Calisthenics Instructor Certification Chapter 4: Muscles of the Human Body

Chapter 4: Muscles of the Human Body

The muscles of the head are divided into two functional groups:

  1. Cutaneous (mimetic) muscles:
    • Attach to the skin and produce facial expressions.
    • Also called mimetic muscles because they convey emotions.
  2. Masticatory muscles:
    • Attach to the mandible and move it during chewing.

4.1 Epicranial Aponeurosis

The epicranial aponeurosis (galea aponeurotica) is a broad, tendinous sheet that:

  • Connects the two frontalis muscles anteriorly with the two occipitalis muscles posteriorly.
  • Laterally attaches to the temporal fascia on each side.
  • Is firmly adhered to the overlying scalp; when intact, wound edges in this region remain closely opposed. If it is severed, the wound edges retract.

4.2 Mimetic (Cutaneous) Muscles

Frontalis

  • Origin: Skin of the eyebrows and glabella.
  • Insertion: Anterior margin of the epicranial aponeurosis.
  • Action: Elevates the eyebrows and wrinkles the forehead.

Orbicularis Oculi (Eyelid Sphincter)

  • Arrangement: Circular muscle surrounding the palpebral fissure.
  • Action: Closes the eyelids; assists in tear drainage.
  • Clinical note: Paralysis causes incomplete eyelid closure (lagophthalmos).

Buccinator

  • Origin: Alveolar processes of the maxilla and mandible, and pterygomandibular raphe.
  • Insertion: Angle of the mouth; fibers interlace with orbicularis oris.
  • Action: Compresses the cheek against the teeth (e.g., during blowing or sucking), and helps convey food toward the molars.

Zygomaticus Major

  • Origin: Zygomatic bone.
  • Insertion: Angle of the mouth.
  • Action: Elevates and draws the angle of the mouth laterally (smiling).

Risorius (“Smiling” Muscle)

  • Origin: Fascia over the parotid gland.
  • Insertion: Angle of the mouth.
  • Action: Retracts the angle of the mouth laterally.

Orbicularis Oris (Lip Sphincter)

  • Arrangement: Fibers arranged in circular bundles around the mouth.
  • Action: Closes and protrudes the lips (kissing, speech).

Depressor Labii Inferioris (Lower-Lip Depressor)

  • Location: Lateral to the chin, below the angle of the mouth.
  • Action: Depresses the lower lip (expression of doubt or sadness).

Depressor Anguli Oris (Angulus Depressor)

  • Location: Lateral to the chin, below the mouth angle.
  • Action: Draws the angle of the mouth downward (frowning).

4.3 Masticatory Muscles

Temporalis

  • Origin: Temporal fossa and temporal fascia.
  • Insertion: Coronoid process and anterior border of the ramus of the mandible.
  • Action: Elevates and retracts the mandible (closes the jaw).

Masseter

  • Origin: Zygomatic arch.
  • Insertion: Lateral surface of the ramus and angle of the mandible.
  • Action: Elevates the mandible, bringing the teeth into occlusion.

4.4 Muscles of the Neck

The neck is anatomically divided into an anterior (cervical) region and a posterior (nuchal) region. Neck muscles are commonly grouped as:

  • Anterior (infrahyoid and suprahyoid) muscles
  • Lateral muscles
  • Anterolateral muscles
  • Posterior (paraspinal) muscles

Below are the principal anterior and anterolateral muscles involved in swallowing, head movement, and vascular regulation.

Thyrohyoid

  • Origin: Thyroid cartilage of the larynx
  • Insertion: Body of the hyoid bone
  • Action: Depresses the hyoid bone or elevates the larynx during swallowing

Omohyoid

  • Origin: Superior border of the scapula (via an inferior belly)
  • Insertion: Body of the hyoid bone (via a superior belly)
  • Action: Depresses and stabilizes the hyoid; assists venous return through the internal jugular vein by tensioning the deep cervical fascia

Sternohyoid

  • Origin: Manubrium of the sternum and medial end of the clavicle
  • Insertion: Body of the hyoid bone
  • Action: Depresses the hyoid bone after swallowing

Sternocleidomastoid (SCM)

  • Origin:
    • Sternal head: Manubrium of the sternum
    • Clavicular head: Medial third of the clavicle
  • Insertion: Mastoid process of the temporal bone and superior nuchal line of the occiput
  • Action:
    • Unilateral contraction: Rotates the face to the opposite side and tilts the head to the same side
    • Bilateral contraction: Extends the head at the atlanto-occipital joint
  • Clinical Note:
    • Torticollis (“wry neck”): Spasm or contracture of one SCM causing head tilt and rotation

Scalenes (Anterior, Middle, Posterior)

  • Location: Lateral aspect of the neck, forming the floor of the posterior triangle
  • Origins & Insertions:
    • Anterior scalene: Transverse processes of C3–C6 → first rib
    • Middle scalene: Transverse processes of C2–C7 → first rib (posterior to subclavian artery)
    • Posterior scalene: Transverse processes of C4–C6 → second rib
  • Actions:
    • Elevate the first and second ribs during forced inspiration
    • Laterally flex the neck to the same side
  • Clinical Note:
    • Thoracic outlet syndrome: Hypertrophy or spasm of scalenes can compress the brachial plexus or subclavian vessels

4.5 Trunk Muscles

The muscles of the trunk are divided into:

  • Thoracic muscles
  • Abdominal muscles

4.5.1 Thoracic Muscles

A. Intrinsic (Autochthonous) Muscles
These lie between the ribs and are the primary muscles of thoracic respiration:

  • Intercostal Muscles
    • Fill the spaces between adjacent ribs.
    • During inspiration, they elevate the ribs and sternum, increasing thoracic volume.
    • During expiration, they depress the ribs, decreasing thoracic volume.

Mechanics of breathing:
Movements occur at the costovertebral joints. On inspiration, ribs swing upward (“pump-handle”) and outward (“bucket-handle”), while the sternum moves away from the spine, expanding the thoracic cavity. Expiration reverses these movements.

B. Extrinsic (Heterochthonous) Muscles
These originate on the thoracic wall but insert on the upper limb or shoulder girdle:

  • Pectoralis Major
    • Origin: Clavicle, sternum, and sternal half of the costal cartilages (also the aponeurosis of the external oblique).
    • Insertion: Lateral lip of the bicipital groove of the humerus.
    • Action: Adducts and medially rotates the humerus; draws the trunk upward when the arms are fixed (e.g., climbing).
  • Serratus Anterior
    • Origin: External surfaces of the first eight or nine ribs.
    • Insertion: Anterior aspect of the medial border of the scapula.
    • Action: Protracts and rotates the scapula upward, essential for raising the arm above horizontal; stabilizes the scapula against the thoracic wall.

4.6 Abdominal Muscles

The abdominal wall muscles are grouped as anterior, lateral, and posterior. They support the abdominal viscera, assist respiration, and govern trunk movements.

A. Lateral Muscles

Muscle Origin Insertion / Action
External Oblique Outer surfaces of ribs 5–12 Aponeurosis to the linea alba; iliac crest; pubic tubercle.
Action: Compresses abdomen; flexes and rotates trunk to the opposite side.
Internal Oblique Thoracolumbar fascia; iliac crest; inguinal ligament Aponeurosis to the linea alba; lower three ribs.
Action: Compresses abdomen; flexes and rotates trunk to the same side.

B. Anterior Muscle

  • Rectus Abdominis
    • Origin: Xiphoid process of the sternum and costal cartilages of ribs 5–7.
    • Insertion: Pubic symphysis and pubic crest.
    • Features: Contains 3–4 tendinous intersections (“tendinous inscriptions”) that strengthen the muscle.
    • Action: Flexes the vertebral column; compresses abdominal contents.

C. Rectus Sheath & Linea Alba

  • The aponeuroses of the lateral muscles interweave at the midline to form:
    • Rectus sheath: Encloses the rectus abdominis.
    • Linea alba: A fibrous seam between the two rectus sheaths that prevents the muscles from diverging and is a common site for hernias.

D. Umbilicus

  • The navel is a scar marking where the umbilical cord passed in utero.
  • It lies at a naturally weak point in the linea alba, predisposing to umbilical hernias.

4.7 Functional Roles of Abdominal Muscles

  • Trunk movements:
    • Flexion: Rectus abdominis
    • Lateral flexion & rotation: External and internal obliques
  • Respiratory assistance: Contract to support diaphragmatic action during forced expiration.
  • Intra-abdominal pressure:
    • Contraction of all abdominal muscles raises pressure, aiding in defecation, micturition, and parturition (childbirth).
    • The linea alba and tendinous intersections help distribute force and maintain midline integrity.

 

4.8 Muscles of the Back

The back muscles lie in three layers:

  1. Superficial layer (scapulorachial muscles): move the shoulder girdle
  2. Intermediate layer (thoracorachial muscles): accessory respiratory muscles and contribute to trunk movement
  3. Deep layer (true paraspinal muscles): maintain posture and the spinal curvatures

A. Superficial Layer

Trapezius

  • Origin: External occipital protuberance, nuchal ligament, spinous processes C7–T12
  • Insertion: Lateral third of the clavicle, acromion, and spine of scapula
  • Action: Retracts scapula; upper fibers elevate, lower fibers depress shoulder

Latissimus Dorsi

  • Origin: Iliac crest, thoracolumbar fascia, spinous processes T7–L5, lower ribs
  • Insertion: Floor of the intertubercular sulcus of humerus
  • Action: Extends, adducts, and medially rotates humerus; stabilizes trunk

Levator Scapulae

  • Origin: Transverse processes C1–C4
  • Insertion: Superior angle and medial border of scapula
  • Action: Elevates scapula and tilts its glenoid cavity inferiorly

Rhomboid Minor & Major

  • Origin:
    • Minor: Ligamentum nuchae and spinous processes C7–T1
    • Major: Spinous processes T2–T5
  • Insertion: Medial border of scapula (minor above spine, major below)
  • Action: Retract and rotate scapula to depress glenoid cavity; fix scapula to thoracic wall

B. Intermediate Layer

Serratus Posterior Superior

  • Origin: Nuchal ligament, spinous processes C7–T3
  • Insertion: Superior borders of ribs 2–5
  • Action: Elevates those ribs, aiding inspiration

Serratus Posterior Inferior

  • Origin: Spinous processes T11–L2
  • Insertion: Inferior borders of ribs 9–12
  • Action: Depresses those ribs, aiding forced expiration

C. Deep Layer (Intrinsic Paraspinal Muscles)

These muscles lie in the groove on either side of the vertebral column and maintain posture and control segmental movements.

Erector Spinae Group (from lateral to medial)

  • Iliocostalis
    • Origin: Iliac crest, sacrum, ribs
    • Insertion: Angles of ribs and transverse processes of cervical vertebrae
    • Action: Extends and laterally flexes vertebral column
  • Longissimus
    • Origin: Transverse processes of lumbar vertebrae, sacrum
    • Insertion: Transverse processes of thoracic and cervical vertebrae; mastoid process
    • Action: Extends head and vertebral column; laterally flexes
  • Spinalis
    • Origin: Spinous processes of upper lumbar and lower thoracic vertebrae
    • Insertion: Spinous processes of upper thoracic and cervical vertebrae
    • Action: Extends vertebral column

Transversospinalis Group

  • Semispinalis (capitis, cervicis, thoracis)
    • Origin: Transverse processes of C4–T12
    • Insertion: Spinous processes of vertebrae 4–6 levels above (and occiput)
    • Action: Extends head and thoracic/cervical spine; rotates toward opposite side
  • Multifidus
    • Origin: Sacrum, iliac spine, transverse processes of T1–T12, articular processes of C4–C7
    • Insertion: Spinous processes 2–4 levels above
    • Action: Stabilizes vertebrae; small extension and rotation
  • Rotatores (brevis and longus)
    • Origin: Transverse processes of vertebrae
    • Insertion: Lamina and transverse process of the immediate (brevis) or next (longus) superior vertebra
    • Action: Proprioceptive; assist with rotation and extension of the spine

Splenius Group

  • Splenius Capitis
    • Origin: Nuchal ligament, spinous processes C7–T3
    • Insertion: Mastoid process, superior nuchal line
    • Action: Unilateral: laterally flexes and rotates head to same side; Bilateral: extends head
  • Splenius Cervicis
    • Origin: Spinous processes T3–T6
    • Insertion: Transverse processes C1–C3
    • Action: Similar to splenius capitis for cervical vertebrae

4.9 Shoulder Muscles

Deltoid

  • Origin: Lateral third of the clavicle; acromion; spine of scapula
  • Insertion: Deltoid tuberosity of the humerus
  • Action:
    • Abduction of the arm to horizontal
    • Anterior fibers: flexion and medial rotation of the humerus
    • Posterior fibers: extension and lateral rotation of the humerus

Infraspinatus

  • Origin: Infraspinous fossa of the scapula
  • Insertion: Middle facet of the greater tubercle of the humerus
  • Action: Lateral (external) rotation of the humerus; stabilizes the glenohumeral joint

Teres Major

  • Origin: Posterior surface of the inferior angle of the scapula
  • Insertion: Medial lip of the intertubercular sulcus of the humerus
  • Action: Medial rotation and adduction of the humerus; assists extension

Subscapularis

  • Origin: Subscapular fossa of the scapula
  • Insertion: Lesser tubercle of the humerus
  • Action: Medial rotation of the humerus; main stabilizer of the anterior shoulder

4.10 Arm Muscles

Triceps Brachii

  • Heads & Origins:
    • Long head: Infraglenoid tubercle of the scapula
    • Lateral head: Posterior humerus above radial groove
    • Medial head: Posterior humerus below radial groove
  • Insertion: Olecranon of the ulna
  • Action: Extension of the forearm at the elbow; long head also extends and adducts the humerus

Anconeus

  • Origin: Lateral epicondyle of the humerus
  • Insertion: Lateral aspect of the olecranon and proximal ulna
  • Action: Assists triceps in forearm extension; stabilizes the elbow

Coracobrachialis

  • Origin: Coracoid process of the scapula
  • Insertion: Middle third of the medial humeral shaft
  • Action: Flexion and adduction of the humerus

Brachialis

  • Origin: Distal half of the anterior humerus
  • Insertion: Ulnar tuberosity and coronoid process of the ulna
  • Action: Powerful flexion of the forearm at the elbow

Biceps Brachii

  • Heads & Origins:
    • Long head: Supraglenoid tubercle of the scapula (via intertubercular sulcus)
    • Short head: Coracoid process of the scapula
  • Insertion: Radial tuberosity and bicipital aponeurosis into the deep fascia of the forearm
  • Action: Flexes the elbow; supinates the forearm; assists shoulder flexion

4.11 Forearm Muscles

Posterior (Extensor) Compartment

  • Brachioradialis
    • Origin: Lateral supracondylar ridge of the humerus
    • Insertion: Styloid process of the radius
    • Action: Flexes the elbow, especially in mid-pronation
  • Extensor Carpi Radialis Longus (ECRL)
    • Origin: Lateral supracondylar ridge of the humerus
    • Insertion: Base of the second metacarpal
    • Action: Extends and abducts the wrist
  • Extensor Carpi Ulnaris (ECU)
    • Origin: Lateral epicondyle of the humerus; posterior border of the ulna
    • Insertion: Base of the fifth metacarpal
    • Action: Extends and adducts the wrist
  • Extensor Digitorum (Common Extensor of the Fingers)
    • Origin: Lateral epicondyle of the humerus
    • Insertion: Extensor expansions of medial four digits
    • Action: Extends the fingers and assists wrist extension and ulnar deviation

Anterior (Flexor) Compartment

  • Flexor Carpi Ulnaris (FCU)
    • Heads:
      • Humeral head: Medial epicondyle of the humerus
      • Ulnar head: Olecranon and posterior border of the ulna
    • Insertion: Pisiform, hook of the hamate, and base of the fifth metacarpal
    • Action: Flexes and adducts (ulnar deviates) the wrist
  • Flexor Carpi Radialis (FCR)
    • Origin: Medial epicondyle of the humerus
    • Insertion: Bases of second and third metacarpals
    • Action: Flexes and abducts (radial deviates) the wrist
  • Palmaris Longus
    • Origin: Medial epicondyle of the humerus
    • Insertion: Palmar aponeurosis and flexor retinaculum
    • Action: Tenses the palmar fascia; weak wrist flexor
  • Flexor Digitorum Superficialis (FDS)
    • Heads: Humeroulnar head from medial epicondyle and coronoid process; radial head from anterior radius
    • Insertion: Shafts of middle phalanges of digits II–V
    • Action: Flexes the proximal interphalangeal joints and aids in wrist flexion

Deep Anterior Forearm Muscles

  • Flexor Digitorum Profundus
    • Origin: Proximal three-quarters of the anterior and medial surfaces of the ulna and interosseous membrane
    • Insertion: Bases of the distal phalanges of digits II–V
    • Action: Flexes the distal interphalangeal joints of the fingers; assists in wrist flexion
  • Flexor Pollicis Longus
    • Origin: Anterior surface of the radius and adjacent interosseous membrane
    • Insertion: Base of the distal phalanx of the thumb (digit I)
    • Action: Flexes the interphalangeal joint of the thumb; assists in wrist flexion
  • Pronator Quadratus
    • Origin: Distal quarter of the anterior surface of the ulna
    • Insertion: Distal quarter of the anterior surface of the radius
    • Action: Pronates the forearm by pulling the distal radius over the ulna
  • Pronator Teres
    • Origin:
      • Humeral head: Medial epicondyle of the humerus
      • Ulnar head: Coronoid process of the ulna
    • Insertion: Middle of the lateral surface of the radius
    • Action: Pronates and flexes the forearm at the elbow

Deep Posterior Forearm Muscles

  • Supinator
    • Origin: Lateral epicondyle of the humerus; supinator crest and fossa of the ulna
    • Insertion: Lateral, posterior, and anterior surfaces of the proximal third of the radius
    • Action: Supinates the forearm by rotating the radius laterally
  • Abductor Pollicis Longus
    • Origin: Posterior surfaces of the ulna, radius, and interosseous membrane
    • Insertion: Base of the first metacarpal (thumb)
    • Action: Abducts and extends the carpometacarpal joint of the thumb; assists wrist abduction
  • Extensor Pollicis Brevis
    • Origin: Posterior surface of the distal third of the radius and interosseous membrane
    • Insertion: Base of the proximal phalanx of the thumb
    • Action: Extends the metacarpophalangeal joint of the thumb; assists wrist abduction
  • Extensor Pollicis Longus
    • Origin: Posterior surface of the middle third of the ulna and interosseous membrane
    • Insertion: Base of the distal phalanx of the thumb
    • Action: Extends all joints of the thumb; assists wrist extension
  • Extensor Indicis
    • Origin: Posterior surface of the ulna and interosseous membrane
    • Insertion: Extensor expansion of the index finger
    • Action: Extends the index finger; assists wrist extension
  • Extensor Digiti Minimi
    • Origin: Lateral epicondyle of the humerus (via common extensor tendon)
    • Insertion: Extensor expansion of the little finger
    • Action: Extends the little finger; assists wrist extension

4.12 Muscles of the Pelvis and Medial Thigh

Iliopsoas

A combined muscle of the psoas major and iliacus that acts as the primary hip flexor.

  • Psoas Major Origin: Bodies and transverse processes of T12–L5
  • Iliacus Origin: Iliac fossa and iliac crest
  • Insertion (both): Lesser trochanter of the femur
  • Action: Flexes and externally rotates the thigh; in stance, tilts the pelvis anteriorly

Tensor Fasciae Latae (TFL)

  • Origin: Anterior superior iliac spine (ASIS) and iliac crest
  • Insertion: Iliotibial band (IT tract) to the lateral condyle of the tibia
  • Action: Tenses the IT tract, stabilizes the extended knee; assists hip flexion and abduction

Sartorius

  • Origin: ASIS
  • Insertion: Medial surface of the proximal tibia (pes anserinus)
  • Action: Flexes, abducts, and externally rotates the thigh; flexes the leg at the knee

Pectineus

  • Origin: Superior ramus of the pubis
  • Insertion: Pectineal line of the femur (just below lesser trochanter)
  • Action: Adducts and flexes the thigh; assists external rotation

Adductor Longus

  • Origin: Body of the pubis inferior to the pubic crest
  • Insertion: Middle third of the linea aspera of the femur
  • Action: Adducts and externally rotates the thigh

Gracilis

  • Origin: Body and inferior ramus of the pubis
  • Insertion: Medial surface of the tibia (pes anserinus)
  • Action: Adducts the thigh; flexes and internally rotates the leg

4.13 Quadriceps Femoris (Anterior Thigh)

A powerful four-part muscle group that extends the leg at the knee and flexes the thigh.

Head Origin Insertion Action
Rectus Femoris ASIS (straight head) & ilium above acetabulum (reflected head) Patella and tibial tuberosity via the patellar ligament Flexes thigh; extends leg
Vastus Lateralis Greater trochanter & lateral linea aspera Patella → tibial tuberosity Extends leg
Vastus Medialis Intertrochanteric line & medial linea aspera Patella → tibial tuberosity Extends leg; stabilizes patella medially
Vastus Intermedius Anterior and lateral femoral shaft Patella → tibial tuberosity Extends leg

4.14 Anterior and Lateral Leg Muscles

Tibialis Anterior

  • Origin: Lateral tibial condyle and shaft
  • Insertion: Medial cuneiform and base of the first metatarsal
  • Action: Dorsiflexes and inverts the foot; supports medial longitudinal arch

Extensor Digitorum Longus

  • Origin: Lateral tibial condyle; anterior fibula; interosseous membrane
  • Insertion: Distal phalanges of toes II–V
  • Action: Extends toes II–V; dorsiflexes foot

Fibularis (Peroneus) Longus

  • Origin: Head and upper two-thirds of lateral fibula
  • Insertion: Base of first metatarsal and medial cuneiform (passes under foot)
  • Action: Everts and plantarflexes the foot; supports transverse arch

Fibularis (Peroneus) Brevis

  • Origin: Lower two-thirds of lateral fibula
  • Insertion: Tuberosity of the fifth metatarsal
  • Action: Everts and plantarflexes the foot

4.15 Gluteal and Posterior Lower Limb Muscles

Gluteus Maximus

  • Origin: Posterior ilium, sacrum, coccyx, sacrotuberous ligament, thoracolumbar fascia, and the fascia of gluteus medius
  • Insertion: Iliotibial tract and gluteal tuberosity of the femur
  • Action:
    • Powerful extension and external rotation of the thigh
    • Assists in maintaining knee extension via tension on the iliotibial tract
    • Extends the trunk when the femur is fixed (e.g., rising from sitting)

Hamstrings (Ischiocrural Muscles)

All arise from the ischial tuberosity (except short head of biceps femoris) and cross both hip and knee joints.

Muscle Origin Insertion Action
Biceps Femoris Long head: ischial tuberosityShort head: linea aspera & lateral supracondylar line of femur Head of fibula Flexes and externally rotates the leg; long head also extends the thigh
Semitendinosus Ischial tuberosity Medial tibial condyle via pes anserinus tendon Flexes and internally rotates the leg; extends the thigh
Semimembranosus Ischial tuberosity Posterior medial condyle of tibia (and oblique popliteal ligament) Flexes and internally rotates the leg; extends the thigh; stabilizes knee joint capsule

4.16 Superficial Posterior Leg Muscles

Gastrocnemius

  • Heads & Origin:
    • Medial head: medial femoral condyle and popliteal surface
    • Lateral head: lateral femoral condyle
  • Insertion: Calcaneus via the Achilles (calcaneal) tendon (with soleus and plantaris)
  • Action:
    • Plantarflexes the foot at the ankle when the knee is extended
    • Flexes the leg at the knee

Plantaris

  • Origin: Lateral supracondylar line of femur and oblique popliteal ligament
  • Insertion: Posterior calcaneus via the Achilles tendon
  • Action: Weak plantarflexion of the foot and knee flexion; often harvested for tendon grafts

Soleus

  • Origin: Posterior fibular head, proximal tibia, and the soleal line
  • Insertion: Calcaneus via the Achilles tendon
  • Action: Powerful plantarflexion of the foot; important for postural control

4.17 Iliotibial Tract

  • A thickened lateral portion of the fascia lata.
  • Proximal attachment: Fascia of tensor fasciae latae and gluteus maximus
  • Distal attachment: Lateral condyle of the tibia (Gerdy’s tubercle) and lateral patellar retinaculum
  • Function: Stabilizes the extended knee in standing and assists lateral support during gait

 

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