Upper and Lower Extremity MIRS Required Second-Order Information

Introduction and Axio-Appendicular Muscles

The following study notes cover the required second-order information for the Upper and Lower Extremity MIRS students. Attachment sites for muscles (MM) have been simplified to focus primarily on bony attachments. Information provided in parentheses regarding complicated attachments is supplemental and not required for the examination.

Posterior Axio-Appendicular Muscle

• Latissimus dorsi

  • Proximal Attachment: Thoracic and lumbar vertebra, thoracolumbar fascia, ileum, ribs, and scapula.

  • Distal Attachment: Humerus.

  • Innervation: Thoracodorsal nerve.

  • Main Action: Extends, adducts, and medially rotates the arm. It also assists in respiration.

Anterior Axio-Appendicular Muscles

• Pectoralis major

  • Proximal Attachment: Clavicular head: clavicle; Sternocostal head: sternum, costal cartilages.

  • Distal Attachment: Humerus.

  • Innervation: Lateral and medial pectoral nerves.

  • Main Action: Adducts and medially rotates humerus; flexes and extends humerus.

• Pectoralis minor

  • Proximal Attachment: $3^{rd}–5^{th}$ ribs.

  • Distal Attachment: Scapula.

  • Innervation: Medial pectoral nerve.

  • Main Action: Stabilizes scapula by drawing it inferiorly and anteriorly against the thoracic wall.

• Subclavius

  • Proximal Attachment: $1^{st}$ rib and its costal cartilage.

  • Distal Attachment: Clavicle.

  • Innervation: Nerve to subclavius.

  • Main Action: Anchors and depresses the clavicle.

• Serratus anterior

  • Proximal Attachment: $1^{st}–8^{th}$ ribs.

  • Distal Attachment: Scapula.

  • Innervation: Long thoracic nerve.

  • Main Action: Protracts scapula and holds it against the thoracic wall; superiorly rotates scapula so the glenoid cavity is facing up.

Scapulohumeral and Arm Muscles

Scapulohumeral Muscles

• Deltoid

  • Proximal Attachment: Clavicle; Scapula.

  • Distal Attachment: Humerus.

  • Innervation: Axillary nerve.

  • Main Action: Anterior part: flexes and medially rotates arm; Middle part: abducts arm; Posterior part: extends and laterally rotates arm.

• Supraspinatus

  • Proximal Attachment: Supraspinous fossa of scapula.

  • Distal Attachment: Humerus.

  • Innervation: Suprascapular nerve.

  • Main Action: Initiates and assists deltoid in abduction of arm; acts with rotator cuff muscles; stabilizes humeral head in glenoid cavity.

• Infraspinatus

  • Proximal Attachment: Infraspinous fossa of scapula.

  • Distal Attachment: Humerus.

  • Innervation: Suprascapular nerve.

  • Main Action: Laterally rotates arm; may assist in adduction of arm; acts with rotator cuff muscles; stabilizes humeral head in glenoid cavity.

• Teres minor

  • Proximal Attachment: Scapula.

  • Distal Attachment: Humerus.

  • Innervation: Axillary nerve.

  • Main Action: Laterally rotates arm; weak adductor of arm; acts with rotator cuff muscles; stabilizes humeral head in glenoid cavity.

• Teres major

  • Proximal Attachment: Scapula.

  • Distal Attachment: Humerus.

  • Innervation: Lower subscapular nerve.

  • Main Action: Adducts, extends and medially rotates arm.

• Subscapularis

  • Proximal Attachment: Subscapular fossa of scapula.

  • Distal Attachment: Humerus.

  • Innervation: Upper and lower subscapular nerves.

  • Main Action: Medially rotates arm; helps hold head of humerus in glenoid cavity.

Anterior Arm Muscles

• Biceps brachii

  • Proximal Attachment: Short head: Scapula; Long head: Scapula.

  • Distal Attachment: Radius.

  • Innervation: Musculocutaneous nerve.

  • Main Action: Supinates forearm (most powerful in body) and, when it is supine, flexes forearm.

• Coracobrachialis

  • Proximal Attachment: Scapula.

  • Distal Attachment: Humerus.

  • Innervation: Musculocutaneous nerve.

  • Main Action: Helps flex and adduct arm.

• Brachialis

  • Proximal Attachment: Humerus.

  • Distal Attachment: Ulna.

  • Innervation: Musculocutaneous nerve and radial nerve.

  • Main Action: Flexes forearm in all positions.

Posterior Arm Muscles

• Triceps brachii

  • Proximal Attachment: Long head: Scapula; Lateral head: Humerus; Medial head: Humerus.

  • Distal Attachment: Ulna.

  • Innervation: Radial nerve.

  • Main Action: Prime mover of forearm extension.

• Anconeus

  • Proximal Attachment: Humerus.

  • Distal Attachment: Ulna.

  • Innervation: Radial nerve.

  • Main Action: Assists triceps in extending forearm.

Muscles of the Forearm

Anterior Forearm Muscles

• Pronator teres

  • Proximal Attachment: Humerus.

  • Distal Attachment: Radius.

  • Innervation: Median nerve.

  • Main Action: Pronates and flexes forearm.

• Flexor carpi radialis (FCR)

  • Proximal Attachment: Humerus.

  • Distal Attachment: $2^{nd}$ metacarpal.

  • Innervation: Median nerve.

  • Main Action: Flexes and abducts hand.

• Palmaris longus

  • Proximal Attachment: Humerus.

  • Distal Attachment: Flexor retinaculum and palmar aponeurosis.

  • Innervation: Median nerve.

  • Main Action: Flexes hand (at wrist) and tenses palmar aponeurosis.

• Flexor carpi ulnaris (FCU)

  • Proximal Attachment: Humerus and ulna.

  • Distal Attachment: Pisiform, hook of hamate, and $5^{th}$ metacarpal.

  • Innervation: Ulnar nerve.

  • Main Action: Flexes and adducts hand (at wrist).

• Flexor digitorum superficialis (FDS)

  • Proximal Attachment: Humerus and ulna.

  • Distal Attachment: Middle phalanges of $2^{nd}–5^{th}$ digits.

  • Innervation: Median nerve.

  • Main Action: Flexes middle proximal interphalangeal joints (PIP joints) and metacarpophalangeal joints (MCP joints); flexes wrist.

• Flexor digitorum profundus (FDP)

  • Proximal Attachment: Ulna (and interosseous membrane).

  • Distal Attachment: Distal phalanges of $2^{nd}–5^{th}$ digits.

  • Innervation: Ulnar nerve and median nerve.

  • Main Action: Flexes distal interphalangeal joints (DIP joints), proximal interphalangeal joints (PIP joints), and metacarpophalangeal joints (MCP joints) of digits $2^{nd}–5^{th}$; flexes wrist.

• Flexor pollicis longus (FPL)

  • Proximal Attachment: Radius (and interosseous membrane).

  • Distal Attachment: Distal phalanx of thumb.

  • Innervation: Median nerve.

  • Main Action: Flexes phalanges of $1^{st}$ digit (thumb).

• Pronator quadratus

  • Proximal Attachment: Ulna.

  • Distal Attachment: Radius.

  • Innervation: Median nerve.

  • Main Action: Pronates forearm.

Posterior Forearm Muscles

• Brachioradialis

  • Proximal Attachment: Humerus.

  • Distal Attachment: Radius.

  • Innervation: Radial nerve.

  • Main Action: Relatively weak flexion of forearm; maximal when forearm is in midpronated position.

• Extensor carpi radialis longus (ECRL)

  • Proximal Attachment: Humerus.

  • Distal Attachment: $2^{nd}$ metacarpal.

  • Innervation: Radial nerve.

  • Main Action: Extend and abduct hand at the wrist joint.

• Extensor carpi radialis brevis (ECRB)

  • Proximal Attachment: Humerus.

  • Distal Attachment: $3^{rd}$ metacarpal.

  • Innervation: Radial nerve.

  • Main Action: Extend and abduct hand at wrist joint.

• Extensor digitorum

  • Proximal Attachment: Humerus.

  • Distal Attachment: Extensor expansions.

  • Innervation: Radial nerve.

  • Main Action: Extends medial four digits at metacarpophalangeal joints (MCP joints) and interphalangeal joints (IP joints).

• Extensor digiti minimi (EDM)

  • Proximal Attachment: Humerus.

  • Distal Attachment: Extensor expansion of $5^{th}$ digit.

  • Innervation: Radial nerve.

  • Main Action: Extends $5^{th}$ digit at metacarpophalangeal joint and at interphalangeal joint (IP joint).

• Extensor carpi ulnaris (ECU)

  • Proximal Attachment: Humerus.

  • Distal Attachment: $5^{th}$ metacarpal.

  • Innervation: Radial nerve.

  • Main Action: Extends and adducts hand.

• Supinator

  • Proximal Attachment: Humerus (and other ligaments of the elbow).

  • Distal Attachment: Radius.

  • Innervation: Radial nerve.

  • Main Action: Supinates forearm.

• Extensor indicis

  • Proximal Attachment: Ulna (and interosseous membrane).

  • Distal Attachment: Extensor expansion of $2^{nd}$ digit.

  • Innervation: Radial nerve.

  • Main Action: Extends $2^{nd}$ digit (enabling its independent extension); helps extend hand at wrist.

• Abductor pollicis longus (APL)

  • Proximal Attachment: Ulna, radius, (and interosseous membrane).

  • Distal Attachment: $1^{st}$ metacarpal.

  • Innervation: Radial nerve.

  • Main Action: Abducts thumb.

• Extensor pollicis longus (EPL)

  • Proximal Attachment: Ulna and (interosseous membrane).

  • Distal Attachment: Distal phalanx of thumb.

  • Innervation: Radial nerve.

  • Main Action: Extends distal interphalangeal joint (DIP joint) at thumb; extends metacarpophalangeal (MCP joint) and carpometacarpal joints.

• Extensor pollicis brevis (EPB)

  • Proximal Attachment: Radius and (interosseous membrane).

  • Distal Attachment: Proximal phalanx of thumb.

  • Innervation: Radial nerve.

  • Main Action: Extends metacarpophalangeal joint (MCP joint of thumb); extends carpometacarpal joint.

Intrinsic Hand Muscles

• Abductor pollicis brevis

  • Proximal Attachment: Scaphoid and trapezium.

  • Distal Attachment: Proximal phalanx of thumb.

  • Innervation: Median nerve.

  • Main Action: Abducts thumb.

• Flexor pollicis brevis

  • Proximal Attachment: Scaphoid and trapezium.

  • Distal Attachment: Proximal phalanx of thumb.

  • Innervation: Median nerve and ulnar nerve.

  • Main Action: Flexes thumb.

• Opponens pollicis

  • Proximal Attachment: Scaphoid and trapezium.

  • Distal Attachment: $1^{st}$ metacarpal.

  • Innervation: Median nerve.

  • Main Action: To oppose thumb.

• Adductor pollicis

  • Proximal Attachment: $2^{nd}$ and $3^{rd}$ metacarpals, capitate.

  • Distal Attachment: Proximal phalanx of thumb.

  • Innervation: Ulnar nerve.

  • Main Action: Adducts thumb.

• Abductor digiti minimi

  • Proximal Attachment: Pisiform.

  • Distal Attachment: Proximal phalanx of $5^{th}$ digit.

  • Innervation: Ulnar nerve.

  • Main Action: Abducts $5^{th}$ digit.

• Flexor digiti minimi brevis

  • Proximal Attachment: Hook of hamate and flexor retinaculum.

  • Distal Attachment: Proximal phalanx of $5^{th}$ digit.

  • Innervation: Ulnar nerve.

  • Main Action: Flexes proximal phalanx of $5^{th}$ digit.

• Opponens digiti minimi

  • Proximal Attachment: Hook of hamate and flexor retinaculum.

  • Distal Attachment: $5^{th}$ metacarpal.

  • Innervation: Ulnar nerve.

  • Main Action: Brings $5^{th}$ digit into opposition with thumb.

• Lumbricals $1$ and $2$

  • Proximal Attachment: Tendons of flexor digitorum profundus.

  • Distal Attachment: Lateral sides of extensor expansions.

  • Innervation: Median nerve.

  • Main Action: Flex metacarpophalangeal joints (MCP joints); extend interphalangeal joints (IP joints) of $2^{nd}–5^{th}$ digits.

• Lumbricals $3$ and $4$

  • Proximal Attachment: Tendons of flexor digitorum profundus.

  • Distal Attachment: Extensor expansions.

  • Innervation: Ulnar nerve.

  • Main Action: Flex metacarpophalangeal joints (MCP joints); extend interphalangeal joints (IP joints) of $2^{nd}–5^{th}$ digits.

• Dorsal interossei ($1–4$)

  • Proximal Attachment: Metacarpals.

  • Distal Attachment: Proximal phalanges; extensor expansions.

  • Innervation: Ulnar nerve.

  • Main Action: Abduct $2^{nd}–4^{th}$ digits.

• Palmar interossei ($1–3$)

  • Proximal Attachment: Metacarpals.

  • Distal Attachment: Proximal phalanges; extensor expansions.

  • Innervation: Ulnar nerve.

  • Main Action: Adduct $2^{nd}$, $4^{th}$, and $5^{th}$ digits.

Gluteal Region Muscles

Superficial Gluteal Muscles

• Gluteus maximus

  • Proximal Attachment: Ilium; Sacrum; coccyx; (sacrotuberous ligament).

  • Distal Attachment: Most fibers end in iliotibial tract, which inserts into lateral condyle of tibia; some fibers insert on gluteal tuberosity.

  • Innervation: Inferior gluteal nerve.

  • Main Action: Extends hip joint (especially from flexed position) and assists in lateral rotation; fixes hip joint and assists in rising from sitting position.

• Gluteus medius

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Superior gluteal nerve.

  • Main Action: Abduct and medially rotate hip joint; keep pelvis level when ipsilateral limb is weight bearing and advance opposite (unsupported) side during its swing phase.

• Gluteus minimus

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Superior gluteal nerve.

  • Main Action: Same as Gluteus medius.

• Tensor fasciae latae

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Iliotibial tract, which attaches to tibia.

  • Innervation: Superior gluteal nerve.

  • Main Action: Same as Gluteus medius.

Deep Gluteal Muscles

• Piriformis

  • Proximal Attachment: Sacrum; (sacrotuberous ligament).

  • Distal Attachment: Femur.

  • Innervation: Branches of anterior rami of $S1$ and $S2$.

  • Main Action: Laterally rotate extended hip joint and abduct hip joint when flexed; stabilize hip joint.

• Obturator internus

  • Proximal Attachment: Obturator membrane and surrounding bones.

  • Distal Attachment: Femur.

  • Innervation: Nerve to obturator internus.

  • Main Action: Laterally rotate extended hip joint and abduct hip joint when flexed; stabilize hip joint.

• Superior and inferior gemelli

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Superior gemellus: nerve to obturator internus; Inferior gemellus: nerve to quadratus femoris.

  • Main Action: Laterally rotate extended hip joint and abduct hip joint when flexed; stabilize hip joint.

• Quadratus femoris

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Nerve to quadratus femoris.

  • Main Action: Laterally rotates hip joint; stabilizes hip joint.

Thigh Muscles

Anterior Thigh Muscles Acting on Hip Joint

• Pectineus

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Femoral nerve; may receive a branch from obturator nerve.

  • Main Action: Adducts and slightly flexes hip joint; assists with lateral rotation.

• Psoas major

  • Proximal Attachment: $T12$ and all lumbar vertebrae and discs between them.

  • Distal Attachment: Femur.

  • Innervation: Anterior rami of lumbar nerves ($L1, L2, L3$).

  • Main Action: Act conjointly in flexion and lateral rotation of hip joint and in stabilizing this joint when standing.

• Psoas minor

  • Proximal Attachment: Sides of $T12–L1$ vertebrae and intervertebral discs.

  • Distal Attachment: Femur.

  • Innervation: Anterior rami of lumbar nerves ($L1, L2$).

  • Main Action: Conjoint action in flexion and lateral rotation of hip joint.

• Iliacus

  • Proximal Attachment: Os coxae, sacrum, (and anterior sacro-iliac ligaments).

  • Distal Attachment: Tendon of psoas major and femur.

  • Innervation: Femoral nerve.

  • Main Action: Conjoint action in flexion and lateral rotation of hip joint.

• Sartorius

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Tibia (as part of pes anserinus).

  • Innervation: Femoral nerve.

  • Main Action: Flexes, abducts, and lateral rotates hip joint; flexes knee joint (medially rotates leg when knee joint is flexed).

Anterior Thigh Muscles Acting on Knee Joint

• Rectus femoris

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Via the common quadriceps tendon to the patella.

  • Innervation: Femoral nerve.

  • Main Action: Extends knee joint; steadies hip joint and helps iliopsoas flex hip joint.

• Vastus lateralis

  • Proximal Attachment: Femur.

  • Distal Attachment: Via common quadriceps tendon to patella.

  • Innervation: Femoral nerve.

  • Main Action: Extends knee joint.

• Vastus medialis

  • Proximal Attachment: Femur.

  • Distal Attachment: Via common quadriceps tendon to patella.

  • Innervation: Femoral nerve.

  • Main Action: Extends knee joint.

• Vastus intermedius

  • Proximal Attachment: Femur.

  • Distal Attachment: Via common quadriceps tendon to patella.

  • Innervation: Femoral nerve.

  • Main Action: Extends knee joint.

Medial Thigh Muscles

• Adductor longus

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Obturator nerve.

  • Main Action: Adducts hip joint.

• Adductor brevis

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Obturator nerve.

  • Main Action: Adducts hip joint and, to some extent, flexes it.

• Adductor magnus

  • Proximal Attachment: Adductor part: Os coxae; Hamstring part: Os coxae.

  • Distal Attachment: Adductor part: Femur; Hamstring part: Adductor tubercle of femur.

  • Innervation: Adductor part: obturator nerve and branches of posterior division; Hamstring part: tibial part of sciatic nerve.

  • Main Action: Adducts hip joint. Adductor part: flexes hip joint; Hamstring part: extends hip joint.

• Gracilis

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Tibia (as part of pes anserinus).

  • Innervation: Obturator nerve.

  • Main Action: Adducts hip joint; flexes knee joint, medially rotating it when flexed.

• Obturator externus

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Femur.

  • Innervation: Obturator nerve.

  • Main Action: Laterally rotates hip joint; stabilizes hip joint.

Posterior Thigh Muscles

• Semitendinosus

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Tibia (as part of pes anserinus).

  • Innervation: Tibial division of sciatic nerve.

  • Main Action: Extend hip joint; flex knee joint and medially rotate it when flexed.

• Semimembranosus

  • Proximal Attachment: Os coxae.

  • Distal Attachment: Tibia.

  • Innervation: Tibial division of sciatic nerve.

  • Main Action: Extend hip joint; flex knee joint and medially rotate it when flexed.

• Biceps femoris

  • Proximal Attachment: Long head: Os coxae; Short head: Femur.

  • Distal Attachment: Fibula.

  • Innervation: Long head: tibial division of sciatic nerve; Short head: common fibular division of sciatic nerve.

  • Main Action: Flexes knee joint and laterally rotates it when flexed; long head extends hip joint.

Muscles of the Leg

Anterior Leg Muscles

• Tibialis anterior

  • Proximal Attachment: Tibia (and interosseous membrane).

  • Distal Attachment: Medial cuneiform and $1^{st}$ metatarsal.

  • Innervation: Deep fibular nerve.

  • Main Action: Dorsiflexes ankle joint (talocrural joint) and inverts foot.

• Extensor digitorum longus

  • Proximal Attachment: Tibia, fibula, (and the interosseous membrane).

  • Distal Attachment: Middle and distal phalanges of lateral four digits.

  • Innervation: Deep fibular nerve.

  • Main Action: Extends lateral four digits and dorsiflexes ankle joint (talocrural joint).

• Extensor hallucis longus

  • Proximal Attachment: Fibula (and interosseous membrane).

  • Distal Attachment: Distal phalanx of great toe (hallux).

  • Innervation: Deep fibular nerve.

  • Main Action: Extends great toe and dorsiflexes ankle joint (talocrural joint).

• Fibularis tertius

  • Proximal Attachment: Fibula (and the interosseous membrane).

  • Distal Attachment: $5^{th}$ metatarsal.

  • Innervation: Deep fibular nerve.

  • Main Action: Dorsiflexes ankle joint (talocrural joint) and aids in eversion of foot.

Lateral Leg Muscles

• Fibularis longus

  • Proximal Attachment: Fibula.

  • Distal Attachment: $1^{st}$ metatarsal and medial cuneiform.

  • Innervation: Superficial fibular nerve.

  • Main Action: Everts foot and weakly plantarflexes ankle joint (talocalcaneal joint).

• Fibularis brevis

  • Proximal Attachment: Fibula.

  • Distal Attachment: $5^{th}$ metatarsal.

  • Innervation: Superficial fibular nerve.

  • Main Action: Everts foot and weakly plantarflexes ankle joint.

Posterior Leg Compartment Muscles (Superficial)

• Gastrocnemius

  • Proximal Attachment: Lateral head: Femur; Medial head: Femur.

  • Distal Attachment: Calcaneus via calcaneal tendon.

  • Innervation: Tibial nerve.

  • Main Action: Plantarflexes ankle joint (talocrural joint) when knee joint is extended.

• Soleus

  • Proximal Attachment: Fibula and tibia.

  • Distal Attachment: Calcaneus via calcaneal tendon.

  • Innervation: Tibial nerve.

  • Main Action: Plantarflexes ankle joint (talocrural joint); stabilizes ankle joint.

• Plantaris

  • Proximal Attachment: Femur.

  • Distal Attachment: Calcaneus via calcaneal tendon.

  • Innervation: Tibial nerve.

  • Main Action: Weakly assists gastrocnemius in plantarflexing ankle joint (talocrural joint).

Posterior Leg Compartment Muscles (Deep)

• Popliteus

  • Proximal Attachment: Femur and lateral meniscus.

  • Distal Attachment: Tibia.

  • Innervation: Tibial nerve.

  • Main Action: Weakly flexes knee joint and unlocks it by rotating femur on fixed tibia; medially rotates tibia.

• Flexor hallucis longus

  • Proximal Attachment: Fibula (and interosseous membrane).

  • Distal Attachment: Distal phalanx of great toe (hallux).

  • Innervation: Tibial nerve.

  • Main Action: Flexes great toe at all joints; weakly plantarflexes ankle joint (talocrural joint); supports medial longitudinal arch of foot.

• Flexor digitorum longus

  • Proximal Attachment: Tibia.

  • Distal Attachment: Distal phalanges of lateral four digits.

  • Innervation: Tibial nerve.

  • Main Action: Flexes lateral four digits; plantarflexes ankle joint (talocrural joint); supports longitudinal arches of foot.

• Tibialis posterior

  • Proximal Attachment: (Interosseous membrane,) Tibia, and fibula.

  • Distal Attachment: Some tarsals and metatarsals.

  • Innervation: Tibial nerve.

  • Main Action: Plantarflexes ankle joint (talocrural joint); inverts foot; maintains medial longitudinal arch.

Muscles of the Foot

First Layer of Plantar Foot Muscles

• Abductor hallucis

  • Proximal Attachment: Calcaneus; (flexor retinaculum; plantar aponeurosis).

  • Distal Attachment: Phalanx of $1^{st}$ digit.

  • Innervation: Medial plantar nerve.

  • Main Action: Abducts and flexes $1^{st}$ digit (great toe, hallux).

• Flexor digitorum brevis

  • Proximal Attachment: Calcaneus (and plantar aponeurosis).

  • Distal Attachment: Middle phalanges of lateral four digits.

  • Innervation: Medial plantar nerve.

  • Main Action: Flexes lateral four digits.

• Abductor digiti minimi

  • Proximal Attachment: Calcaneus.

  • Distal Attachment: Proximal phalanx of $5^{th}$ digit.

  • Innervation: Lateral plantar nerve.

  • Main Action: Abducts and flexes little toe ($5^{th}$ digit).

Second Layer of Plantar Foot Muscles

• Quadratus plantae

  • Proximal Attachment: Calcaneus.

  • Distal Attachment: Tendon of flexor digitorum longus.

  • Innervation: Lateral plantar nerve.

  • Main Action: Assists flexor digitorum longus in flexing lateral four digits.

• Lumbricals

  • Proximal Attachment: Tendons of flexor digitorum longus.

  • Distal Attachment: Dorsal expansions (over lateral four digits).

  • Innervation: Medial one: medial plantar nerve; Lateral three: lateral plantar nerve.

  • Main Action: Flex proximal IP joint; extend middle and distal IP joints of lateral four digits.

Third Layer of Plantar Foot Muscles

• Flexor hallucis brevis

  • Proximal Attachment: Cuboid and lateral cuneiforms.

  • Distal Attachment: (Proximal phalanx of) $1^{st}$ digit.

  • Innervation: Medial plantar nerve.

  • Main Action: Flexes $1^{st}$ digit.

• Adductor hallucis

  • Proximal Attachment: Oblique head: Metatarsals $2–4$; Transverse head: (Plantar of metatarsophalangeal—MTP—joints).

  • Distal Attachment: Tendons of both heads attach to proximal phalanx of $1^{st}$ digit.

  • Innervation: Lateral plantar nerve.

  • Main Action: Traditionally said to adduct $1^{st}$ digit; assists in maintaining transverse arch of foot by pulling metatarsals medially.

• Flexor digiti minimi brevis

  • Proximal Attachment: $5^{th}$ metatarsal.

  • Distal Attachment: Proximal phalanx of $5^{th}$ digit.

  • Innervation: Lateral plantar nerve.

  • Main Action: Flexes $5^{th}$ digit.

Fourth Layer of Plantar Foot Muscles

• Plantar interossei (three muscles)

  • Proximal Attachment: Metatarsals $3–5$.

  • Distal Attachment: Phalanges of $3^{rd}–5^{th}$ digits.

  • Innervation: Lateral plantar nerve.

  • Main Action: Adduct and flex digits $3–5$.

• Dorsal interossei (four muscles)

  • Proximal Attachment: Metatarsals $1–5$.

  • Distal Attachment: $1^{st}$: proximal phalanx of $2^{nd}$ digit; $2^{nd}–4^{th}$: proximal phalanx of $2^{nd}–4^{th}$ digits.

  • Innervation: Lateral plantar nerve.

  • Main Action: Abduct and flex digits $2^{4}$.

Dorsal Foot Muscles

• Extensor digitorum brevis

  • Proximal Attachment: Calcaneus; (interosseous talocalcaneal ligament; inferior extensor retinaculum).

  • Distal Attachment: Long extensor tendons of three intermediate digits.

  • Innervation: Deep fibular nerve.

  • Main Action: Aids the extensor digitorum longus in extending the three intermediate toes.

• Extensor hallucis brevis

  • Proximal Attachment: Calcaneus.

  • Distal Attachment: Proximal phalanx of great toe (digit $1$).

  • Innervation: Deep fibular nerve.

  • Main Action: Aids the extensor hallucis longus in extending the great toe.