Joint Biomechanics - Shoulder

Question 1

Shoulder Complex

Answer 1

• Glenohumeral (GH)
• Sternoclavicular (SC)
• Acromioclavicular (AC)
• Scapula thoracic (ScTh)
  Clavicle only fixed articulation acting as a lever maintain distance between sternum and scapula.

Question 2

Scapular Plane

Answer 2

Approximately 30° anterior to frontal plane.

Abduction is in 30° anterior to frontal plane based on the anatomical position of the scapula.

Question 3

Static Stabilizers of the Shoulder

Answer 3

• Glenoid labrum: The fossa is half of the surface area of the head. Labrum increases the surface area of the fossa by 50% and provides negative pressure.
• Coracohumeral ligament: maintains the head in the fossa.
• Glenohumeral ligament: anterior, inferior, superior.
• Joint capsule: Reinforcement
  Prevent excessive translation of the humeral head in the glenoid fossa.

Question 4

Dynamic Stabilizers of the Shoulder

Answer 4

Rotator cuff muscles.
Stabilization of the humeral head, preventing anterior translation, and counteracting deltoid action.
Stability of the shoulder depends on how the head is centered in the fossa. Muscles contract and “move” the head anteriorly or posteriorly in the fossa.
Based on vector forces, supraspinatus is the most stabilizing in resting position.

Question 5

Synergists of Elevation of the Scapula

Answer 5

Upper Trapezius
Lavator Scapulae
Rhomboid major and minor

Question 6

Synergists of Depression of the Scapula

Answer 6

Pectoralis minor
Serratus anterior
Lower trapezius

Question 7

Synergists of Scapula Protraction

Answer 7

Serratus anterior

Pectoralis minor

Question 8

Synergists of Scapula Retraction

Answer 8

Middle trapezius

Rhomboids major and minor

Question 9

Synergists of Scapula Upwards Rotation

Answer 9

Upper and lower trapezius

Serratus anterior

Question 10

Synergists of Scapula Downwards Rotation

Answer 10

Rhomboids major and minor
Levator scapulae
Pectoralis minor

Question 11

Kinematic Relationships at the Glenohumeral Joint - Abduction/Adduction

Answer 11

Plane: Scapular plane
Axis: antero-posterior
Roll and slide along joint’s longitudinal diameter - Opposite direction

Question 12

Kinematic Relationships at the Glenohumeral Joint - Internal/External Rotation

Answer 12

Plane: Horizontal
Axis: Longitudinal
Roll and slide along joint’s transverse diameter - Opposite direction.

Question 13

Kinematic Relationships at the Glenohumeral Joint - Flexion/Extension, Internal/External Rotation in 90° Abduction

Answer 13

Plane: Near sagittal
Axis: Near medial-lateral
Primarily a spin between humeral head and glenoid fossa.

Question 14

Arthrokinematics of Shoulder Abduction

Answer 14

Convex on concave: during shoulder abduction, the head of the humerus rolls superiorly and slides inferiorly on the glenoid. Supraspinatus is majorly responsible for the slide.
Capsular tension helps to stabilize the humerus.

Question 15

1st Principal of Shoulder Arthrokinematics (Abduction)

Answer 15

Scapula humeral rhythm:
In average, in healthy people, there is a 2:1 ratio between shoulder elevation and scapula and scapula upwards rotation. 2 degrees glenohumeral against 1 degree scapula thoracic.

Question 16

2nd Principal of Shoulder Arthrokinematics (Abduction)

Answer 16

What dictates the 60° scapula upward rotation are the combined kinematics at the Sternoclavicular and Acromioclavicular joints:

1. Simultaneous elevation of the clavicle at the SC joint combined with upward rotation of the scapula at the AC joint. (permits the upward rotation of the scapula).
2. Clavicle retracts at the SC joint during full shoulder abduction.
3. Clavicle rotates posteriorly around it’s own long axis.

Question 17

3rd Principal of Shoulder Arthrokinematics (Abduction)

Answer 17

During shoulder abduction, there is a scapula posterior tilt, and a slight external rotation.

• Proper orientation of the glenoid to the humeral head
• Distance between the acromion and the humeral head.

Question 18

4th Principal of Shoulder Arthrokinematics (Abduction)

Answer 18

During shoulder abduction, the humerus externally rotates - Subacromial space decreases gradually until 90° (minimal space). Automatic external rotation allows for the greater tubercle to go posterior to the acromion and therefore increase the subacromial space, to be able to continue the movement.

Question 19

Subacromial Space

Answer 19

Space between acromion and humeral head.

Question 20

Dynamic Centralization of the Humeral Head

Answer 20

Tension in both tendons and capsula (either by contracting or stretching) helps stabilize the head.
- Subscapularis and anterior capsula stabilize on anterior side.
- Infraspinatus and posterior capsula stabilize on posterior side.
Balance between internal rotation muscles and external rotation muscles is very important!

Question 21

Muscle Synergy for Shoulder Abduction - Glenohumeral

Answer 21

Middle deltoid is the prime mover. Supraspinatus is one of the main synergists.
Subscapularis and infraspinatus are important stabilizers for the head of the humerus.

Question 22

Synergists of Shoulder External Rotation

Answer 22

Deltoid (posterior)
Infraspinatus
Teres minor

Question 23

Synergists of Shoulder Internal Rotation

Answer 23

Deltoid (anterior)
Latissimus Dorsi
Teres Major
Subscapularis
Pectoralis major (all)

Question 24

Synergists of Shoulder Flexion

Answer 24

Biceps Brachii
Deltoid (anterior)
Pectoralis major (upper)
Coracobrachialis

Question 25

Synergists of Shoulder Extension

Answer 25

Triceps brachii
Deltoid (posterior)
Latissimus dorsi
Teres major
Pectoralis major (lower)

Question 26

Synergists of Horizontal Abduction

Answer 26

Deltoid (posterior)

Question 27

Synergists of Horizontal Adduction

Answer 27

Deltoid (anterior)
Pectoralis major (upper)

Question 28

Normal Shoulder Joint Range of Motion

Answer 28

Flexion: 180°
Extension: 45°
Abduction: 180°
Adduction: 0°
Internal Rotation at 90°: 70°
External Rotation at 90°: 90°