Shoulder Joint Motion

• Notes on the sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic joints

Sternoclavicular (SC) Joint

• Clavicle elevation/depression is determined by the movement of the lateral clavicle during SC joint rotation around an anteroposterior axis.
• During shoulder elevation, the clavicle rolls superiorly and glides inferiorly.
• During shoulder depression, the clavicle rolls inferiorly and glides superiorly.
• Protraction/retraction happens in the transverse plane.
• Retraction involves the clavicle's concave articular surface rolling and sliding posteriorly on the sternum's convex surface.
• Retraction stretches the anterior costoclavicular ligament and anterior capsular ligaments.
• Protraction involves the clavicle's concave articular surface rolling and sliding anteriorly on the sternum's convex surface.
• Protraction is limited by tightness in the posterior costoclavicular ligament, posterior capsular ligament, and scapular retractor muscles.
• Anterior/posterior spin occurs in the sagittal plane
• The clavicle rotates posteriorly during the end range of shoulder elevation.
• Anterior rotation is blocked by the first rib and sternum.
• Clavicle rotation causes the lateral clavicle end to flip upwards.
• The anterior/posterior sternoclavicular ligament limits anterior/posterior translation at the medial clavicle end, with each part limiting the opposite translation.
• The interclavicular ligament limits excessive depression of the distal clavicle and superior glide of the medial clavicle end.

Acromioclavicular (AC) Joint

• Has three degrees of motion.
• Maintains scapular contact with the ribcage and orients the glenoid fossa.
• Motion is described as the acromion moving on the distal clavicle.
• The closed pack position is at 90 degrees of glenohumeral abduction.
• Anterior/posterior tipping allows for scapular elevation and depression, with the inferior angle moving posteriorly.
• Upward/downward rotation occurs around an anteroposterior axis, perpendicular to the scapula's plane.
• Glenoid fossa is the reference point.
• Coracoclavicular ligament limits upward/downward rotation.
• Upward rotation involves rolling and sliding up, while downward rotation involves rolling and sliding down.
• Internal/external rotation is coupled with scapular protraction and retraction, bringing the fossa anteromedially or posteriolaterally.
• Internal rotation occurs during scapular protraction and external rotation during retraction.
• This involves the acromion sliding on the distal clavicle end.
• The coracoclavicular ligament firmly connects the clavicle to the scapula.
• The trapezoid portion resists posterior forces on the distal clavicle with its lateral and horizontal fibers.
• The conoid portion resists superior/inferior AC joint movement with its medial and posterior fibers.
• The conoid portion unwinds with posterior rotation, allowing clavicular backward rotation for full range of motion.

Suprahumeral Joint (Subacromial Space)

• Involves humeral head movement beneath the coracoacromial arch.
• The biceps tendon slides in the bicipital groove, held by the transverse humeral ligament.

Scapulothoracic (ST) Joint

• Lies in the scapular plane, 20-45 degrees anterior to the frontal plane.
• Articulation of the scapula to the thorax via the serratus anterior muscle.
• It creates a stable and movable base for the humerus and increases the range of motion.
• Scaption is scapular movement in the scapular plane, decreasing capsular restrictions and increasing functionality.

Closed Chain Motions of SC, AC, and ST Joints

• Mechanically linked, so movement in one affects the others.
• Scapulothoracic protraction leads to acromioclavicular internal rotation and sternoclavicular protraction.
• Scapulothoracic retraction leads to acromioclavicular external rotation and sternoclavicular retraction.
• Scapulothoracic elevation leads to acromioclavicular anterior tipping and sternoclavicular elevation.
• Scapulothoracic depression leads to acromioclavicular posterior tipping and sternoclavicular depression.
• Scapulothoracic upward rotation leads to acromioclavicular upward rotation, sternoclavicular elevation to end range (150-180 degrees), then posterior spin.
• Scapulothoracic downward rotation leads to acromioclavicular downward rotation and sternoclavicular anterior spin until about 150 degrees, then depression.

Glenohumeral (GH) Joint

• Sacrifices stability for mobility.
• The close-packed position is full elevation and external rotation.
• Abduction and adduction occur in the coronal plane around an anteroposterior axis:
  • Abduction involves superior roll and inferior glide.
  • Adduction involves inferior roll and superior glide.
• Internal/external rotation occurs in the transverse plane around a vertical axis:
  • Internal rotation involves anterior roll and posterior glide.
  • External rotation involves posterior roll and anterior glide.
• Flexion/extension occurs in the sagittal plane around a horizontal axis with a slight inferior glide at end-range flexion.
• The glenoid labrum enhances the depth of the glenoid fossa by 50% for improved joint congruency and is an attachment site for glenohumeral ligaments and the biceps long head tendon.

Rotator Interval Capsule

• Located between the supraspinatus and subscapularis tendons.
• Consists of the superior glenohumeral capsule, superior glenohumeral ligament, and coracohumeral ligament.
• Maintains the humeral head in the fossa when the arm is dependent.
• It blends with the capsule and provides static stability.

Scapular Stability

• Controlled movement and dynamic stabilization of the scapula allows the humeral head to remain centered in the glenoid fossa.
• The serratus anterior maintains contact between the scapula and ribcage from the medial scapular border to ribs 1-8 and is primarily an upward rotator and protractor.
• The pectoralis minor downwardly rotates and depresses the scapula.

Scapulohumeral Rhythm

• A sequence of combined glenohumeral and scapulothoracic motion resulting in maximum elevation range.
• The overall ratio is 2 degrees of glenohumeral motion to 1 degree of scapulothoracic motion.
• 0-90 degrees of elevation: 60 degrees from the glenohumeral joint and 30 degrees from the scapulothoracic joint.
  • The upper traps, lower traps, and serratus anterior work synergistically to upwardly rotate the scapula.
  • The deltoid and supraspinatus work synergistically to elevate the humerus.
  • The AC and SC joints contribute to the movement.
• 90-180 degrees elevation: 120 degrees of joint motion from glenohumeral and 60 degrees of joint motion from scapulothoracic.
  • The costoclavicular ligament limits further movement at the sternoclavicular joint (clavicular elevation).
  • As tension increases in the coracoclavicular ligament, the conoid portion pulls from its posteroinferior attachment on the clavicle, causing posterior rotation.

Static vs. Dynamic Stabilization

• Static stabilization relies on passive structures:
  • Articular surface configuration
  • Capsule
  • Ligaments
  • Negative intra-articular pressure
  • Passive capsuloligamentous forces (rotator interval and capsule)
  • Bony block (glenoid fossa orientation)
• Dynamic stabilization relies on active forces:
  • Muscle force, gravity
  • Dynamic muscular control (concentric, eccentric, and isometric contractions)
  • Forces of prime movers
  • Force of gravity
  • Force of muscle stabilizers
  • Joint reaction forces
  • Friction

Prime Movers vs. Synergists

• Prime movers for shoulder elevation:
  • Abduction: Deltoid
  • Flexion: Anterior deltoid
• Synergists for shoulder elevation:
  • Abduction: Supraspinatus
  • Flexion: Coracobrachialis, pectoralis major, and biceps

Rotator Cuff

• Its role is to approximate the humeral head in the glenoid fossa.
• The supraspinatus initiates 15-30 degrees of shoulder abduction, placing the humerus in the correct position for the deltoid to take over.
• Supraspinatus is most active during initial abduction due to its line of pull and moment arm.
• Stabilizes the humeral head in the glenoid fossa and can produce up to full abduction range of motion, steering the humeral head against other rotator cuff muscles.
• Also maintains joint congruency and counters gravity on the dependent arm

Infraspinatus and Teres Minor

• Prime movers for external rotation

Subscapularis

• Prime mover for internal rotation

Deltoid vs. Supraspinatus During Shoulder Abduction

• Supraspinatus is active during the full range of shoulder abduction.
• The deltoid can cause superior migration of the humeral head into the acromion without the supraspinatus
• The supraspinatus and deltoid function as a force couple, with the supraspinatus having a longer mechanical advantage than the deltoid.
• The deltoid has a better line of pull after the initial 30 degrees of abduction.
• The supraspinatus can abduct through a full range of motion but is weaker than the deltoid.

Muscles During Shoulder Abduction

• Supraspinatus performs the initial 30 degrees of abduction, after which the deltoid takes over due to a better line of pull.
• Supraspinatus provides stability against superior translation, while the infraspinatus and teres minor provide stability and external rotation, countering the deltoid's upward pull.
• The subscapularis provides anterior stability and internal rotation, balancing the posterior forces.
• The latissimus dorsi muscles are progressively stretched during shoulder abduction and generate passively directed tension on the humeral head.

Force Couple of Deltoid and Infraspinatus/Teres Minor/Subscapularis During Shoulder Elevation

• Prevents superior migration of humeral head
• Creates better humeral head rotation in the glenoid fossa
• The deltoid's upward pull is balanced by the downward pull of the infraspinatus, teres minor, and subscapularis.

Force Couple of Upper Trapezius, Lower Trapezius, and Serratus Anterior During Shoulder Elevation

• Acts on the scapulothoracic joint
• The force production from each muscle will produce upward rotation of the scapula.
• Serratus anterior pulls upward and laterally, while the lower trapezius pulls downward, creating a torque that rotates the scapula upward.
• The upper trapezius begins the movement by elevating and stabilizing the superior angle of the scapula.

Force Couple of Teres Major and Rhomboids During Upward Rotation of Scapula

• The rhomboids stabilize the scapula on the thorax, allowing the teres major to bring the humerus into extension, internal rotation, and adduction.
• The rhomboids create downward rotation of the scapula, whereas the teres major alone would pull scapula to humerus.

Force Couple of Trapezius and Deltoid

• The action of the deltoid during arm elevation would result in downward rotation of the scapula if unopposed by the upper trapezius.
• The upper trapezius pulls the scapula superiorly and slightly posteriorly, while the middle deltoid abducts the humerus, creating a combined effect of upward rotation of the scapula.

Active and Passive Insufficiency

• Active insufficiency:
  • The agonist is shortened over all joints.
  • The muscle cannot generate additional force in this position.
• Passive insufficiency:
  • The muscle is lengthened over all joints.
  • The range of motion is decreased for the antagonist muscle.