Muscle Actions

Trapezius

Rotates, elevates, adducts, and depresses the scapula

Latissimus dorsi

Extends, adducts, and medially rotates humerus

Levator scapulae

Elevates scapula

Rhomboids

Retracts (adducts) and elevates scapula

Splenius capitis

Draw head backward, extending neck; Draw and rotate head to one side

Semispinalis capitis

Extend head and neck and rotate them to opposite sides

Erector spinae

Primary extensors of the vertebral column and head

Bilaterally: straighten the back, returning it to upright position from a flexed position, and pull the head posteriorly; control vertebral column flexion by contracting and relaxing in a coordinated fashion

Unilaterally: bend the vertebral column laterally; contractions of muscles attached to the head turn the head to the actively contracting side

Deltoid

Major abductor of arm, clavicular fibers assist in flexing the arm, posterior fibers assist in extending the arm

Pectoralis minor

Depresses tip of shoulder, protracts scapula

Pectoralis major

Adduction, medial rotation, and flexion of the humerus at the shoulder joint

Serratus anterior

Protracts and rotates scapula and holds it against thoracic wall

Sternocleidomastoid

Individually: tilts head toward shoulder on same side, rotating head to turn face to opposite side

Together: draw head forward (flex head, elevate thorax)

Internal intercostal

Most active during expiration; supports intercostal space; moves ribs inferiorly (depression)

External intercostal

Most active during inspiration; supports intercostal space; moves ribs superiorly

Diaphragm

Draws central tendon down and forward during inspiration

External oblique

Compress abdominal contents; both muscles flex trunk; each muscle bends trunk to same side, turning anterior part of abdomen to opposite side

Internal oblique

Compress abdominal contents; both muscles flex trunk; each muscle bends trunk and turns anterior part of abdomen to same side

Transversus abdominis

Compress and support abdominal contents

Rectus abdominis

Compress abdominal contents; flex vertebral column; tense abdominal wall