Serratus Anterior

Original Editor - Esraa Mohamed Abdullzaher

Top Contributors - Lucinda hampton, Kholoud Abd Elghany, Kim Jackson, Joao Costa, Lilian Ashraf, Esraa Mohamed Abdullzaher and Oyemi Sillo

Introduction

The serratus anterior (SA) is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax and inserts along the superior angle, medial border, and inferior angle of the scapula. Its main part lies deep under the scapula and the pectoral muscles.

• It acts on the scapula and is the prime mover in both scapular protraction and scapular upward rotation.
• Is a key scapular stabilizer, keeping the shoulder blades against the ribcage when at rest and during movement..

Image 1: Serratus Anterior.

Origin

It originates on the top lateral surface of the eight or nine upper ribs.  It then wraps posteromedially around the ribcage, passing beneath the scapula to insert on the underside of the scapula on its medial border.

As the muscle extends from the ribs, it is divided by tendinous septa into nine finger-like groupings of muscle fibers called “digitations.”

• The fibers run obliquely (to varying degrees) between these septa, forming a multipennate muscle architecture.
• Upper three digitations are termed the superior fibers , lower six digitations are termed the inferior fibers.
• The lowest four digitations of the serratus anterior interdigitate with the fibers of the external oblique.

Image 2: Serratus anterior muscle seen from a lateral view.

Insertion

It inserts on the front border of the scapula. The muscle is divided into three parts :

• Upper / Superior : 1st to 2nd rib → superior angle of scapula.
• Middle / Intermedius : 2nd to 3rd rib → medial border of scapula.
• Lower / Inferior : 4th to 9th rib → medial border and inferior angle of scapula. It is the most powerful and prominent part.

Image 3: Insertional area of SA on scapula.

Nerve and Blood Supply

1. The long thoracic nerve, which arises from C5 to C7 nerve roots of the brachial plexus.
2. Lateral thoracic artery, the superior thoracic artery and the thoracodorsal artery. 

Function

The main actions are protraction and upward rotation of the scapulothoracic joint.

It’s also a key scapular stabilizer, keeping the shoulder blades against the ribcage when at rest and during movement. It also acts with the upper and lower fibers of the trapezius muscle to sustain upward rotation of the scapula, which allows for overhead arm movement. See Dynamic Stabilisers of the Shoulder Complex and Scapulohumeral Rhythm.

When the shoulder blade is in fixed position the accessory inspiratory muscles (used in respiratory distress) are activated e.g breathing after a boxing bout, person with emphysema. The serratus anterior lifts the ribcage and thus supports breathing.The other accessory inspiratory muscles include sternocleidomastoid, scalene muscles, pectoralis major, pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis lumborum, quadratus lumborum

Image 4: The serratus anterior is also known as the “boxer’s muscle,” as it is largely responsible for the protraction of the scapula which occurs when throwing a punch.

Image 5: When the shoulder blade is in fixed position the accessory inspiratory muscles (used in respiratory distress) are activated.

Clinical Relevance

The most common causes of serratus muscle pain include: tension; stress; overuse.

• It is is common in sports with repetitive motions, eg swimming, tennis, or weightlifting (especially with heavy weights).
• This pain may also result from serratus anterior myofascial pain syndrome (SAMPS), a rare myofascial pain syndrome.

Image 6: Bench press, stresses SA

When the serratus anterior is inhibited, lengthened or weakened, scapular protraction and scapular upward rotation are impaired. This limits overhead range of motion and reduces overall scapular/shoulder stability, increasing the risk of injury to the rotator cuff and shoulder girdle. Common causes for this include:

• Upper Crossed Syndrome
• Damage to the long thoracic nerve can weaken, or even paralyze, the serratus anterior. Neurogenic scapular winging (i.e. scapular winging caused by nerve damage) requires a doctor to verify if scapular winging is neurogenic.
• Weakness of the serratus anterior leads to altered line of pull of the rotator cuff muscle which could increase the risk of subacromial impingement syndrome.

Assessment

Palpation

The serratus anterior muscle is very thin and covers the side of the ribcage. The main part of the serratus anterior lies deep to the scapula and the pectoral muscles and is easily palpated between the pectoralis major and latissimus dorsi muscles.

Image 7: SA seen her between the pectoralis major and latissimus dorsi muscles.

• You can feel it by putting your hand just below the arm pit.
• It also helps to experience how your ribs feel, so that you can distinguish the ribs and this thin and superficial muscle.

Muscle Test

The 3 tests below can be used to examine the serratus anterior muscle weakness and scapula winging

1. Serratus Anterior Strength Test or Push Out Test is used to examine the serratus anterior muscle weakness and scapula winging.
2. Shoulder abduction test. Therapist applies a downwards resisting force against the scapular plane abduction of the shoulder at about 120–130° and against upward rotation of the scapula. A weak serratus anterior would result in the patient failing to resist the therapist’s force resulting in the shoulder breaking into adduction and the scapula can't rotate upwards significantly.
3. In the third test the patient is in a seated or supine position, with his arm flexed 90-100 degrees and his elbow fully extended. The therapist resists the maximally protracting force by the patient. In case of serratus anterior muscle weakness the patient’s scapula is pushed into a retracted and internally rotated position leading to scapular winging appearance.See video below.

Exercises for Activating Serratus Anterior

The serratus anterior is trained indirectly in all anterior deltoid exercises. It works to some extent in the select few chest exercises that involve scapular protraction A few examples include:

• Isometric Exercises: Plank (emphasizing protraction); Straight arm plank (emphasizing protraction)
• Incline shoulder raise
• Flat shoulder raise
• Overhead shrug
• Push-up plus
• Serratus anterior punch

Image 8: Plank exercise.

Resources

Watch this comprehensive video from Kenhub (3 minutes)