, and clinical relevance to movement and injury prevention. Focus on understanding the muscle actions and recommended exercises for effective learning and application.