The hip joint is one of the most important joints in the body for movement, like walking or dancing.

Part 1: The Bony Pelvis & The Hip Bone

The bony pelvis is a basin-shaped ring of bones connecting the vertebral column to the femurs, formed by the sacrum, coccyx, and the two hip bones (Os coxae).

The Hip Bone (Os Coxa)

Each large, irregularly shaped hip bone is a fusion of three primary bones that completes by the end of puberty:

• Ilium: The largest, most superior part, forming the prominent "wings" of the pelvis.
• Ischium: Forms the posteroinferior (lower-back) part of the hip bone.
• Pubic Bone (Pubis): Forms the anterior part of the hip bone.

Detailed Anatomy of the Hip Bone

• Ilium:
  • Iliac Crest: The palpable superior border, terminating anteriorly as the Anterior Superior Iliac Spine (ASIS) and posteriorly as the Posterior Superior Iliac Spine (PSIS).
  • Other Spines: Anterior Inferior Iliac Spine (AIIS) and Posterior Inferior Iliac Spine (PIIS).
  • Surfaces: The large, concave internal Iliac Fossa; the rough outer Gluteal Surface for gluteal muscle attachment; and the medial Auricular Surface for articulation with the sacrum.
  • Notches: The Greater Sciatic Notch, a large indentation for passage of the sciatic nerve.
• Ischium:
  • Ischial Tuberosity: The large, roughened "sitting bone" that supports body weight when seated.
  • Ischial Spine: A pointed projection posterior to the acetabulum, separating the Greater and Lesser Sciatic Notches.
  • Ramus of the Ischium: Projects forward to join with the pubis.
• Pubis:
  • Body of Pubis: The central part that meets the other pubic bone at the Pubic Symphysis.
  • Superior & Inferior Rami: Bars of bone that help form the acetabulum and obturator foramen.
  • Key Markings: Includes the Pubic Tubercle and Obturator Crest for ligament and muscle attachments.

The femur is the longest, strongest, and heaviest bone in the body, transmitting weight from the hip to the tibia.

Key Ligaments of the Hip Joint

• Iliofemoral Ligament (Y-ligament of Bigelow): The strongest ligament in the body, located anteriorly. It prevents hyperextension of the hip.
• Pubofemoral Ligament: Located anteroinferiorly, it limits excessive abduction and extension.
• Ischiofemoral Ligament: Located posteriorly, it limits internal rotation and adduction.
• Ligament of the Head of the Femur (Ligamentum Teres): Located inside the joint, connecting the fovea capitis to the acetabulum.
• Transverse Acetabular Ligament: Bridges the acetabular notch, completing the socket.

The powerful muscles of the lower limb are designed for stability, locomotion, and maintaining an upright posture. We will cover them regionally, starting with the hip and gluteal region.

The Iliopsoas is the strongest hip flexor in the body. It's a composite muscle formed by the Psoas Major and Iliacus, which merge to insert on the lesser trochanter of the femur.

• Psoas Major: Originates from the lumbar vertebrae.
• Iliacus: Originates from the iliac fossa.
• Main Actions: As the main flexor of the hip, it is essential for walking, running, and lifting the leg.

These muscles are essential for hip movement, stability, and posture, divided into superficial and deep layers.

Deep Gluteal Muscles (Short External Rotators)

This group of six smaller muscles lies deep to the gluteus maximus. They collectively function as powerful lateral rotators of the thigh and help stabilize the head of the femur in the acetabulum.

Piriformis

• Origin: Anterior surface of sacrum.
• Insertion: Superior border of greater trochanter.
• Innervation: Nerve to Piriformis (S1, S2).
• Actions: Laterally rotates, abducts (when hip is flexed), and extends the thigh.

Superior Gemellus

• Origin: Ischial spine.
• Insertion: Medial surface of greater trochanter (with Obturator Internus tendon).
• Innervation: Nerve to Obturator Internus (L5, S1).
• Actions: Laterally rotates and abducts the thigh.

Obturator Internus

• Origin: Pelvic surface of obturator membrane.
• Insertion: Medial surface of greater trochanter.
• Innervation: Nerve to Obturator Internus (L5, S1).
• Actions: Laterally rotates and abducts the thigh.

Inferior Gemellus

• Origin: Ischial tuberosity.
• Insertion: Medial surface of greater trochanter (with Obturator Internus tendon).
• Innervation: Nerve to Quadratus Femoris (L4, L5, S1).
• Actions: Laterally rotates and abducts the thigh.

Obturator Externus

• Origin: External surface of obturator membrane.
• Insertion: Trochanteric fossa of femur.
• Innervation: Obturator Nerve (L3, L4).
• Actions: Laterally rotates and adducts the thigh.

Quadratus Femoris

• Origin: Lateral border of ischial tuberosity.
• Insertion: Quadrate tubercle on intertrochanteric crest.
• Innervation: Nerve to Quadratus Femoris (L4, L5, S1).
• Actions: A powerful lateral rotator and adductor of the thigh.

Clinical Note (Piriformis Syndrome): The sciatic nerve usually passes inferior to the piriformis muscle. If the muscle is tight or inflamed, it can compress the nerve, causing pain, tingling, and numbness down the back of the leg (sciatica).

Summary Table of Gluteal Muscles

The powerful muscles of the thigh are divided into three compartments: anterior (extensors), medial (adductors), and posterior (flexors/hamstrings).

Anterior Compartment of the Thigh (Extensors)

• Innervation: Femoral Nerve (L2, L3, L4)
• Main Actions: Primarily extension of the knee; some flexion of the hip.

Quadriceps Femoris

A group of four muscles (Rectus Femoris, Vastus Lateralis, Vastus Medialis, Vastus Intermedius) that converge on the patellar tendon. It is the powerful extensor of the knee. The Rectus Femoris is unique as it also flexes the hip.

Sartorius

The longest muscle in the body. It flexes, abducts, and laterally rotates the thigh, and also flexes the knee (the "tailor's muscle" for crossing legs).

Posterior Compartment of the Thigh (Hamstrings)

• Innervation: Sciatic Nerve (Tibial portion), except for the short head of Biceps Femoris.
• Main Actions: Primarily flexion of the knee and extension of the hip.

Biceps Femoris

The lateral hamstring muscle, with a long and a short head. It flexes the knee and laterally rotates the leg.

Semitendinosus

A medial hamstring muscle. It flexes the knee and medially rotates the leg.

Semimembranosus

A medial hamstring muscle, deep to the Semitendinosus. It flexes the knee and medially rotates the leg.

Summary Table of Thigh Muscles

The muscles of the leg are divided into four compartments by interosseous membrane and fascial septa: anterior, lateral, posterior superficial, and posterior deep.

Anterior Compartment of the Leg

• Innervation: Deep Fibular (Peroneal) Nerve (L4, L5, S1)
• Main Actions: Primarily dorsiflexion of the ankle and extension of the toes.

This compartment includes the Tibialis Anterior (main dorsiflexor and invertor), Extensor Digitorum Longus (extends lateral four toes), Extensor Hallucis Longus (extends great toe), and Fibularis Tertius (dorsiflexes and everts).

Lateral Compartment of the Leg

• Innervation: Superficial Fibular (Peroneal) Nerve (L5, S1, S2)
• Main Actions: Primarily eversion of the foot; some plantarflexion.

This compartment contains two muscles: Fibularis (Peroneus) Longus and Fibularis (Peroneus) Brevis. Together, they are the main everters of the foot.

Posterior Compartment of the Leg

• Innervation: Tibial Nerve (L4-S2)
• Main Actions: Primarily plantarflexion of the ankle, inversion of the foot, and flexion of the toes.

Superficial Group

This group forms the bulk of the calf and inserts via the calcaneal (Achilles) tendon. It includes the two-headed Gastrocnemius, the powerful underlying Soleus, and the small Plantaris. Together, Gastrocnemius and Soleus are known as the Triceps Surae and are powerful plantarflexors.

Deep Group

These muscles lie deep to the superficial group. They include the Popliteus (unlocks the knee), Flexor Digitorum Longus (flexes lateral four toes), Flexor Hallucis Longus (flexes the great toe), and the Tibialis Posterior (the main invertor of the foot).

Summary Table of Leg Muscles

The intrinsic muscles of the foot are divided into dorsal (top) and plantar (sole) groups, responsible for fine motor control, supporting the arches, and assisting in locomotion.

Plantar Muscles of the Foot

These muscles are organized into four layers from superficial to deep. They are primarily innervated by the Medial and Lateral Plantar Nerves (branches of the Tibial Nerve) and are crucial for supporting the arches and controlling fine movements of the toes.

Layer 1 (Superficial)

Abductor Hallucis

Abducts and flexes the great toe. (Innervated by Medial Plantar N.)

Flexor Digitorum Brevis

Flexes the lateral four toes at the PIP joints. (Innervated by Medial Plantar N.)

Abductor Digiti Minimi

Abducts and flexes the little toe. (Innervated by Lateral Plantar N.)

Layer 2

Quadratus Plantae

Assists the Flexor Digitorum Longus (FDL) tendon in flexing the toes by straightening its line of pull. (Innervated by Lateral Plantar N.)

Lumbricals (4)

Flex the MTP joints and extend the IP joints of the lateral four toes. (Innervated by both Medial and Lateral Plantar Nerves).

Layer 3

Flexor Hallucis Brevis

Flexes the great toe at the MTP joint. (Innervated by Medial Plantar N.)

Adductor Hallucis

Has two heads (oblique and transverse); adducts the great toe. (Innervated by Lateral Plantar N.)

Flexor Digiti Minimi Brevis

Flexes the little toe. (Innervated by Lateral Plantar N.)

Layer 4 (Deepest)

Plantar Interossei (3)

Adduct toes 3-5 (PAD - Plantar Adduct). (Innervated by Lateral Plantar N.)

Dorsal Interossei (4)

Abduct toes 2-4 (DAB - Dorsal Abduct). (Innervated by Lateral Plantar N.)

Note on extrinsic tendons: Although their muscle bellies are in the leg, the tendons of Tibialis Anterior and Fibularis Longus cross the ankle and insert deep in the foot, providing significant dynamic support to the arches.

Summary Table of Foot Muscles

Use this map to move through the lesson in order:

1. Anatomy of the Lower Extremities
2. The Hip Joint
3. The Femur (Thigh Bone)
4. Muscles of the Lower Limb
5. Hip Muscles: The Iliopsoas Group
6. 1. Muscles of the Gluteal Region (Buttocks)
7. 2. Muscles of the Thigh
8. 3. Muscles of the Leg
9. 4. Muscles of the Foot

For deeper study, compare this lesson with current guidance and standard references from recognised medical and public-health organisations.

• World Health Organization
• Centers for Disease Control and Prevention
• Merck Manuals