Explain me muscles of upper and lower limbs.

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Muscles of the Upper and Lower Limbs

Sources: Gray's Anatomy for Students; THIEME Atlas of General Anatomy and Musculoskeletal System

UPPER LIMB

The muscles of the upper limb are organized into functional-topographical groups: shoulder girdle, shoulder joint, arm, forearm, and hand. The arm and forearm are each divided into anterior (flexor) and posterior (extensor) compartments by intermuscular septa, bones, and fascia.

Cross-sections of the arm (top) and forearm (bottom) showing anterior and posterior compartments - Gray's Anatomy for Students

1. Muscles of the Shoulder Girdle

These muscles move and stabilize the scapula and clavicle relative to the trunk.

Muscles that migrated from the head:

Trapezius
Sternocleidomastoid
Omohyoid

Posterior trunk/shoulder girdle muscles:

Rhomboid major and minor - retract the scapula
Levator scapulae - elevates the scapula

Anterior trunk/shoulder girdle muscles:

Subclavius
Pectoralis minor - protracts and depresses the scapula
Serratus anterior - protracts the scapula, holds it against the thorax ("winging" occurs on palsy)

2. Muscles of the Shoulder Joint (Acting on the Humerus)

Muscles of the shoulder - (A) posterior, (B) anterior, (C) rotator cuff - Gray's Anatomy for Students

The Rotator Cuff (SITS) - the four muscles that wrap the glenohumeral joint and provide dynamic stability:

Muscle	Action	Nerve
Supraspinatus	Initiates abduction (0-15°)	Suprascapular n.
Infraspinatus	External rotation	Suprascapular n.
Teres minor	External rotation	Axillary n.
Subscapularis	Internal rotation	Upper/lower subscapular n.

Other shoulder joint muscles:

Deltoid - main abductor of shoulder (middle fibers), flexion (anterior), extension (posterior)
Pectoralis major - flexion, adduction, medial rotation
Latissimus dorsi - extension, adduction, medial rotation
Teres major - adduction, medial rotation
Coracobrachialis - flexion and adduction

3. Muscles of the Arm

Separated into two compartments by the humerus and intermuscular septa.

Anterior compartment (flexors of the elbow):

Muscle	Origin	Action	Nerve
Biceps brachii	Coracoid process + supraglenoid tubercle	Elbow flexion, supination	Musculocutaneous
Brachialis	Anterior humerus	Elbow flexion (prime mover)	Musculocutaneous
Coracobrachialis	Coracoid process	Shoulder flexion/adduction	Musculocutaneous

Posterior compartment (extensors of the elbow):

Muscle	Action	Nerve
Triceps brachii (3 heads: long, medial, lateral)	Elbow extension	Radial n.
Anconeus	Assists elbow extension	Radial n.

4. Muscles of the Forearm

Anterior compartment (flexors - median and ulnar nerves):

Superficial layer:

Pronator teres - pronation + weak elbow flexion
Flexor carpi radialis - wrist flexion, radial deviation
Palmaris longus - weak wrist flexion (absent in ~15%)
Flexor carpi ulnaris - wrist flexion, ulnar deviation
Flexor digitorum superficialis - flexes middle phalanges of fingers 2-5

Deep layer:

Flexor digitorum profundus - flexes distal phalanges (median + ulnar)
Flexor pollicis longus - flexes distal phalanx of thumb (median)
Pronator quadratus - pronation (median)

Posterior compartment (extensors - radial nerve):

Superficial:

Extensor carpi radialis longus and brevis
Extensor digitorum - extends fingers 2-5
Extensor digiti minimi - extends little finger
Extensor carpi ulnaris

Deep:

Supinator - supination
Abductor pollicis longus
Extensor pollicis brevis and longus
Extensor indicis

5. Intrinsic Muscles of the Hand

Thenar muscles (act on thumb - median nerve):

Abductor pollicis brevis
Flexor pollicis brevis
Opponens pollicis - opposition of thumb

Hypothenar muscles (act on little finger - ulnar nerve):

Abductor digiti minimi
Flexor digiti minimi brevis
Opponens digiti minimi

Other intrinsic hand muscles:

Lumbricals (1st and 2nd - median; 3rd and 4th - ulnar) - flex MCP, extend IP joints
Dorsal interossei (4) - abduct fingers (DAB), ulnar nerve
Palmar interossei (3) - adduct fingers (PAD), ulnar nerve
Adductor pollicis - adducts thumb, ulnar nerve

LOWER LIMB

The lower limb is specialized for weight-bearing and bipedal locomotion, making its muscles much more massive and powerful than their upper limb counterparts. The hip and gluteal muscles are particularly large because they support and stabilize the entire body weight.

Thigh (3 compartments) and leg (3 compartments) cross-sectional organization - Gray's Anatomy for Students

1. Hip and Gluteal Muscles

Inner hip muscles (Iliopsoas):

Psoas major - arises from lumbar vertebrae (L1-L5)
Iliacus - arises from iliac fossa
Together they form the iliopsoas, the most powerful hip flexor. Supplied by femoral nerve (L2-L4) and direct branches (L1-L3).

Outer hip / Gluteal muscles:

Muscle	Action	Nerve
Gluteus maximus	Hip extension, lateral rotation	Inferior gluteal (L5-S2)
Gluteus medius	Hip abduction, stabilizes pelvis	Superior gluteal (L4-S1)
Gluteus minimus	Hip abduction, medial rotation	Superior gluteal (L4-S1)
Tensor fasciae latae	Hip flexion, abduction, medial rotation	Superior gluteal (L4-S1)
Piriformis	Lateral rotation	Sacral plexus (L5-S2)
Obturator internus	Lateral rotation	Sacral plexus
Gemelli (superior + inferior)	Lateral rotation	Sacral plexus
Quadratus femoris	Lateral rotation	Sacral plexus

Adductor group (medial compartment of thigh, act mainly on hip):

Obturator externus, Pectineus, Adductor longus, Adductor brevis, Adductor magnus, Adductor minimus, Gracilis
Innervated by the obturator nerve (L2-L4); also femoral nerve for pectineus

2. Muscles of the Thigh

The thigh has three compartments:

Anterior compartment - Knee extensors (femoral nerve L2-L4):

Muscle	Notes
Quadriceps femoris	The large extensor; 4 heads - rectus femoris, vastus lateralis, vastus medialis, vastus intermedius
Sartorius	Longest muscle in the body; flexes, abducts, laterally rotates thigh; also flexes knee

Posterior compartment - Hamstrings (sciatic nerve L5-S2): The hamstrings extend the hip and flex the knee.

Muscle	Notes
Biceps femoris	Has a long and short head
Semimembranosus	Also medially rotates the tibia
Semitendinosus	Long tendon, medially rotates tibia

3. Muscles of the Leg

The leg has three compartments:

Anterior compartment - Dorsiflexion and toe extension (deep fibular/peroneal nerve):

Tibialis anterior - dorsiflexion + inversion
Extensor digitorum longus - extends toes 2-5, dorsiflexion
Extensor hallucis longus - extends big toe, dorsiflexion
Fibularis tertius

Lateral compartment - Eversion (superficial fibular nerve):

Fibularis (peroneus) longus - eversion + plantar flexion
Fibularis (peroneus) brevis - eversion

Posterior compartment - Plantar flexion (tibial nerve):

Superficial (Triceps surae - the most powerful plantar flexors):

Gastrocnemius (medial and lateral heads) - plantarflexion + knee flexion
Soleus - plantarflexion only
Plantaris - weak assistant

Gastrocnemius + Soleus = Triceps surae → insert via the Achilles/calcaneal tendon

Deep:

Tibialis posterior - plantarflexion + inversion, supports medial arch
Flexor digitorum longus - flexes toes 2-5, plantarflexion
Flexor hallucis longus - flexes big toe, plantarflexion
Popliteus - unlocks the knee (laterally rotates femur on fixed tibia)

4. Intrinsic Muscles of the Foot

Dorsal surface:

Extensor digitorum brevis
Extensor hallucis brevis

Plantar surface (three compartments):

Medial:

Abductor hallucis
Flexor hallucis brevis

Lateral:

Abductor digiti minimi
Flexor digiti minimi brevis
Opponens digiti minimi

Central:

Flexor digitorum brevis
Adductor hallucis (transverse and oblique heads)
Quadratus plantae
Four lumbricals
Three plantar interossei
Four dorsal interossei

The intrinsic foot muscles provide dynamic support for the longitudinal arch and fine-tune toe movements during walking.

Key Innervation Summary

Region	Key Nerve	Muscles
Shoulder girdle	Accessory (CN XI)	Trapezius
Arm (anterior)	Musculocutaneous (C5-C7)	Biceps, brachialis, coracobrachialis
Arm (posterior)	Radial (C5-C8)	Triceps, anconeus
Forearm (anterior)	Median + ulnar nerves	Flexors, pronators
Forearm (posterior)	Radial nerve	Extensors, supinator
Hand (thenar)	Median nerve	Thenar muscles, lumbricals 1+2
Hand (hypothenar)	Ulnar nerve	Hypothenar, interossei, lumbricals 3+4
Hip flexors	Femoral (L2-L4)	Iliopsoas, sartorius, rectus femoris
Hip extensors	Inferior gluteal (L5-S2)	Gluteus maximus
Hip abductors	Superior gluteal (L4-S1)	Gluteus medius/minimus, TFL
Thigh (anterior)	Femoral nerve	Quadriceps
Thigh (posterior)	Sciatic nerve	Hamstrings
Thigh (medial)	Obturator nerve	Adductors
Leg (anterior)	Deep fibular nerve	Dorsiflexors
Leg (lateral)	Superficial fibular nerve	Everters
Leg (posterior)	Tibial nerve	Plantar flexors

Sources: THIEME Atlas of General Anatomy and Musculoskeletal System (ISBN 9781626237186); Gray's Anatomy for Students (ISBN 9780323934237)

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