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Chapter 11 – Muscular System: Comprehensive Bullet-Point Notes

How Skeletal Muscles Produce Movement

  • Force produced by contracting skeletal muscle is transmitted to bones via tendons ➔ bone(s) move
  • Basic mechanical schema
    • Bones = levers
    • Joints = fulcrums (pivot points)
    • Effort (E) = muscle contraction pulling on tendon
    • Load (L) = weight of body part/objects being moved
  • Most muscles span at least one joint and attach to two bones
    • Origin = attachment on stationary bone (usually proximal)
    • Insertion = attachment on moveable bone (usually distal)
    • Contraction pulls insertion toward origin

Lever Systems & Mechanical Advantage

  • Lever = rigid bar that moves about a fulcrum; in body, bone acts as lever
  • Three classes (location of F = fulcrum, E = effort, L = load)
    1. First-class (F between E & L) – rare; e.g., atlanto-occipital joint extending head (posterior neck muscles supply effort)
    2. Second-class (L between F & E) – always powerful; e.g., plantar-flexion when rising on toes (gastrocnemius/soleus supply effort, ball of foot = fulcrum, body weight = load)
    3. Third-class (E between F & L) – most common; favors speed & ROM; e.g., elbow flexion (biceps brachii supplies effort, elbow joint = fulcrum, hand = load)
  • Mechanical advantage (MA) = MA = \frac{L}{E} = \frac{length{effort\,arm}}{length{load\,arm}}
    • 2nd-class levers: MA > 1 (force advantage)
    • 3rd-class levers: MA < 1 (speed & distance advantage)

Fascicle Arrangement & Functional Consequences

  • Muscle fibers grouped into fascicles; orientation relative to tendon determines power vs. ROM
    • Parallel & Fusiform → long excursion, moderate force (e.g., sternohyoid, digastric)
    • Circular (sphincter) → control openings (e.g., orbicularis oculi)
    • Triangular (convergent) → versatile pull directions (e.g., pectoralis major)
    • Pennate types (uni-, bi-, multi-) → high fiber density, great force, limited ROM (e.g., extensor digitorum longus [uni], rectus femoris [bi], deltoid [multi])

Coordination Within Muscle Groups

  • Muscles seldom work alone; typically arranged in antagonistic pairs around joints
    • Agonist (prime mover) – chiefly responsible for action
    • Antagonist – opposes agonist; provides control & smoothness
    • Synergist – assists prime mover; adds force or stabilizes origin (fixator subtype)

Principles for Naming Skeletal Muscles

  • Location (e.g., tibialis anterior)
  • Relative size (maximus, minimus, longus, brevis, vastus, major/minor, latissimus, longissimus, magnus)
  • Shape (deltoid, trapezius, serratus, rhomboid, orbicularis, piriformis, quadratus, platys, gracilis, pectinate)
  • Direction of fascicles (rectus = parallel, transverse = perpendicular, oblique = diagonal)
  • Number of origins (biceps = 2, triceps = 3, quadriceps = 4)
  • Origin & insertion points (sternocleidomastoid – sternum/clavicle → mastoid process)
  • Action produced (flexor, extensor, abductor, adductor, levator, depressor, supinator, pronator, tensor, sphincter, rotator)
  • Combinations are common (e.g., fibularis longus = location + size)

Regional Survey of Major Skeletal Muscles (Selected Origins, Insertions, Actions)

Head & Face

  • Orbicularis oris – O: encircles mouth | I: skin at mouth corners | A: closes/protrudes lips (kissing, whistling)
  • Zygomaticus major/minor, Risorius, Platysma, etc. – move skin for facial expression
  • Masseter – O: zygomatic arch & maxilla | I: mandible | A: elevates mandible (strongest mastication muscle)
  • Extra-ocular set (6) – precise eye movements; all originate in orbit & insert on sclera
    • Superior/Inferior/Medial/Lateral recti; Superior/Inferior obliques
  • Levator palpebrae superioris – lifts upper eyelid

Neck

  • Sternocleidomastoid (SCM) – O: sternum & clavicle | I: mastoid process | A: bilateral neck flexion; unilateral rotation to opposite side
  • Suprahyoid group (digastric, stylohyoid, mylohyoid, geniohyoid) – elevate hyoid, floor of mouth; aid swallowing
  • Infrahyoid group (sternohyoid, omohyoid, sternothyroid, thyrohyoid) – depress hyoid/larynx; stabilize during speech & swallowing
  • Scalene anterior/medius/posterior – elevate ribs 1–2 (deep inspiration); flex/rotate neck

Thorax – Breathing

  • Diaphragm – principle inspiratory muscle; contraction flattens central tendon, expands thoracic cavity
  • External intercostals – elevate ribs; inspiration
  • Internal/Innermost intercostals – forceful expiration (depress ribs)

Shoulder (Pectoral Girdle Movers)

  • Pectoralis minor – ribs → coracoid; protracts & rotates scapula downward
  • Serratus anterior (“boxer’s muscle”) – ribs 1–8 → vertebral border of scapula; protracts & upwardly rotates scapula
  • Trapezius – occiput/C7–T12 → clavicle, acromion, spine; upper: elevate/up-rotate, mid: retract, lower: depress scapula
  • Levator scapulae, Rhomboid major/minor – elevate & retract scapula, downward rotation

Thorax/Shoulder – Humerus Movers (Axial & Scapular)

  • Pectoralis major – adducts & medially rotates arm; clavicular head flexes, sternocostal head extends flexed arm
  • Latissimus dorsi (“swimmer’s muscle”) – extends, adducts, medially rotates arm; draws arm down & back
  • Deltoid (multi-action) – anterior flex/medial rotate; lateral abduct; posterior extend/lateral rotate
  • Rotator cuff (SITS): Subscapularis, Supraspinatus, Infraspinatus, Teres minor – collectively stabilize glenohumeral joint & rotate humerus
  • Teres major & Coracobrachialis – assist extension, adduction, flexion respectively

Arm – Elbow/Radioulnar Movers

  • Biceps brachii – flexes elbow, supinates forearm, weak shoulder flexor
  • Brachialis – primary elbow flexor in any forearm position
  • Brachioradialis – elbow flexor best in mid-pronation
  • Triceps brachii – chief elbow extensor; long head assists shoulder extension
  • Anconeus – minor elbow extender
  • Pronator teres & Pronator quadratus – pronation
  • Supinator – supination

Forearm – Wrist/Hand/Finger Movers

  • Flexor compartment (anterior):
    • Superficial: flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis
    • Deep: flexor pollicis longus, flexor digitorum profundus
  • Extensor compartment (posterior):
    • Superficial: extensor carpi radialis longus/brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris
    • Deep: abductor pollicis longus, extensor pollicis longus/brevis, extensor indicis
  • Intrinsic hand (thenar, hypothenar, lumbricals, interossei) – fine digital control; allow opposition, grip adjustment, ab/adduction of digits

Abdominal Wall

  • Rectus abdominis – trunk flexion, abdominal compression (defecation, childbirth, forced exhalation)
  • External & Internal obliques – bilateral trunk flexion/compression; unilateral lateral flexion & rotation (external rotates trunk to opposite side)
  • Transversus abdominis – deepest; pure abdominal compression
  • Quadratus lumborum – fixes 12th rib during inspiration; lateral trunk flexion; “hip-hike” RMA

Pelvic Floor & Perineum

  • Levator ani complex (pubococcygeus, puborectalis, iliococcygeus) plus Ischiococcygeus form pelvic diaphragm
    • Support pelvic viscera, resist intra-abdominal pressure, aid continence
  • Superficial & deep perineal muscles (bulbospongiosus, ischiocavernosus, transverse perineals, urethral/anal sphincters) govern micturition, sexual function, defecation

Back – Vertebral Column Movers & Postural Muscles

  • Erector spinae (Iliocostalis, Longissimus, Spinalis) – principal extensors, maintain posture, lateral flexion
  • Transversospinalis group (Semispinalis, Multifidus, Rotatores) – fine adjustments, rotation opposite side
  • Segmental (Interspinales, Intertransversarii) – proprioception, stabilization
  • Splenius capitis/cervicis & Scalenes – head/neck extension, lateral flex, rotation

Gluteal Region – Hip Movers

  • Gluteus maximus – powerful hip extensor & external rotator (rising, climbing)
  • Gluteus medius/minimus & Tensor fasciae latae – hip abduction & medial rotation; stabilize pelvis during gait
  • Iliopsoas (psoas major + iliacus) – strongest hip flexor; also trunk flexor when femur fixed
  • Short lateral rotators (piriformis, obturators, gemelli, quadratus femoris) – stabilize & externally rotate femur
  • Adductor group (longus, brevis, magnus, pectineus, gracilis) – adduction; assist flexion or extension depending on fibers

Thigh – Knee Movers (Quadriceps & Hamstrings)

  • Quadriceps femoris (rectus femoris, vastus lateralis/medialis/intermedius) – knee extension; rectus also hip flexion
  • Sartorius – “tailor’s muscle”; hip flex/abduct/lat-rotate + knee flex (sitting cross-legged)
  • Hamstrings (biceps femoris, semitendinosus, semimembranosus) – hip extension, knee flexion; crucial in gait & propulsion

Leg – Ankle & Toe Movers

  • Anterior compartment – tibialis anterior (dorsiflex + inversion), extensor hallucis longus, extensor digitorum longus, fibularis tertius
  • Lateral compartment – fibularis (peroneus) longus & brevis (eversion + plantar flexion)
  • Superficial posterior – gastrocnemius, soleus, plantaris (plantar-flexion; gastrocnemius also knee flexion)
  • Deep posterior – popliteus (unlocks knee), tibialis posterior (inversion + PF), flexor digitorum longus, flexor hallucis longus

Foot – Intrinsic Muscles

  • Dorsum: extensor hallucis brevis, extensor digitorum brevis – extend hallux & toes 2-4
  • Plantar layers:
    1. Abductor hallucis, flexor digitorum brevis, abductor digiti minimi
    2. Quadratus plantae, lumbricals
    3. Flexor hallucis brevis, adductor hallucis, flexor digiti minimi brevis
    4. Dorsal & plantar interossei (toe ab/adduction)

Common Musculoskeletal Injuries & Disorders

  • Running injuries – mostly knee; often due to poor training. Managed with PRICE, NSAIDs, corticosteroid injections, rehab
  • Compartment syndrome – ↑ pressure in fascial compartment ➔ vascular & neural compromise; untreated ➔ scar/contracture
  • Plantar fasciitis – chronic irritation of plantar aponeurosis at calcaneal origin; managed with ice/heat, stretching, weight loss, orthoses, steroids, surgery

Ethical / Practical Considerations

  • Understanding lever classes & fascicle design informs ergonomic design, rehabilitation protocols, and athlete training
  • Muscle nomenclature standardization allows clear communication across clinical & research settings
  • Awareness of compartment syndrome highlights importance of timely diagnosis to prevent permanent disability