#002 The Scapula - The Unsung Hero of Upper Limb Movement -

Clinical Anatomy & Kinesiology｜TEAM ACTX

1. Introduction

When evaluating a patient with shoulder pain, where are you looking?

The rotator cuff? The position of the humeral head? Impingement signs? Of course, these are important. However, if you do not evaluate the position and movement of the scapula, which serves as the "foundation," you may overlook the essence of the problem.

The scapula is a unique bone to which 17 muscles attach. It has only one bony connection to the skeleton, the acromioclavicular joint with the clavicle, while the rest of it contacts the thoracic cage in a "floating state" held by muscles. This structure creates the scapula's overwhelming range of motion while simultaneously presenting the "difficulty of balancing stability and mobility."

In this article, we will systematically organize the basic anatomy, kinesiology, and biomechanics of the scapula, and how to apply them in clinical practice.

2. Basic Anatomy

Shape

The scapula is a triangular flat bone, appearing close to an inverted triangle when viewed from the posterior.

Major structures:

Region: Features | Glenoid fossa: Shallow concave surface contacting the humeral head. Approximately 5-7° upward tilt | Spine: Posterior ridge. Attachment site for trapezius and deltoid | Acromion: Lateral extension of the spine. Forms the acromioclavicular joint with the clavicle | Coracoid process: Anterior projection. Attachment site for pectoralis minor, short head of biceps brachii, and coracobrachialis | Medial/Lateral/Superior borders: Attachment sites for serratus anterior, rhomboids, etc. | Superior/Inferior/Lateral angles: Bony landmarks for movement

Joints

• Acromioclavicular joint (ACJ): Connects to the lateral end of the clavicle at the lateral angle

• Scapulothoracic joint (STJ): Not an anatomical joint, but a functionally important "pseudo-joint"

Major muscles attached (17 muscles)

Anterior (subscapular fossa)

• Subscapularis

Posterior (supraspinous fossa/infraspinous fossa)

• Supraspinatus, infraspinatus, teres minor, teres major

Coracoid process/Acromion

• Pectoralis minor, short head of biceps brachii, coracobrachialis, deltoid

Medial border/inferior angle

• Serratus anterior, rhomboid major, rhomboid minor, levator scapulae

Scapular spine/acromion

• Trapezius (upper, middle, lower)

3. Functional Anatomy and Kinesiology

Six directions of scapular movement

Scapular movements at the scapulothoracic joint are classified into six directions.

Movement - Primary Mover
Elevation - Levator scapulae, upper trapezius
Depression - Lower trapezius, lower serratus anterior
Protraction - Serratus anterior, pectoralis minor
Retraction - Rhomboids, middle trapezius
Upward rotation - Upper and lower trapezius, serratus anterior
Downward rotation - Rhomboids, levator scapulae, pectoralis minor

Mechanism of upward rotation: Force Couple

Upward rotation of the scapula does not occur through a single muscle, but is generated by a force couple of three muscle groups.

1. Upper trapezius: Pulls the superior angle of the scapula superiorly and laterally

2. Lower trapezius: Pulls the medial end of the scapular spine inferiorly

3. Serratus anterior (lower part): Pulls the inferior angle anteriorly, laterally, and superiorly

By combining these three forces, the scapula rotates upward efficiently. If any one of these malfunctions, the axis of rotation shifts, leading to compensatory movements or injury.

Scapulohumeral Rhythm

The ratio of movement between the humerus and the scapula during full elevation (180°) of the upper limb.

• Total range of motion: 120° humerus + 60° scapula = 180°

• Ratio: 2:1 (humerus: scapula)

However, this is an average value; during the initial phase of elevation (0–30°), the humerus is dominant, and thereafter, upward rotation of the scapula gradually increases.

Scapular tilt and rotation

In three dimensions, the scapula also possesses the following movements:

• Anterior tilt/Posterior tilt (tilt in the anteroposterior direction)

• Internal rotation/External rotation (rotation in the anteroposterior direction relative to the trunk)

It is known that abnormalities in these movements (e.g., increased anterior tilt and internal rotation) increase the risk of subacromial impingement.

④ Biomechanics

What the 'floating structure' of the scapula provides

The structure in which the scapula is held to the rib cage solely by muscles provides two functions.

① Ensuring degrees of freedom Because there are no bony constraints, the scapula can move over a wide range while following the curvature of the rib cage. This enables the large range of motion of the upper limb.

② Shock absorption Muscles absorb and disperse shocks transmitted from the upper limb, such as during a fall. Because there are few bony connections, force is less likely to be concentrated directly on the bone.

Resting position of the scapula

The normal position of the scapula in a resting standing position is considered to be as follows:

• The medial border is approximately 5–7 cm lateral to the spinous processes

• The glenoid fossa is slightly anteriorly tilted (approximately 10–20°)

• The scapular plane is approximately 30–45° anterior to the frontal plane

Movement in this 'scapular plane' (scaption) provides the highest congruency between the humerus and the glenoid fossa, and is considered a clinically safe and efficient direction of movement.

⑤ Clinical relevance

Scapular Dyskinesis

A state where the normal movement pattern of the scapula is disrupted is called scapular dysfunction.

Common patterns

• Winged scapula: Decreased function of the serratus anterior

• Medial border prominence: Decreased function of the rhomboids and serratus anterior

• Excessive elevation: Overactivity of the upper trapezius

• Insufficient upward rotation: Decreased function of the lower trapezius and serratus anterior

Evaluation methods

• Visual inspection of left-right differences in the resting position of the scapula

• Dynamic observation during upper limb elevation (from behind)

• Lateral Scapular Slide Test (LSST)

• Scapular Dyskinesis Test (SDT)

Relationship with rotator cuff tears

In patients with rotator cuff tears, increased anterior tilt and internal rotation of the scapula, as well as decreased upward rotation, have been reported. In parallel with approaching the rotator cuff itself, correcting scapular alignment and stabilization become the pillars of treatment.

Winged Scapula

A state where the medial border is prominent. Main causes:

• Long thoracic nerve palsy: The serratus anterior is paralyzed, causing the medial border to protrude posteriorly (medial winging)

• Accessory nerve palsy: The trapezius is paralyzed, causing the scapular spine and superior angle to protrude posteriorly (lateral winging)

Confirmation of nerve damage via electrophysiological testing and the utilization of compensatory muscles are central to treatment.

⑥ Latest evidence

A significant amount of research has been accumulated regarding the relationship between scapular dysfunction and shoulder disorders.

Kibler et al. (2013) published an international consensus statement on scapular dyskinesis, demonstrating that scapular assessment and intervention are essential for the rehabilitation of shoulder injuries¹.

A systematic review by Ludewig & Reynolds (2009) consistently reported increased anterior tilt, decreased upward rotation, and increased internal rotation of the scapula in patients with shoulder joint disorders, indicating that these patterns are associated with narrowing of the subacromial space².

Furthermore, a meta-analysis by Struyf et al. (2014) showed that scapular-focused interventions have a significant effect on improving pain and function in patients with shoulder joint disorders³.

⑦ Today's Clinical ― Clinical points you can use starting tomorrow ―

✅ Make dynamic observation from behind a habit Have the patient elevate their upper limb and observe the movement of the scapula from behind. Look for the movement of the medial border and inferior angle, side-to-side differences, and any "jerking" during the motion. A great deal of information can be gained from this alone.

✅ Evaluate the lower trapezius and serratus anterior separately If you consider the cause of upward rotation dysfunction by separating it into "weakness of the lower trapezius" and "weakness of the serratus anterior," the precision of your training prescription will improve. Be conscious of selective assessment and intervention for each.

✅ Look beyond "shoulder pain = rotator cuff" For shoulders that do not improve even after intervening on the rotator cuff, re-evaluate the scapular alignment and dynamic stability. If the "foundation" is not set, the load on the rotator cuff will not decrease.

⑧ Summary

• The scapula is a unique bone to which 17 muscles attach and which is held to the thorax solely by muscles

• The only bony connection is the acromioclavicular joint. This "floating structure" creates freedom of movement and shock absorption

• Upward rotation is achieved by the force couple of the upper/lower trapezius and the serratus anterior

• The scapulohumeral rhythm is 2:1. However, the ratio changes between the early and late phases

• Scapular dysfunction is closely related to rotator cuff disorders and impingement

• Dynamic observation from behind and mechanical muscle assessment increase clinical precision

⑨ References

1. Kibler WB, et al. Clinical implications of scapular dyskinesis in shoulder injury: the 2013 consensus statement from the 'Scapular Summit'. Br J Sports Med. 2013;47(14):877-885. PMID: 23580420. DOI: 10.1136/bjsports-2013-092425.

2. Ludewig PM, Reynolds JF. The association of scapular kinematics and glenohumeral joint pathologies. J Orthop Sports Phys Ther. 2009;39(2):90-104. PMID: 19194022. DOI: 10.2519/jospt.2009.2808.

3. Struyf F, et al. Scapular-focused treatment in patients with shoulder impingement syndrome: a randomized clinical trial. Clin Rheumatol. 2013;32(1):73-85. PMID: 22961232. DOI: 10.1007/s10067-012-2093-2.

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