Shoulder Anatomy and Movement Quiz

Shoulder Complex - Chapter 5

• This chapter covers the anatomy and biomechanics of the shoulder complex, including the shoulder girdle, glenohumeral joint, acromioclavicular joint, and sternoclavicular joint.
• The clavicle's long axis is positioned approximately 20 degrees posterior to the frontal plane.
• The humeral head is situated approximately 30 degrees posteriorly relative to the medial-lateral axis through the elbow.
• The scapula's plane of motion is called scaption, which combines abduction and flexion.
• The sternoclavicular joint is a base joint (saddle, plane, modified ball and socket) that secures the clavicle, scapula, and upper extremity to the axial skeleton. It is characterized as having a high level of stability.
• The sternoclavicular joint has an articular disc that divides the joint into medical and lateral cavities, which strengthens the articulation. Its main function is to absorb shock. The ligaments support the joint, notably the anterior and posterior sternoclavicular ligaments, and the interclavicular and costoclavicular ligaments. Stabilizing muscles include the sternocleidomastoid (anteriorly) sternothyroid (posteriorly), sternohyoid (posteriorly), and subclavius (inferiorly) muscles.
• The degrees of freedom for the sternoclavicular joint is 3.
• The acromioclavicular joint is a plane (gliding) joint with 3 degrees of freedom. It is composed of the clavicle and scapula.
• The acromioclavicular joint's capsule is reinforced by superior and inferior acromioclavicular ligaments and the coracoclavicular ligament and an articular disc.
• The muscles that stabilize the acromioclavicular joint include the deltoid, and trapezius muscles.

Primary Movements of the Scapulothoracic Joint

• The scapula moves in elevation/depression, protraction/retraction, and upward/downward rotation.

Scapulothoracic Joint

• It is not a true joint; it is the articulation between the anterior surface of the scapula and the posterior-lateral thorax; important for shoulder function.
• The scapula is situated on ribs 2-7. The medial border of the scapula is approximately 6cm away from the spine.
• The scapular muscles have a crucial role in shock absorption.
• The stabilizing muscles of the scapulothoracic joint are the rhomboids, trapezius, and serratus anterior.

Glenohumeral Joint

• The Glenohumeral joint is a ball-and-socket type joint.
• The glenoid fossa is positioned anterior-laterally and slightly upward by about 4 degrees.
• The joint's capsule is made up of various ligaments that play a substantial role in maintaining structural stability.
• The rotator cuff muscles (Supraspinatus, Infraspinatus, Teres minor, and subscapularis) provide dynamic stability during arm movements and control the arthrokinematics at the Glenohumeral joint.

GH Joint Kinematics

• The GH joint has 3 degrees of freedom (abduction, flexion/extension, internal/external rotation).
• Abduction: The convex head of the humerus rolls superiorly and slides inferiorly.
• Adduction: The head returns to neutral.
• Flexion/Extension: The humeral head spins in the glenoid fossa.
• Internal/External Rotation: Humeral head rolls, relative to the concave glenoid fossa.

Coracoacromial Arch

• The coracoacromial arch is formed by the coracoacromial ligament and the acromion process.
• This area is important in limiting movement, and is a significant site for potential impingement in the shoulder.

Overall Kinematics: Shoulder Abduction

• Principles 1-6 describe the overall biomechanics of shoulder abduction.
• Principle 1: 3:2:1 relationship of rotation between the shoulder, glenohumeral, and scapulothoracic joints.
• Principle 2: A 60-degree combined range of motion occurs.
• Principle 3: Clavicular retraction assists the acromioclavicular joint with optimal positioning within the frontal plane.
• Principle 4: Scapular rotation adjustments (posterior tilt and external rotation) occur at the acromioclavicular joint and the sternoclavicular joints.
• Principle 5: Clavicular elevation is halted by a strong coracoclavicular ligament, while the scapular upward rotators continue to drive the scapulothoracic upward motion.
• Principle 6: GH joint external rotation (25-50 degrees) occurs primarily prior to shoulder abduction (70-80 degrees).

Scapulo-Humeral Rhythm

• Superior roll and compression of the humeral head, scapula upward rotation, and humeral ER are all important for smooth shoulder abduction motions.
• The deltoid and rotator cuff muscles are essential for shoulder abduction motions.

Muscle Interactions During Shoulder Abduction

• Sequential activation of upper and lower trapezius, serratus anterior, supraspinatus, deltoid, infraspinatus, and teres minor muscles during shoulder abduction. This is a simultaneous interaction of all the supporting muscles.

Muscle and Joint Interaction

• Proximal stabilizers act on the spine, ribs, and cranium, while distal stabilizers act on the scapula, clavicle, and humerus (or forearm).

Stabilizing Muscles

• Each stabilizing muscle acts to elevate, depress, retract, or protract the shoulder components to prevent injury and facilitate smooth movement. This includes muscle groups like the upper trapezius, levator scapulae, rhomboids, lower trapezius, latissimus dorsi, pectoralis minor, and subclavius, and to assist with a range of motions of the glenohumeral joint.

Serratus Anterior Weakness

• Weakness in the serratus anterior muscle can cause the scapula to wing out, affecting shoulder stability and function.

Rotator Cuff Muscles

• These muscles (infraspinatus, teres minor, supraspinatus, subscapularis) provide dynamic stability to the Glenohumeral joint.
• They're crucial to achieve active control in maintaining proper arthrokinematics during shoulder movements (abduction, external / internal rotation).

Glenoid Labrum Tears, Shoulder Instability

• Glenoid labrum tears can result from shoulder instability, often associated with repetitive motions, injuries, or an acute event. Pain and other symptoms often result.
• Shoulder instability encompasses various disruptions in the structural stability of the shoulder, which can manifest as looseness or recurring subluxation or dislocations. This condition may also be caused by poor posture.

Test your knowledge on the anatomy and biomechanics of the shoulder complex. This quiz covers muscle functions, movement patterns, and joint mechanics related to shoulder movement. Perfect for anatomy students and fitness professionals.