Skeletal Muscle Actions

Muscles of Facial Expression:

• Epicranius (Frontal Belly/Frontalis):

  • Action: Raises eyebrows, wrinkles forehead.

• Orbicularis Oris:

  • Action: Purses lips.

  • Also Known As: "Kissing muscle".

  • Type: Sphincter muscle.

• Orbicularis Oculi:

  • Action: Closes eye.

  • Also Known As: "Winking muscle".

  • Type: Sphincter muscle.

• Zygomaticus:

  • Action: Raises lateral mouth corners.

  • Also Known As: "Smiling muscle."

• Buccinator:

  • Action: Compresses cheek (not visible superficially).

  • Also Known As: "Trumpeter’s muscle".

  • Categorization: Chewing muscle.

• Mentalis:

  • Action: Wrinkles chin.

  • Also Known As: "Pouting muscle".

Muscles of Mastication (Chewing):

• Definition: Mastication = Chewing

• Temporalis:

  • Action: Closes jaw at TMJ (temporomandibular joint).

• Masseter:

  • Action: Closes jaw at TMJ (temporomandibular joint).

  • The primary chewing muscle.

Muscles Involved in Head Movement and Trunk Extension:

• Sternocleidomastoid (SCM):

  • Shape: Anterior V-shaped.

  • Action: Flexes neck (B), rotates head (U).

• Scalenes (Anterior, Middle, Posterior):

  • Position: More lateral.

  • Action: Flex neck (B), rotates neck (B).

• Splenius Capitus:

  • Action: Extends/hyperextends head (capitus = head, splenius = bandage).

• Erector Spinae (Iliocostalis, Longissimus, Spinalis):

  • Action: Extends/hyperextends the spine/trunk.

  • Significance: Important postural muscle.

Muscles Involved in Scapula or Arm Movement:

• Pectoralis Major:

  • Action: Flexes, adducts, medially rotates shoulder.

• Pectoralis Minor:

  • Position: Small muscle deep to pec major.

  • Action: Depresses scapula. Raises ribcage if the scapula is fixed (this expands ribcage for deeper inhalation).

• Serratus Anterior:

  • Action: Protracts scapula (moves the scapula anteriorly around the ribcage).

  • Features: Has obvious serration on the lateral ribs.

  • Also Known As: "Boxer’s muscle".

• Deltoid:

  • Shape: Large, superficial triangle-shaped muscle.

  • Action: Abducts shoulder. Rounds out the lateral shoulder.

• Trapezius:

  • Location: Superior superficial diamond-shaped back muscle.

  • Action: Upper traps elevate shoulders.

• Latissimus Dorsi:

  • Location: Inferior superficial back muscle.

  • Also Known As: "Swimmer’s" or "hand-cuff" muscle due to its actions.

• Rhomboids (Major & Minor):

  • Action: Retract scapulae (move scapulae closer together).

  • Also Known As: "Christmas tree" muscle because of the fiber arrangement.

• Teres Major:

  • Action: extend, adducts, and internally rotate the humerus

• Levator Scapulae:

  • Action: Elevates scapula (raises shoulders).

• Rotator Cuff Muscle Group:

  • Function: Stabilize the head of humerus in glenoid fossa so the larger, stronger shoulder muscles can work more efficiently.

  • Individual actions on the shoulder joint are not required (e.g. medial or lateral rotation).

  • Muscles:

    • Teres Minor

    • Supraspinatus

    • Infraspinatus

    • Subscapularis

Muscles That Move the Trunk and Compress the Abdominal Viscera:

• Function: These muscles form the abdominal wall; all 4 muscles compress abdominal contents.

• Rectus Abdominus:

  • Action: Flexes trunk.

  • Also Known As: "6-pack" muscle.

• External Obliques:

  • Position: Lateral to RA.

  • Action: Flexes & also laterally flex trunk.

• Internal Obliques:

  • Position: Deep to external obliques.

  • Action: Flexes & also laterally flex trunk.

• Transversus Abdominus:

  • Action: Only compresses abdominal contents.

Muscles of Respiration (NOT IN THE LAB MANUAL):

• Intercostals (External & Internal):

  • Action: Move the ribs during inhalation & exhalation.

• Diaphragm:

  • Action: 1° breathing muscle, increases ribcage volume (for inhalation).

Muscles Crossing the Elbow Joint That Act on the Forearm:

• Triceps Brachii:

  • Action: Extends elbow (tri = 3 heads).

• Biceps Brachii:

  • Action: Flexes/supinates elbow because it attaches to radius (bi = 2 heads).

• Brachioradialis:

  • Action: Flexes elbow (esp. in neutral position).

  • Also Known As: "Drinking muscle".

• Brachialis:

  • Action: Flexes elbow (deep to biceps).

  • Function: Synergist to biceps.

Forearm Muscles That Act on Hand/Fingers:

• Flexors (Anterior Forearm):

  • Flexor Carpi Radialis: Lies on thumb (radial) side.

  • Flexor Carpi Ulnaris: Lies on pinky (ulnar) side.

  • Palmaris Longus: Central, often absent.

  • Pronator Teres: Not a wrist flexor. Pronates forearm.

• Extensors (Posterior Forearm):

  • Extensor Carpi Radialis (combine longus & brevis): Lies on thumb (radial) side.

  • Extensor Carpi Ulnaris: Lies on pinky (ulnar) side.

  • Extensor Digitorum: Also extends the fingers.

Muscles That Act on the Thigh/Leg:

• Iliopsoas (Combined Iliacus & Psoas Major):

  • Feature: Have common insertion on the femur.

  • Action: Flex hip.

• Sartorius:

  • Characteristics: Strap-like muscle crossing over the quadriceps, longest muscle in body.

  • Also Known As: "Tailor’s muscle".

• Adductor Longus:

  • Action: Adducts femur (bring thighs together).

• Gracilis:

  • Action: Adducts femur (bring thighs together).

  • Also Known As: "Pant seam" muscle.

• Tensor Fascia Latae (TFL):

  • Position: Small muscle on anterolateral hip, inserts on the ITB (iliotibial band).

  • Action: Flexes/abducts thigh.

  • Also Known As: "Pocket muscle".

• Gluteus Maximus:

  • Action: Extends the hip (large, superficial gluteal muscle).

• Gluteus Medius:

  • Action: Abducts hip, stabilizes/levels pelvis when walking/standing.

• Quadriceps Femoris (4):

  • Action: all extend the knee (cross knee to insert on the tibial tuberosity).

  • Rectus Femoris: Most superficial quad muscle, central quad (rectus = straight).

  • Vastus Lateralis: Lateral to RF (vastus = large).

  • Vastus Medialis: Medial to RF, “tear-drop” shape.

  • Vastus Intermedius: Small muscle, lies deep to rectus femoris.

• Hamstring Group (3):

  • Action: Extend hip, flex the knee.

  • Biceps Femoris: Single muscle with 2 heads, lies laterally.

  • Semitendinosus: Medial HS, (per name, more tendon at knee).

  • Semimembranosus: Medial HS, (per name, more CT membrane at knee).

Muscles That Act on the Foot and Ankle:

• Fibularis/Peroneus (Includes Longus & Brevis):

  • Action: Evert foot (sole faces outward).

• Tibialis Anterior:

  • Action: Dorsiflexes ankle (toes to shin). Runs along lateral edge of shin.

• Extensor Digitorum Longus:

  • Action: Extends toes. See 4 tendons extend to toes. Lies just lateral to Tib anterior.

• Gastrocnemius:

  • Position: Superficial calf.

  • Action: Plantar flexes ankle, flexes knee (crosses knee).

• Soleus:

  • Position: Deep to gastrocs.

  • Action: Only plantarflexes ankle (doesn’t cross knee).

• Achilles (Calcaneal) Tendon:

  • Note: (not in Lab Manual).