muscles of the head, neck and trunk actions

infraspinatus

lateral rotation of humerus; helps hold head of humerus in glenoid cavity; stabilizes shoulder

latissimus dorsi

prime mover of arm extension; adducts and medially rotates arm; brings arm down in power stroke, as in striking a blow

pectoralis major

prime mover of arm flexion; adducts, medially rotates arm; with arm fixed, pulls chest upward (thus also acts in forced inspiration)

diaphragm

prime mover of inspiration flattens on contraction, increasing vertical dimensions of thorax; increases intra-abdominal pressure

masseter

prime mover of jaw closure; elevates mandible

serratus anterior

prime mover to protract and hold scapula against chest wall; rotates scapula, causing inferior angle to move laterally and upward; essential to raising arm; fixes scapula for arm abduction

lateral pterygoid

protracts jaw (moves it anteriorly); in conjunction with medial pterygoid, aids in grinding movements of teeth

mentalis

protrudes lower lip; wrinkles chin

external intercostals

pull ribs toward one another to elevate rib cage; aid in inspiration

rhomboids- major and minor

pull scapula medially (retraction); stabilize scapula; rotate glenoid cavity downward

levator labii superioris

raises and furrows upper lip; opens lips

zygomaticus- major and minor

raises lateral corners of mouth upward (smiling muscle)

sternocleidomastoid

simultaneous contraction of both muscles of pair causes flexion of neck forward, generally against resistance (as when lying on the back); acting independently, rotate head toward shoulder on opposite side

medial pterygoid

synergist of temporalis and masseter, elevates mandible; in conjunction with lateral pterygoid, aids in grinding movements

platysma

tenses skin of neck; depresses mandible; pulls lower lip back and down (i.e. produces downward sag of the mouth)

orbicularis oculi

various parts can be activated individually; closes eyes, produces blinking, squinting, and draws eyebrows inferiorly

epicranius- frontal belly

with aponeurosis fixed, it raises eyebrows

pectoralis minor

with ribs fixed, draws scapula forward and inferiorly; with scapula fixed, draws rib cage superiorly

depressor anguli oris

zygomaticus antagonist; draws corners of mouth downward and laterally

deltoid

acting as a whole, prime mover of arm abduction; when only specific fibers are active, can aid in flexion, extension, and rotation of humerus

digastric

acting in concert, elevate hyoid bone; open mouth and depress mandible

semispinalis (capitis, cervicis and thoracis)

acting together, extend head and vertebral column; acting independently (right vs. left) causes rotation toward the opposite side

sternothyroid

acting with sternohyoid and omohyoid depresses larynx and hyoid bone if mandible is fixed; may also flex skull

omohyoid

acting with sternohyoid and sternothyroid depresses larynx and hyoid bone if mandible is fixed; may also flex skull

sternohyoid

acting with sternothyroid and omohyoid depresses larynx and hyoid bone if mandible is fixed; may also flex skull

splenius (capitis and cervicis)

as a group, extend or hyperextend head; when only one side is active, head is rotated and bent toward the same side

internal oblique

as for external oblique

teres minor

as for infraspinatus

temporalis

closes jaw; elevates and retracts mandible

orbicularis oris

closes lips; purses and protrudes lips (kissing and whistling muscle)

transversus abdominis

compresses abdominal contents

external oblique

compresses abdominal wall; also aids muscles of back in trunk rotation and lateral flexion; used in oblique curls

thyrohyoid

depresses hyoid bone; elevates larynx if hyoid is fixed

internal intercostals

draw ribs together to depress rib cage; aid in forced expiration; antagonistic to external intercostals

risorius

draws corner of lip laterally; tenses lip; zygomaticus synergist

buccinator

draws corner of mouth laterally; compresses cheek (as in whistling); holds food between teeth during chewing

corrugator supercilii

draws eyebrows medially and inferiorly; wrinkles skin of forehead vertically

depressor labii inferioris

draws lower lip inferiorly

quadratus lumborum

each flexes vertebral column laterally; together extend the lumbar spine and fix rib 12; maintains upright posture

levator scapulae

elevates and adducts scapula; with fixed scapula, laterally flexes neck to the same side

stylohyoid

elevates and retracts hyoid bone

mylohyoid

elevates hyoid bone and base of tongue during swallowing

erector spinae (iliocostalis, longissimus and spinalis)

extend and bend the vertebral column laterally; fibers of the longissimus also extend head

trapezius

extends head; raises, rotates, and retracts (adducts) scapula and stabilizes it; superior fibers elevate scapula (as in shrugging the shoulders); inferior fibers depress it

teres major

extends, medially rotates, and adducts humerus; synergist of latissimus dorsi

epicranius- occipital belly

fixes apponeurosis and pulls scalp posteriorly

scalenes- anterior, middle and posterior

flex and slightly rotate neck; elevate ribs 1-2 (aid in inspiration)

rectus abdominis

flexes and rotates vertebral column; increases abdominal pressure; fixes and depresses ribs; stabilizes pelvis during walking; used in sit-ups and curls

supraspinatus

initiates abduction of humerus; stabilizes shoulder joint