The Shoulder Joint and Kinesiology

The Shoulder Joint

Overview

  • The shoulder joint is a complex joint that allows a wide range of motion, crucial for upper limb activities.

Anatomical Connections

  • Attachment to the Axial Skeleton: The shoulder joint is primarily connected to the axial skeleton through the scapula and clavicle.
    • Scapular movements typically accompany movements of the humerus.
  • Scapula Movement in Relation to Humerus:
    • Humeral Flexion and Abduction require:
    • Scapula elevation
    • Upward rotation
    • Abduction
    • Humeral Adduction and Extension result in:
    • Scapula depression
    • Downward rotation
    • Adduction
    • Scapula Abduction occurs with:
    • Humeral internal rotation
    • Horizontal adduction
    • Scapula Adduction occurs with:
    • Humeral external rotation
    • Horizontal abduction

Motion Characteristics

  • The shoulder joint exhibits a wide range of motion in multiple planes due to its inherent laxity.
    • This feature is linked to common stability issues; instability problems may lead to:
    • Rotator cuff impingement
    • Subluxations and dislocations
  • Trade-off in Mobility and Stability:
    • Increased mobility often correlates with decreased stability; conversely, increased stability can reduce mobility.

Bone Structure and Major Features

  • Key Bones in the Shoulder Joint:
    • Scapula, Clavicle, Humerus
    • Important landmarks include:
    • Acromion process
    • Coracoid process
    • Greater tubercle
    • Lesser tubercle
    • Intertubercular (bicipital) groove
    • Glenoid cavity (fossa)
  • The scapula serves as the primary articular surface for the shoulder joint, while the deltoid tuberosity on the humerus serves as an attachment for shoulder muscles.

Glenohumeral Joint Specifics

  • Glenoid Labrum:
    • Enhances stability by deepening the fossa.
    • Acts as a buttress against excessive humeral head translation and may sustain injury during sudden overhead movements or trauma.
  • Inferior Glenohumeral Ligament:
    • This ligament is relatively lax until extreme ranges of motion are approached due to the joint's wide range of motion.

Ranges of Motion in Glenohumeral Joint

  • Abduction: 90 to 100 degrees
  • Adduction: 0 degrees or 75 degrees anterior to the trunk
  • Extension: 40 to 60 degrees
  • Flexion: 90 to 100 degrees
  • Internal and External Rotation: 70 to 90 degrees

Horizontal Movement Ranges

  • Horizontal abduction: 45 degrees
  • Horizontal adduction: 135 degrees

Interaction Between Shoulder Girdle and Glenohumeral Joint

  • Total Shoulder Movement:
    • Total abduction (170 to 180 degrees) includes:
    • 60 degrees scapula upward rotation
    • 25 degrees scapula elevation
    • 95 degrees glenohumeral abduction
  • Scapulohumeral Rhythm:
    • There exists a synergistic relationship between the glenohumeral joint and shoulder girdle; generally, this is expressed as a two-to-one ratio, where for every 2 degrees of glenohumeral motion, there is 1 degree of scapular motion.

Glenohumeral Joint Vulnerability

  • Injury Risk Factors:
    • Anatomical design factors contribute to injury susceptibility:
    • Shallowness of glenoid fossa
    • Lax ligaments
    • Muscular weakness or lack of endurance
    • Common injuries include:
    • Anterior or anteroinferior glenohumeral subluxations and dislocations
    • Rare posterior dislocations and common posterior instability problems.

Rotator Cuff Overview

  • Composition: The rotator cuff comprises four primary muscles:
    • Subscapularis
    • Supraspinatus
    • Infraspinatus
    • Teres Minor
  • Functions:
    • These muscles stabilize the humeral head within the glenoid fossa during motions of the humerus, particularly during dynamic activities.
    • Ample overhead activity without proper technique or conditioning can result in rotator cuff issues like tendinitis and impingement.

Movement Descriptions

  • Abduction: Lateral movement of the humerus away from the body.
  • Adduction: Movement of the humerus towards the midline.
  • Flexion and Extension:
    • Flexion: Movement of the humerus straight anteriorly.
    • Extension: Movement of the humerus straight posteriorly.
  • Horizontal Movements:
    • Horizontal Adduction: Humerus moves in a horizontal plane towards the chest.
    • Horizontal Abduction: Humerus moves away from the chest.
  • Rotations:
    • Internal Rotation: Movement medial around the humeral long axis towards the midline.
    • External Rotation: Movement lateral around the humeral long axis away from the midline.
  • Diagonal Movements:
    • Diagonal Abduction: Movement diagonally away from the midline.
    • Diagonal Adduction: Movement diagonally towards the midline.

Muscle Contraction Principles

  • Concentric Contraction: The muscle shortens during contraction, pulling from origin to insertion.

Classification of Muscles**

  • Intrinsic vs. Extrinsic:
    • Intrinsic Glenohumeral Muscles: Originate on the scapula and clavicle (Deltoid, Coracobrachialis, Teres Major, Rotator Cuff).
    • Extrinsic Glenohumeral Muscles: Originate outside the shoulder area (Latissimus Dorsi, Pectoralis Major).

Muscle Functions and Attachments

  • Deltoid Muscle:
    • Actions:
    • Anterior Fibers: Abduction, flexion, horizontal adduction, internal rotation.
    • Posterior Fibers: Abduction, extension, horizontal abduction, external rotation.
  • Pectoralis Major Muscle:
    • Upper Fibers: Internal rotation, horizontal adduction, abduction (with arm at 90 degrees).
    • Lower Fibers: Internal rotation, horizontal adduction, extension from a flexed position.
  • Latissimus Dorsi Muscle:
    • Actions: Adduction, extension, internal rotation, horizontal abduction.
    • Origin: Posterior crest of ilium, sacrum, lumbar vertebrae, lower ribs.
  • Teres Major Muscle:
    • Actions: Extension, internal rotation, adduction.
  • Coracobrachialis Muscle:
    • Actions: Flexion, adduction, horizontal adduction.

Rotator Cuff Muscle Actions

  • Supraspinatus: Abduction and stabilization of the humeral head.
  • Infraspinatus: External rotation, horizontal abduction, and stabilization.
  • Teres Minor: External rotation, horizontal abduction, and stabilization.
  • Subscapularis: Internal rotation, stabilization, and adduction.

Primary Agonists for Glenohumeral Movements

  • Flexion: Anterior Deltoid, Upper Pectoralis Major
  • Extension: Teres Major, Latissimus Dorsi, Lower Pectoralis Major
  • Abduction: Deltoid, Supraspinatus, Upper Pectoralis Major
  • Adduction: Latissimus Dorsi, Teres Major, Lower Pectoralis Major
  • Internal Rotation: Latissimus Dorsi, Teres Major, Subscapularis, Pectoralis Major
  • External Rotation: Infraspinatus, Teres Minor
  • Horizontal Abduction: Posterior Deltoid, Middle Deltoid, Infraspinatus, Teres Minor
  • Horizontal Adduction: Anterior Deltoid, Pectoralis Major, Coracobrachialis
  • Diagonal Adduction: Anterior Deltoid, Coracobrachialis, Biceps Brachii (short head), Pectoralis Major (upper and lower fibers)
  • Diagonal Abduction: Posterior Deltoid, Infraspinatus, Teres Minor, Triceps Brachii (long head)