Comprehensive Anatomy Notes: Muscles
and Movements of the Human Body
1. Introduction
Muscles are the active contractile tissues of the human body responsible for generating
movement, maintaining posture, and producing heat. They are classified as skeletal,
cardiac, or smooth, with skeletal muscles enabling voluntary movements of the limbs
and trunk. Understanding muscle anatomy, origin, insertion, innervation, and
biomechanical function is essential for medical students, physiotherapists, and
clinicians. Muscles work in groups, forming agonist-antagonist pairs and synergists to
produce precise, coordinated movements. Knowledge of these patterns is critical for
rehabilitation, sports medicine, and surgical planning.
2. Classification of Skeletal Muscles
2.1. By Structure
1. Fusiform: Spindle-shaped, e.g., biceps brachii.
2. Pennate: Fibers attach obliquely to tendon; can be unipennate (extensor
digitorum), bipennate (rectus femoris), or multipennate (deltoid).
3. Convergent: Broad origin, narrow insertion, e.g., pectoralis major.
4. Circular: Surround openings, e.g., orbicularis oris.
2.2. By Function
● Agonist: Prime mover for an action.
● Antagonist: Opposes the action of agonist.
● Synergist: Assists agonist.
● Fixator: Stabilizes origin of agonist.
3. Upper Limb Muscles
3.1. Shoulder Muscles
● Deltoid: Origin clavicle and scapula, inserts into humerus; abduction of arm.
● Rotator Cuff (Supraspinatus, Infraspinatus, Teres Minor, Subscapularis):
Stabilizes shoulder, rotates arm.
Movement Patterns:
● Flexion/extension: Anterior/posterior deltoid
● Abduction: Deltoid and supraspinatus
● Rotation: Infraspinatus (external), subscapularis (internal)
3.2. Arm Muscles
● Anterior (Flexors): Biceps brachii (flexion, supination), brachialis (flexion),
coracobrachialis (flexion, adduction)
● Posterior (Extensors): Triceps brachii (extension of elbow), anconeus (assists
extension)
3.3. Forearm Muscles
● Anterior Compartment: Flexor carpi radialis, flexor carpi ulnaris, palmaris longus,
flexor digitorum superficialis/profundus; all responsible for wrist and finger flexion.
● Posterior Compartment: Extensor carpi radialis longus/brevis, extensor digitorum,
extensor pollicis longus/brevis; responsible for wrist and finger extension.
3.4. Hand Muscles
● Thenar Muscles: Opponens pollicis, abductor pollicis brevis, flexor pollicis brevis;
enable thumb opposition.
● Hypothenar Muscles: Abductor digiti minimi, flexor digiti minimi brevis, opponens
digiti minimi; control little finger movement.
● Lumbricals and Interossei: Fine motor control of fingers.
4. Lower Limb Muscles
4.1. Hip Muscles
● Gluteus Maximus: Extends thigh, lateral rotation.
● Gluteus Medius/Minimus: Abducts thigh, medial rotation.
● Iliopsoas: Flexes thigh at hip joint.
4.2. Thigh Muscles
● Anterior Compartment: Quadriceps femoris (rectus femoris, vastus lateralis,
medialis, intermedius) – knee extension.
● Posterior Compartment: Hamstrings (biceps femoris, semitendinosus,
semimembranosus) – hip extension, knee flexion.
● Medial Compartment: Adductor longus, magnus, gracilis – thigh adduction.
4.3. Leg Muscles
● Anterior Compartment: Tibialis anterior (dorsiflexion), extensor hallucis longus,
extensor digitorum longus.
● Posterior Compartment: Gastrocnemius, soleus (plantarflexion), tibialis posterior
(foot inversion).
● Lateral Compartment: Fibularis longus/brevis – foot eversion.
4.4. Foot Muscles
● Intrinsic Muscles: Flexor digitorum brevis, abductor hallucis, lumbricals – assist in
toe movements and maintaining arches.
5. Biomechanics of Muscle Action
● Lever Systems: Muscles act on bones as levers; bones are rigid bars, joints are
fulcrums, and muscles generate force.
● Types of Muscle Contractions:
○ Isotonic: Muscle length changes (concentric/shortening,
eccentric/lengthening)
○ Isometric: Muscle length constant, tension increases
● Synergistic Action: Multiple muscles work together to produce smooth movement
6. Muscle Innervation and Blood Supply
● Upper Limb: Brachial plexus (C5–T1)
○ Axillary: Deltoid, teres minor
○ Musculocutaneous: Anterior arm
○ Radial: Posterior arm and forearm
○ Median/Ulnar: Forearm and hand flexors, intrinsic hand muscles
● Lower Limb: Lumbar-sacral plexus (L2–S3)
○ Femoral: Quadriceps
○ Obturator: Adductors
○ Sciatic: Posterior thigh and leg muscles
○ Tibial/Common Fibular: Posterior/anterior compartments of leg
Blood Supply: Muscles receive arterial blood via major limb arteries (brachial, radial,
ulnar, femoral, popliteal, tibial) with capillary networks supporting metabolic activity.
7. Muscle Adaptation and Clinical Considerations
1. Hypertrophy: Response to resistance training
2. Atrophy: Disuse or denervation leads to loss of muscle mass
3. Spasm/Cramp: Sudden involuntary contraction, often due to electrolyte
imbalance
4. Tendon Injuries: Rupture or tendinopathy affects muscle function
5. Rehabilitation: Focus on strength, flexibility, and neuromuscular coordination
8. Common Muscle Injuries
● Upper Limb: Rotator cuff tears, biceps tendon rupture, forearm flexor/extensor
strains
● Lower Limb: Hamstring strains, quadriceps contusions, Achilles tendon rupture
● Prevention: Warm-up, stretching, strength training, ergonomic posture
9. Summary of Major Movements
Movement
Prime Muscles
Joint Involved
Innervation
Shoulder abduction
Deltoid,
supraspinatus
Glenohumeral
Axillary,
Suprascapular
Elbow flexion
Biceps brachii,
brachialis
Humeroulnar
Musculocutaneous
Wrist extension
Extensor carpi
radialis/brevis
Radiocarpal
Radial
Hip flexion
Iliopsoas
Hip
Femoral
Knee extension
Quadriceps
Knee
Femoral
Ankle dorsiflexion
Tibialis anterior
Talocrural
Deep fibular
Plantarflexion
Gastrocnemius,
soleus
Talocrural
Tibial
10. Conclusion
Muscles are the engines of human movement, coordinating with bones, joints, and
nerves to perform complex actions. Upper limb muscles emphasize mobility and
precision, whereas lower limb muscles prioritize stability and locomotion. Detailed
knowledge of anatomy, function, innervation, and clinical relevance is essential for
effective diagnosis, rehabilitation, and surgical intervention. Proper care and training
prevent injury, enhance performance, and maintain functional independence.
