Thorax and Abdominal Muscles: Anatomy and Function
Muscles of the Thorax and Abdomen
Pectoralis Major
Origin: Clavicular, sternal, and abdominal parts.
Insertion: Crest of the humerus (subtrochanteric crest).
Function:
Clavicular part: Flexion.
Sternal part: Adduction and internal rotation.
Abdominal part: Extension.
Pectoralis Minor
Origin: Coracoid process of the scapula.
Insertion: Anterior surfaces of the 3rd, 4th, and 5th ribs.
Function: Pulls the ribs up towards the coracoid process, bringing the scapula forward and downward.
Serratus Anterior
Origin:
Upper part: Ribs I and II.
Middle part: Ribs II and IV.
Lower part: Ribs IV and IX.
Insertion:
Upper part: Superior angle of the scapula.
Middle part: Medial border of the scapula.
Inferior part: Inferior angle of the scapula.
Function:
Upper part: Elevates the scapula.
Middle and lower parts: Depresses the scapula, elevates the ribs.
External Oblique
Origin: Lateral aspect of ribs 5th to 12th, interdigitating with the serratus anterior and latissimus dorsi muscles.
Insertion: Iliac crest to the external part of the aponeurosis of the rectus abdominis.
Function:
Unilateral action: Tilts the trunk to the same side, rotates the trunk to the opposite side.
Bilateral action: Trunk flexion.
Internal Oblique
Origin: Iliac crest.
Insertion: Posterior fibers insert into the xiphoid process; middle and lower fibers insert into the linea alba.
Function:
Unilateral action: Tilts the trunk to the same side, rotates the trunk to the same side.
Bilateral action: Trunk flexion.
Rectus Abdominis
Origin: Superior aspect of the pubis via a short tendon (2-3 cm).
Insertion: Anterior surfaces of the 5th, 6th, and 7th costal cartilages and the xiphoid process.
It is a polygastric muscle formed by 4 muscular bellies separated by 3 tendinous bands.
Function:
- Maintains upright posture and keeps the viscera in position.
- Produces flexion of the spine through the ribs.
- Contraction increases intra-abdominal pressure, aiding in defecation and urination.
- Unilateral contraction produces lateral trunk tilt to the same side.
- Limits maximum inspiration and favors expiration.
Transversus Abdominis
Origin: Inner surfaces of the last 5 or 6 ribs, costiform processes of L1-L5, iliac crest, and the inguinal ligament.
Insertion: Midline, forming an aponeurotic curve at the level of the navel, covering the posterior aspect of the rectus abdominis.
Function:
- Constricts the abdomen.
- Increases intra-abdominal pressure.
- Aids in urination, defecation, vomiting, coughing, childbirth, and forced expiration.
Latissimus Dorsi
Origin: Continuous line along the spinous processes from the 7th thoracic vertebra to the sacrum and the crest of the sacrum.
Insertion: All fibers terminate at the intertubercular groove of the humerus.
Function:
- On the arm: Extension, adduction, and internal rotation.
- With fixed arms: Raises the trunk.
- With both ends fixed: Maintains a firm position.
Infraspinatus
Origin: Infraspinous fossa of the scapula.
Insertion: Greater tubercle of the humerus, posterior to the supraspinatus muscle.
Function: External rotation of the arm. Has limited ability to abduct.
Trapezius
Origin:
Upper fibers: Occipital bone to the spinous process of the 7th cervical vertebra.
Middle fibers: Spinous processes of the 7th cervical vertebra to the 3rd thoracic vertebra.
Lower fibers: Spinous processes of the 3rd to the 12th thoracic vertebrae.
Insertion:
Upper fibers: Lateral third of the clavicle.
Middle fibers: Acromion.
Lower fibers: Spine of the scapula.
Function:
Upper fibers: Elevation and rotation of the scapula.
Middle fibers: Adduction of the scapula.
Lower fibers: Scapular rotation, directing the glenoid cavity upwards.